SIGNAL BROADCASTING IN A SATELLITE TV CHANNEL
Chapter 1
INTRODUCTION
1.1 Background
Television (TV) is a telecommunication medium for transmitting and receiving moving
images that can be monochrome
(black-and-white)
or colored, with accompanying sound. "Television" may also refer
specifically to a television
set, television programming, and television transmission.
The etymology of the word has a mixed Latin and
Greek origin, meaning "far sight": Greek tele far, and Latin visio,
sight (from video, vis- to see, or to view in the first
person).
Broadcast television is the sending of video and
audio signals to devices (such as television sets) within a broadcast area that
can receive the broadcast signals. Television broadcast technology was
developed in the 1940s. The success of the television marketplace is due to
standardized, reliable, and relatively inexpensive television receivers and a
large selection of media sources. The first television transmission standards
used analog radio transmission to provide black and white video service. These
initial television technologies have evolved to allow for both black and white
and color television signals, along with advanced services such as stereo audio
and closed caption text. This was a very important evolution as new television
services (such as color television) can be on the same radio channel as black
and white television services.
Television systems initially used analog
television technology. While analog television transmission provided relatively
good video and audio signals, it does not easily allow the sending and
receiving of multimedia signals and services. As a result, TV broadcast systems
have been converted to use digital signal transmission. Each digital TV
broadcast channel can provide several digital TV channels along with advanced
television services.
New technologies allow transmission of
high-definition television (HDTV). HDTV is the term used to describe a
high-resolution video and high quality audio signal as compared to standard
NTSC or PAL video transmission. HDTV signals can be in analog or digital form.
Digital HDTV systems have the added benefits of providing data and other
multimedia services.
This figure shows a television broadcast system.
This television system consists of a television production studio, a high-power
transmitter, a communications link between the studio and the transmitter, and
network feeds for programming. The production studio controls and mixes the sources
of information including videotapes, video studio, computer created images
(such as captions), and other video sources. A high-power transmitter
broadcasts a single television channel. The television studio is connected to
the transmitter by a high bandwidth communications link that can pass video and
control signals. This communications link may be a wired (coax) line or a
microwave link. Many television stations receive their video source from a
television network. This allows a single video source to be relayed to many
television transmitters.
1.2 Objective
The purpose of this system is to
define the transmission system by television signaling, among various types of
standard television broadcasting handled by signaling technic used on
broadcasting stations. The purpose of signaling method is to define on simple
signal transmits and shows a simple acknowledgement to all.
1.3 Scope
This project is not a full
overview on Television transmission signaling. Day-by-day signaling of
Television transmission technic and the devices are changing. So the pattern of
knowledge also changed by the way of hunting technology. And all over the scope
becomes higher to higher for acquiring knowledge.
1.4 Television system
Television,
system of sending and receiving pictures and sound by means of electronic
signals transmitted through wires and optical fibers or by Electromagnetic radiation. These
signals are usually broadcast from a central source, a television station, to
reception devices such as television sets in homes or relay stations such as
those used by cable television service providers. Television is the most
widespread form of communication in the world. Though most people will never
meet the leader of a country, travel to the moon, or participate in a war, they
can observe these experiences through the images on their television.
A television program is created by focusing a
television camera on a scene. The camera changes light from the scene into an electric
signal, called the video signal, which varies depending on the strength, or
brightness, of light received from each part of the scene. In color television,
the camera produces an electric signal that varies depending on the strength of
each color of light.
Three or four cameras are typically used to produce
a television program. The video signals from the cameras are processed in a
control room, and then combined with video signals from other cameras and
sources, such as videotape recorders, to provide the variety of images and
special effects seen during a television program.
Audio signals from microphones placed in or near
the scene also flow to the control room, where they are amplified and combined.
Except in the case of live broadcasts (such as news and sports programs) the
video and audio signals are recorded on tape and edited, assembled with the use
of computers into the final program, and broadcast later. In a typical
television station, the signals from live and recorded features, including
commercials, are put together in a master control room to provide the station's
continuous broadcast schedule. Throughout the broadcast day, computers start
and stop videotape machines and other program sources, and switch the various
audio and visual signals. The signals are then sent to the transmitter.
The transmitter amplifies the video and audio
signals, and uses the electronic signals to modulate, or vary, carrier waves
(oscillating electric currents that carry information). The carrier waves are
combined (duplexed), and then sent to the transmitting antenna, usually placed on the tallest
available structure in a given broadcast area. In the antenna, the oscillations
of the carrier waves generate electromagnetic waves of energy that radiate
horizontally throughout the atmosphere. The waves excite weak electric currents
in all television-receiving antennas within range. These currents have the
characteristics of the original picture and sound currents. The currents flow
from the antenna attached to the television into the television receiver, where
they are electronically separated into audio and video signals. These signals
are amplified and sent to the picture tube and the speakers, where they produce
the picture and sound portions of the program.
ATN
News TV.
ATN
News is a 24/7 news channel in Bangla News . Dynamism is the key word for ATN
News. It has opened up opportunities for national and international
interactions in the broadcasting world through satellite transmission. Besides
supplying visual material within & outside the country, ATN News aims at
providing information and education through its programmers all over the world.
In the process it also ensures that the country’s culture and heritage are put
to exposure for understing and appreciation.
Technical
Information of ATN News TV.
For
Regional:
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Satellite
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Apstar
IIR (Telstar 10)
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Satellite
Orbital Location
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76.5
Degree East
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Downlink
Frequency
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3997.75
MHz
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Symbol
Rate
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3200
KBPS, FEC 3/4
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Downlink
Polarization
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Horizontal
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Modulation
Type
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QPSK
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Transponder
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C10A
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ATN
News is the first ever private satellite
TV channel in Bangladesh. The channel started operating in the year 2009. The
channel extended its transmission to Europe and beyond. It was converted to
digital system globally. ATN News started broadcasting news bulletins. ATN News changed its analog format of
transmission to digital satellite newsgathering system (SNG). It started
English news bulletins. ATN News earned wide appreciation for its coverage of
the Iraq war by introducing hourly news bulletins. Among the TV channels of
Bangladesh ATN News has taken up the
challenge of providing its viewers with the latest information about
development at home and abroad.
ATN
News is also highly popular in South
East Asia, Far East, Middle East, Europe, the United States of America, Canada,
Australia and Africa.. This has been reflected in the volume of advertisement
it receives from within and outside the country.
ATN News is a 24-hour TV channel and
telecasts its News keeping in mind the internationally accepted broadcast
criterion- Education, Information and Entertainment.
Chapter 2
TV RECORDING
AND EDITING
2.1 TV recording
TV DVRs generally
uses the electronic programming
guide (EPG).
2.2 Hardware features
Hardware features of security DVRs vary between
manufacturers and may include but are not necessarily limited to
§ Designed for rack mounting or
desktop configurations.
§ Single or
multiple video inputs with connector types consistent with the analogue or
digital video provided such as coaxial cable, twisted pair or optical fiber cable.
The most common number of inputs are 1, 2, 4, 8, 16 and 32. Systems may be
configured with a very large number of inputs by networking or bussing in dividual
DVRs together.
§ Looping video
outputs for each input which duplicates the corresponding input video signal
and connector type. These output signals are used by other video equipment such
as matrix switchers, multiplexers, and video monitors.
§ Controlled
outputs to external video display monitors.
§ Front panel
switches and indicators that allow the various features of the machine to be
controlled.
§ Network
connections consistent with the network type and utilized to control features
of the recorder and to send and/or receive video signals.
§ Connections to
external control devices such as keyboards.
§ A connection to
external pan-tilt-zoom drives that position cameras.
§ Internal CD,
DVD, VCR devices typically for archiving video.
§ Connections to
external storage media.
§ Alarm event
inputs from external security detection devices, usually one per video input.
§ Alarm event
outputs from internal detection features such as motion detection or loss of
video.
2.3 Software features
Software features
vary between manufacturers and may include but are not necessarily limited to
§ User selectable
image capture rates either on
an all input basis or input by input basis. The capture rate feature may be
programmed to automatically adjust the capture rate on the occurrence of an
external alarm or an internal event
§ Selectable image resolution either on an all
input basis or input by input basis. The image resolution feature may be
programmed to automatically adjust the image resolution on the occurrence of an
external alarm or an internal event.
§ Compression
methods determine quality of playback. H.264 hardware compression offers fast
transfer rates over the internet with high quality video.
§ Motion
detection: Provided on an input by input basis, this feature detects motion in
the total image or a user definable portion of the image and usually provides
sensitivity settings. Detection causes an internal event that may be output to
external equipment and/or be used to trigger changes in other internal
features.
§ Lack of motion
detection. Provided on an input by input basis, this feature detects the
movement of an object into the field of view and remaining still for a user
definable time. Detection causes an internal event that may be output to
external equipment and/or used to trigger changes in other internal features.
§ Direction of
motion detection. Provided on an input by input basis, this feature detects the
direction of motion in the image that has been determined by the user as an
unacceptable occurrence. Detection causes an internal event that may be output
to external equipment and/or be used to trigger changes in other
internal features.
§ Routing of input
video to video monitors based on user inputs or automatically on alarms or
events.
§ Input, time and
date stamping
§ Alarm and event
logging on appropriate video inputs.
§ Alarm and event
search.
§ One or more
sound recording channels.
§ Archival.
§ Commercial
hopping. Rather than fast-forwarding through commercials, an undocumented
feature of the TiVo box is that the
user can reprogram the tab-to-end button by entering a sequence of buttons on
the remote: SELECT-PLAY-SELECT-3-0-SELECT and listening for the confirming
chimes that signal the feature has been activated (or deactivated). The
tab-to-end button no longer jumps to the end of a recording when so activated:
It skips 30 seconds, which is the length of U.S. commercials. In
combination with the 8-second rewind button, most viewers can completely miss
commercial breaks in programming.
2.4 Getting the
Bit Recorded
Even after getting
the signal converted to bits, it is still difficult to record: the hardest part
is finding a scheme that can record the bits fast enough to keep up with the
signal. For example, to record two channels of audio at 44.1 kHz sample rate with a 16 bit word size,
the recording software has to handle 1,411,200 bits per second.
2.5 Techniques to record to commercial media
For digital cassettes, the read/write head
moves as well as the tape in order to maintain a high enough speed to keep the
bits at a manageable size.
For optical disc recording technologies such as CDs or DVDs, a laser is used to burn microscopic holes into
the dye layer of the medium. A weaker laser is used to read these signals. This
works because the metallic substrate of the disc is reflective, and the
unburned dye prevents reflection while the holes in the dye permit it, allowing
digital data to be represented.
2.6 Digital Audio Recording
2.6.1 Word Size
The number of bits used
to represent a single audio wave (the word
size) directly affects the achievable noise level of a signal recorded with
added dither, or the distortion of an undithered signal. Increasing a
sample's word length by one bit doubles its possible values, like wise
increasing the potential accuracy of each sample and the fidelity of the
recording to the original. 24-bit recording is generally considered a current
practical limit as this word length allows a signal-to-noise
ratio exceeding that of most
analog circuitry, which by necessity must be used in at least two points in the
recording/playback chain.
2.6.2 Sample rate
The sample rate is even more important a consideration
than the word size. If the sample rate is too low, the sampled signal cannot be
reconstructed to the original sound signal. Hence the output will be different
from the input. The process of under sampling results in aliasing whereby the high frequency components
of the sound wave are represented as being lower than they should be. This
causes the output wave shape to be severely altered.
To overcome
aliasing, the sound signal (or other signal) must be sampled at a rate at least
twice that of the highest frequency component in the signal. This is known as
the Nyquist-Shannon sampling
theorem.
For recording
music-quality audio the following PCM sampling rates are the most common:
44.1 kHz
48 kHz 88.2 kHz 96 kHz 176.4 kHz 192 kHz
When making a
recording, experienced audio recording and mastering engineers will normally do
a master recording at a higher sampling rate (i.e. 88.2, 96, 176.4 or
192 kHz) and then do any editing or mixing at that same higher frequency.
High resolution PCM recordings have been released on DVD-Audio (also known as
DVD-A), DAD (Digital Audio Disc—which utilizes the stereo PCM audio tracks of a
regular DVD), Dual Disc (utilizing the DVD-Audio layer), or Blu-ray (Profile
3.0 is the Blu-ray audio standard, although as of mid-2009 it is unclear
whether this will ever really be used as an audio-only format). In addition it
is nowadays also possible and common to release a high resolution recording
directly as either an uncompressed WAV or lossless compressed FLAC file (usually at 24 bits) without
down-converting it .
However if a CD
(the CD Red Book standard is 44.1 kHz 16 bit) is to be made from a
recording, then doing the initial recording using a sampling rate of
44.1 kHz is obviously one approach. Another approach that is usually
preferred is to use a higher sample rate and then down sample to the final format's sample rate.
This is usually done as part of the mastering process. One advantage to the latter
approach is that way a high resolution recording can be released, as well as a
CD and/or lossy compressed file such as mp3—all from the same master recording.
2.6.3 Analog Video RecordingTelevision's first decade was the era of live TV. Most original programming was broadcast when it was produced. This was not by choice - no satisfactory recording method except photographic film was available. Many live shows actually were televised twice for different time zones.
Fig (1): Kinescope (courtesy Museum of Science and Technology
Canada)
It was possible to make a record of a show, a film shot from the screen of
a broadcast quality monitor, called a kinescope
recording. These recordings were frequently made for use by stations
beyond the reach of cable or microwave interconnections with the networks.
Because kinescope recordings were expensive and of poor quality, almost from
the start of television, engineers searched for a way to record video images
on magnetic tape.In the 1950s, audio tape recordings were widely used on radio, and tape recorders were becoming popular on the consumer market. It was logical that if signals representing audio could be stored on tape, their video counterparts also could be recorded. The problem was that the video signal was far more complex, and the frequencies involved were much higher than in audio. |
2.6.4 Analog Videotape Recording Principles
Getting the Signal On Tape
We've got a real problem when we try to record full blown video using a method similar to what we do with audio. Basic physics states that the highest frequency you can record on magnetic tape is determined by two things. One is how fast the tape travels by the head, and the other is how narrow the head gap is (the space between the two poles on a recording head).The formula is
Fmax = Vtape / 2 x Wgap.
 there is a limit to how high a frequency you can record |
This means that the maximum frequency
you can record is equal to the velocity of the tape as is passes by the head,
divided by twice the width of the tape head's gap.
In audio, the highest frequency that has to be recorded is about 20,000
cycles per second. This means that, at 15 inches per second tape speed,
you'll need a head gap of .000375 of an inch (20,000 = 15 / 2 x Wgap,
or Wgap = 15 / 2 x
20000 = .000375.)This was totally manageable in the 1950s, and made for
excellent quality audio recordings. In fact, head gaps could be made as small
as .00006 inches (about 1/16000 of an inch wide), meaning audio recordings as
slow as 3 3/4 inches per second sounded fine.But, we digress. Just keep in mind that .00006 of an inch head gap width. |
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The video signal has
frequencies up to 4.2 million Hz in it. Using our now
well-worn formula, we plug in the numbers, and come up with a tape speed of 504
inches per second! (4200000
Hz x 2 x .00006 gap width = 504 inches per second.) A
half hour recording at this speed would require 75,600
feet of videotape (a little more than 14 miles).
This just won't do. But we tried anyway:
November, 1951: The Electronics Division of Bing
Crosby Enterprises demonstrated a black and white videotape recorder with
twelve fixed recording heads. Using high speed head switching, 1" tape
and 100 inches per second tape speed, a 1.7 MHz recording is made on reels
that hold 16 minutes of
program material - and 8000 feet of tape.
1952: The
BBC showed off VERA (Vision Electronic Recording Apparatus), which uses two
video tracks, tape speeds of 200 inches per second - and quite low resolution.
December, 1953: RCA displayed their black and white
system - one continuous video track, with a17" reel of 1/4" wide
tape, moving at 360 inches per second. Later that month, RCA proved that
they could perform the same feat in colour. They used a 17" reel of
½" wide tape, three video tracks (R, G, and B) along with a sync pulse
track, still running at 360 inches per second, and a recording time of...four
minutes.
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The
breakthrough finally came in 1956 from a team of engineers working at Ampex.
They solved the problem by mounting
the recording heads on a drum that revolved at high speed (14,400
rpm), impressing the signal crosswise on the tape. The
actual head-to-tape speed was very high, but the tape transport moved the
tape at a relatively slow speed. This was the
"quad," so called because of its use of four recording heads. It
used tape 2 inches wide.
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The Quad Did It
The Quad Works (and, why we care...)
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Here's a little
nitty-gritty on what the guys at Ampex designed into their machine.
In one second, the head drum rotates 240 times,
placing 960 short tracks on the videotape. The tape is 2" wide, but room
is left for audio, control track, and blank, separating "guard
bands" between all of the tracks. Therefore, each video track is about 1
4/5 inches long. To allow uninterrupted output from the switched heads, only
1 5/8 inches is used for fresh video - the rest is overlap from the previous
track’s video.
Fig (2): Track layout on a 2"
quadruplex videotape machine
If we do the math on the Ampex system, we find
out that the effective
tape-to-head speed is 1560
inches per second. (960 tracks
per second x 1.625 inches = 1560 i.p.s.)Using our familiar formula (1560 / 2 x .00006 = 13,000,000), this means we can record a
frequency of up to 13 MHz - lots of room for our 4.2 MHz video signal.
It's not quite that simple, however.
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The next problem is one of the strength of a
signal relative to its frequency. If we try to record a full-response signal
directly on tape, we find out that the
higher the frequency of a recorded signal, the greater the playback output
from the tape. Conversely, the lower the frequency, the lower the playback
output.
This causes us grief because we
are trying to put on videotape a signal between 30 Hz and 4,200,000 Hz.
This is a 140,000:1 ratio. And that, in turn, means that the playback voltage at 30 Hz
will be 1/140,000 as strong as that at 4.2 MHz. Expressed as dB (like audio), this
ends up being a range from lowest to highest of about 103 dB. That’s like trying to
record a quiet conversation and an airport runway at the same time. This
is not good. What to do? |
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The solution lies in a device called a
modulator. A modulator is a sort of valve. One signal,
called a carrier,
is sent into the modulator. A second signal, the modulating one, varies a
parameter of the carrier - either (1) its level (or "amplitude") or
(2) its (otherwise constant) frequency. The output is either an
"amplitude modulation" or a "frequency modulation" of the
input signal; known as AM and FM to most radio listeners.
 Fig (3): Types
of modulation (A.M. and F.M.) |
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What we're going to use to solve our problem is
frequency modulation. A carrier of 8.6 MHz is modulated by the incoming video
(which may be up to 4.2 MHz). The modulator output will vary from 4.4 MHz to
12.8 MHz. (8.6 - 4.2 = 4.4 MHz;
8.6 + 4.4 = 12.8 MHz.) Within
these fluctuations is a replica of the program video.
What's important to us is the ratio of the
highest to lowest frequencies on the tape. That's now about 3:1 (12.8 :
4.4). This is a little less than a 10 dB difference. Any noise in any
electronic amplifiers we use will be away back at -60 dB; an excellent safety
margin.
Now, to use this modulated system, we'll need
to be able to record up to 13 MHz of information on our VTR. With the quad
system, and its effective tape to head speed of 1560 inches, we have that
recording capacity. What a coincidence...
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Fig(4):Video frequency modulation
2.6.5 Getting The Signal Off The Tape
The demodulator (If there was a modulator you just had to know there'd be a demodulator!) has the reverse function of the modulator - it takes the high frequency signal from the tape and retrieves the video signal from it. This fundamental video signal is then put through various processing circuitry to make it stable and to adjust its levels, at which point it is like any other video source.Of course, it's more complicated than that. We've glossed over all kinds of things like...
Servos and Time Base Correctors
Keeping Track of the Tracks
When
a videotape is recorded, there must be a way of letting the videotape
recorder know later on just when and where the video tracks were laid down, so
they may be played back reliably. In addition, the motor spinning our rotating video heads (the head drum motor) and the
motor actually pulling the tape through the transport (the capstan motor) must
be kept running at a very constant, identifiable speed, referenced to the
incoming video. We do it with a servo.
Drum and Capstan Servos
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A servo, in general terms, is an automatic control mechanism in which
the output of a device is compared with a stable input reference, so
corrections can be made. Consider the following predicament. You've been chosen to keep the speed of a fan motor constant. To do that, we've supplied you with some equipment - a knob to speed up and slow down the fan, and a little counter that's attached to the fan's motor shaft. Every second, the counter starts counting the revolutions of the fan motor. Your job is to keep the counter always at 100 counts per second - no more, no less. In the first second, the counter counts up to 102; you turn the speed control down a little bit, to slow down the fan. In the next second, the counter only counts up to 97 (you went too far); you turn up the knob a little bit. In the next second, the counter is up to 99; you turn it up a tiny bit more. The subsequent count says you're up to 101; you back off the knob a little bit. And so on... You're a servo. You're continually comparing the speed of the fan to a constant (100), and adjusting the speed to try and match it. Now get rid of the knob, the little counter display, and your hand... and do it all electronically. That's an electronic servo. |
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The Drum Servo
The drum servo is trying to keep a constant speed for the video heads as they go across the videotape. During record, it looks at the incoming video's horizontal sync pulses and ensures that the VTR lays down the proper tracks for each frame of video. During playback, the drum servo can either lock to the playback video itself, or an external reference from a central sync generator (within the broadcast facility), and ensures that it plays back the correct video tracks to reproduce the video.The Capstan Servo
The capstan servo, on the other hand, is trying to
ensure that the tape is
being pulled through the videotape recorder at a very constant speed. During record, the capstan motor runs at a certain
speed, which is compared to the timing of the incoming video's vertical sync pulses.
While this is going on, a control track is being laid down on the
video tape. This is a pulse for every frame of video - about 30 pulses a second. On playback, the capstan servo looks at the control track and
makes sure that the tape is being played back at exactly the right speed. In
broadcast VTRs, this is often referenced to the station's external vertical sync pulses.
The
control track signal is also a basis for setting up the tape's position relative to the video heads' ability to pick up the
tracks recorded on the tape. The speed is adjusted during the "lock
up" period just after pressing the play button, so the video tracks line up precisely with the
video heads sweeping by. This process is known as "tracking",
and it usually takes a couple of seconds to take hold. That's why, when you push play on a VTR, the picture is a little
unstable for a couple of seconds - the VTR's servos are getting their act
together.
Fig (5): Videotape recording basics
Fig (6): Videotape playback basics
2.7 Time Base Correction
All VTRs are mechanical devices, and there is a limit to the timing accuracy on playback that can be achieved. The limit is set by the mechanical accuracy of head wheel bearings and motors, and a certain minimum variation in tape quality along the length and width of the tape. Also, gradual and sudden variations of tape guide adjustment and tape path during extended use may cause instability. Time base correctors are used to take out the remaining mechanical jitters in videotape playback so that the signal is stable for broadcast use.|
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The
first quad VTRs had two devices within them. The AMTEC (Ampex Tape Error
Corrector), the first analog time base corrector, was essentially a series of
electronically variable delay lines used to compensate for the playback time base
error. As the head drum speed deviations were detected, a signal representing
these errors would be sent to the AMTEC, and the video signal would be delayed
as necessary to compensate for the instability. The COLORTEC was later
designed for less major errors, to provide a stable color play
back signal.
  COLORTEC AMTEC |
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Fig (7):
Modern time base corrector (courtesy DPS)
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 Today, modern
timebase correctors not only allow us to compensate for timebase errors, but,
since they're digitizing the video anyway (usually a frame at a time), allow
us to adjust video, black and chroma levels, hue. They also frequently offer
special effects like strobing and freezing of frames and fields. Finally,
this correction technology is used in not only composite video, but component
and S-VHS video formats as well. |
Fig (8): TBCs work
2.8 Analog VTR Formats
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Up until now, we've
been concentrating pretty much on an outdated videotape format - 2"
quad. Why? Because the quad was the venerable workhorse of television
videotape for the first twenty years, until the mid-1970s. But there's more
to it than that.
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 A
2" VTR embodies all the principles used in modern helical scan machines
(which have replaced the quad for the last 25 years), but in ways that are
easy for all to see. Indeed, each component and section of a quad can be
physically removed and studied further. By contrast, modern VTRs are often
highly centralized marvels of construction; often more than one subsystem will
be on a single printed circuit board - or even within one integrated circuit
chip.
Fig (9): Quad head
assembly
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Much has been said
for the "hands-on" approach to learning about videotape technology,
but in fact, so much of this background information is seldom used because of
the comparable high reliability of today's equipment, and the relative lack
of operator adjustments. There are hardly any line up controls on the front
panels of such equipment, except input and output level controls and tracking
adjustments. Even these can often be placed in "automatic" or
preset positions.
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With this in mind, it
is now time to leave nostalgia and history behind us, and take a look at the
myriad formats and features available to us today. The following comparisons
are sometimes a little subjective, but shed some light on the relative merits
of each format, when considered within a broadcasting environment.
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1" Type C
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This format, first
appearing in 1978, was the
first machine to add special effect capabilities (e.g., still frame, slow
motion, and the ability to see pictures while shuttling the tape at high
speed) that were not
possible with the 2" quad VTR. Also, the 1" VTR introduced the use
of automatic scan tracking (which we'll discuss later on). The professional
industry's workhorse from the late 1970s to the early 1990s, this format is
now obsolete. 1-inch offered excellent quality, flawed only by a build up of
noise on successive generations.
 Fig
(10): 1" VTR |
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U-matic (3/4")
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U-matic VTR |
Initially introduced
as a home recording format in the early 1970s (consumers found it too big and
clunky, with a short recording time), this was the
first helical format to be widely accepted and was ultimately to find its
niche in the ENG (electronic news gathering) and industrial markets.
U-matic, like 1" open reel, has become obsolete because there are so
many
 other
higher quality recording formats with smaller cassette sizes and greater
reliability.
Fig
(11): U-matic (3/4”)
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2.9 Component
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Betacam was the
first of the small format machines intended for ENG and EFP (electronic field
production) use. Introduced in the early 1980s, its
use of an upgraded consumer format cassette was the notable feature, as well
as its being the first
professional component recorder - the luminance and chroma signals are
recorded on separate tracks allowing
each of the two signal channels to be optimized for their particular
requirements. Many stations find that Betacam is perfectly useable for
general purpose on-air use, including program and commercial playbacks. The
overall look is very slightly noisy because of the amount of signal
processing going on, but probably below the threshold of visibility for most
people. When used in a completely component environment, the results are very
good. Betacam’s close relation, Betacam SP, uses a metal type tape for higher
quality video reproduction. There are lots of these machines in the field, so
it’s likely that they will be around for some time.
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2.10 Consumer Formats
Betamax
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|
 Not
to be confused with Betacam (the broadcast format), this was the first standardized consumer
video format and
used a composite recording system. Later Betamax versions introduced the
concept of "HiFi" audio using an FM subcarrier. |
Fig (12): Betamax consumer VCR
VHS and S-VHS
This system came out shortly after the Betamax system and, despite its poorer picture quality, outsold Betamax due to longer playing times on the cassettes. The "HiFi" version of this machine uses a recording system for its audio in which a long wavelength FM carrier is recorded under the high frequency FM video carrier.|
The S-VHS system uses a high
coercivity tape, and the maximum video FM carrier frequency was increased,
resulting in picture resolution improvement. For
broadcast work, its main problems are: lots of artifacts induced by signal
processing, and low signal-to-noise ratio. Colour "smears" down the
screen, and there is a definite "haze" over the image because of
the noise reduction circuitry. Even then, noise continues to be a problem due
to the very slow tape speeds. For "first generation" work, S-VHS may
be indeed a good choice. Once you start going down a few, however, the noise
and other artifacts quickly make S-VHS rather unacceptable for broadcast
work, although industrial video suites abound. With the proliferation of
inexpensive and compact MiniDV digital camcorder and playback VTRs, S-VHS has
now become an obsolete format.
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Fig (13): S-VHS VTR
8mm and Hi8
The 8mm format was the first time a consumer video machine made that use of an "imbedded" tracking servo rather than a longitudinal control track. This allows the use of a very slow tape speed, metal evaporated tape, and a small cassette size. You'll also notice that there is no tracking knob on any 8mm format machine. HiFi audio is available through an FM subcarrier recording system, with PCM digital audio as an option built into the format specifications. Even in the Hi8 format, frequency response rolls off fairly quickly above 3 MHz, though some artificial peaking is added to give the impression that things are not that bad. For camcorder work however it is ideal, with one of the best format size/picture quality ratios, for an older analog format. It’s still in use as a consumer camcorder format.Helical Scan Recorders In Depth
Fig (14): Typical cassette loading (VHS)
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Omega wrap 180 Wrap |
All of the above listed formats have one thing in
common - they're all called "helical wrap" machines. One method of
achieving the necessary head-to-tape speed for recording involves wrapping
the tape around a revolving drum-shaped scanner in the form of a helix.
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The tape only
contacts about 50% of the drum surface, thereforeSlow Motion and Stop Motion
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Let's consider for a moment how a helical scan
machine actually records its information on the videotape. A
typical helical format machine, due to its wrap, lays out its video tracks on
a diagonal, across the width of the tape. There's
space left at the top for two longitudinal audio tracks, and some room at the
bottom: for the control track, some little sync head recordings, (on some
formats such at 1" type C) and a third audio track usually used to
record time code information.
All helical scan VTRs record a pattern very similar to this one; there
will be some more illustrations later on to show some differences, but the
basic concepts of a diagonal video track and some longitudinal tracks remains
the same throughout most of the formats.
Fig (19):
 Typical helical scan
track layout |
(1" VTR) |
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 Helical
systems can be used for stop motion playback, by simply stopping the tape and
keeping the head rotating at field rate; theoretically, the video playback
head would just keep scanning the same track over and over again. There's a
hitch, though -
The scan angle (and its length) is now
slightly different since the tape is stopped instead of moving.
After all, the tape moved a small amount during the original field recording,
and now we're trying to reproduce the same video track with no motion. The
result is that a
guard band, or noise bar, is reproduced in the centre of the picture -
we've all seen this on VHS machines without special slow motion and freeze
frame capabilities. What's really happening is that the video playback head
is getting the first
half of one track of video, then going through the space between two tracks,
then picking up video from a second track on
the tape. Attempting "slow motion" with this type of VTR results in
a moving noise
band on the screen, until the VTR is back up to normal playing speed.
|
 Before
we try to correct the "noise band problem," there is one more
aspect that must be considered. In most helical scan systems, the horizontal
sync pulses in each adjacent track are recorded side by side (there are up to
262.5 of them in each video head sweep of one field.)
The advantage of this is obvious when you consider what you're doing when you
attempt to do those noisy freeze frames off videotape - frequently, you're
scanning across two adjacent fields of video (two tracks). If
the horizontal sync pulses as recorded on those two tracks didn't line up,
when you went from one track
to the next, a horizontal sync reference change would occur. This would cause
the monitor picture to break up and re-lock. Sync line up
provides the best possible freeze frame without special circuits or
accessories, and eases the design of special, noise-band free slow motion
systems. |
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Fig
(15): Horizontal sync pulse lineup from one track to the next
However,
we still haven't achieved clean slow motion and freeze frame capability. To do
this, we must ensure that the video head remains centred on the recorded track
even though the tape has been slowed down or stopped.
Dynamic Tracking and Automatic Tracking
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On professional
machines where a still frame or slow motion capability (or automatic
tracking) is required, an extra head is provided. This special head ismounted
on a piezo-electric element that bends in response to an applied voltage.
Here's how it works.
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The playback head begins to trace along the
videotape, reproducing the video field recorded in the track being scanned.
As it moves along the tape, the off-tape signal eventually begins to diminish
in level. This is detected, and
a voltage is applied to the piezo-electric assembly, causing it to bend, thus
maintaining maximum output from the track. This
continues until the end of the field.
 But how does the servo system know which way to bend the piezo? If it detects a reduction in level of off-tape signal, and bends the wrong way, the entire signal will be lost. The solution to this is quite elegant. The piezo assembly, in fact, is bending back and forth a little bit, all the time, at a rate of about 450 Hz. This is called "dithering," and actually varies the output off the tape by about 10%. The servo system sees the off-tape signal passing through a "peak" of output level every time the play head dithers from one end of its range to the other. If the dithering produces a higher output nearer to one end of the sweep, the system knows that it should bend the piezo assembly overall toward the higher output, to equal things out. When this bending reaches an amount that is equal to the distance between adjacent tracks, the system moves the head over to the next track and the process begins again. |
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Slow Motion, Freeze Frames and Those Four Head VHS Decks
Clearly, dynamic tracking isn’t used in your inexpensive home VCR. And, in fact, when you try and do a freeze-frame on a two-head VHS machine, you’ll notice the noise band mentioned earlier. So, are four heads better than two, and why do four head VHS machines make cleaner freeze-frames and clear slo-mos?
Fig (16): VHS video head configuration, showing head offsets
2.11 Color Under System
If broadcast VTRs all need timebase correctors to go
to air, why don't you need one on your VHS machine at home? Because these
machines use the colour under system. The colour under system
(used in all VHS machines) is an interesting if somewhat convoluted process.
For a description of the colour under recording and timebase correction process.
Video 8 and Hi8
This format was designed entirely without reference to any earlier tape formats' cassette size, tape speed or other factors. Video 8 is another colour-under system, with the colour subcarrier at 743.444 KHz, the luminance at 4.98 MHz, and an FM audio track at 1.5 MHz. In addition, there are four low frequency carrier pilot tones that are used to keep an 8mm's spinning heads aligned on the correct track. Therefore, the well known "tracking" control is unnecessary. This also makes for noise-free special effects without additional heads. There are two video tracks (A & B), as well as two longitudinal audio tracks (.6 mm wide, each), which are not suitable for program audio (they are used for editing and cueing information). Tape motion is 14.345 mm/sec (.565 inches/sec).There is additional room on the 8mm format for two PCM-encoded audio tracks, recorded in a helical fashion. The sampling rate for these tracks is 15 KHz and resolution is 8 bits. The PCM system itself, however, incorporates an analog noise reduction system and 10-to-8 bit companding to provide a dynamic range equivalent to that of a conventional 13-bit scheme (about 80 dB or so).
There are two different helical wrap angles for Video 8 depending on whether you're using the tape format for video or the PCM audio. It's 180 degrees for the video (and standard FM audio), and 221 degrees if you're recording the PCM audio as an extension of the video tracks. Since the 8mm PCM is placed on an area of tape apart from the video information, it can be recorded separately. This makes overdubbing of high quality audio onto an already-recorded image possible (unlike VHS or S-VHS Hi-Fi).
The standard 8mm calls for metal-particle tape (required because of the high information density). Metal-evaporated tape offers even greater performance.
Betacam and M-II (Component Recording)
These systems solve the problem of recording the NTSC signal by recording unencoded component signals (called Y, I, Q or Y, R-Y, B-Y), usually on two separate video tracks, one after the other. The process takes advantage of the fact that the chrominance components can share a channel, because of their narrower bandwidth. The tape typically runs six times as fast as consumer machines, though the cassettes used are physically similar. These machines have taken advantage of the newer head and tape technology developed for the consumer video recorders.
Fig (17): Betacam’s CTDM has built-in delays that have to be compensated for during recording and playback
They achieve an
excellent luminance performance, if not quite as good as analog studio NTSC
recorders. Their chrominance performance is superior to most
other VTRs, however, since they treat chrominance information as a baseband
signal, recorded on its own track. This makes it less susceptible to
noise than the high-frequency modulated subcarriers present in NTSC direct
colour recording.
Best results with these systems are achieved by remaining in the
component format whenever possible. This
includes shooting with a camera that has a component output, and editing in
component modes. In production editing, this requires a component switcher as
well.
In editing
situations, the four field colour sequence of the NTSC system is no longer a
restriction - you can now edit on every frame of video. Being only able to
edit on every fourth field was a result of the requirement to achieve the
proper subcarrier phase sequence in edited recordings, but the new component
systems have no subcarrier,
so there is no longer a worry about colour framing.
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This format was
developed by Sony and is designed around the elements of the Betamax machine,
using the same cassette and drum size. Tape motion and drum rotation are in
the same direction. The luminance and chrominance tracks are of the same
width, separated by guard bands. The two tracks have azimuth offsets of +15
and -15 degrees, for a total offset of 30 degrees. The format includes two
normal audio channels, plus a time code and control code track, and a control
track.
The chrominance components recorded are R-Y and
B-Y. These are equal bandwidth colour difference signals that are compressed
in time by a factor of 2:1. This process doubles the bandwidth occupied by
the signals. These compressed signals are recorded such that the chroma track records a line of R-Y and a line of B-Y in the time it
takes to record one line of luminance.This process involves time delays, so the luminance and chrominance
recorded on a pair of tracks do not, in fact, come from quite the same video
line. This timing problem is corrected on playback, so that the
re-expanded chrominance is recombined with its proper luminance signal.
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Since the R-Y and
B-Y signals never share the same track at once, there is no chance of crosstalk
between them. This process of alternating the two components is called
"Component Time Division Multiplexing" (CTDM).
The M-II format is very similar to the
Betacam format.2.12 Digital Video Recording
Video recording
gave broadcasters a valuable tool. As each new analog format appeared, signal
processor and editor packages emerged to stabilize its electromechanical
irregularities. Yet, an undesirable "multiple-generation syndrome"
continued to surface. What is called noise is in reality a combination of phase
modulation, transport jitter, amplifier noise, improper machine lineup, and a
host of other small inconsistencies. Digital recording tries to solve these
problems.
The physical
reality of magnetic recording is ideally suited to digital concepts.
Information is stored on tape according to the polarization of the magnetic
particles in the oxide.
In making an
excellent saturated recording, all particles will be magnetized completely in
either the "north" or "south" direction - exactly what we
want for a digital recording (the "1"s and "0"s of the
digital domain). In addition, error correction and concealment alleviate the effects of tape
irregularities and events
that leave momentary gaps in the recording; this is achieved using some elegant
procedures. Analog recording places adjacent parts of images next to each other
on the tape. The concept in digital recorders is to spread pieces of the image over a large area on the tape, so no adjacent
pixels are written side by side.
2.13 Digital VTR Formats
A listing of the digital video recording systems out there. This list changes all the time, making it almost impossible to keep up, especially in a publication of this kind, but it's reasonably current as of summer, 2003.The D's: D1, D2, D3, D5, D6, D7, D9, D10, D11, D12
D1 was the first
digital videotape format, and it is a component format. The digital video is not compressed. The video in/out can be
RGB or Y/R-Y/B-Y, as well as Betacam component. This format is well suited to
original non-NTSC work, such as graphics and film transfer. D-1 uses a 19mm
metal-particle tape that is available in three different sizes of cassettes -
the large one holds 96 minutes of program. It’s now obsolete.
D1 is based on the
"4:2:2" component system described in CCIR Recommendation 601, which
calls for a 13.5 MHz sample rate for luminance (858 samples/line) and 6.75 MHz
for chroma. The "4:2:2" part is an representation of the sample rate:
there are 4 samples of Y for every 2 samples of R-Y and 2 samples of B-Y. The
samples are time multiplexed in the order B-Y, Y, R-Y, Y, and so on. We'll go
into sampling in greater detail later, but for now, remember that 4:2:2 stuff,
and the sample rate.
D2 is an NTSC
composite format. This format
is well suited as a "workhorse" in the broadcast environment, for
handling program and commercial playbacks, as well as satellite recordings,
since all of it is received by the station as NTSC video. D2 was designed to be
a direct plug-in replacement for 1-inch and Betacam recorders, while D1 needs
to work in a completely non-NTSC environment. D2 operates at the same 4x
subcarrier (14.31818 MHz) sampling rate as most TBCs and frame syncs, and some
existing equipment has been adapted to plug directly into the D2 machines.
D2, by the way,
uses the same three cassette sizes as D1, but the tape lengths are different
and the cassette can hold twice as much material as D1 - three hours for the
large "D2L" cassette.
While all of this
looks good as a theory, the fact of the matter is that D2 never really caught
on, due to its initial upgrade cost, and that it was quickly supplanted by D3
and later digital recorders. It’s now obsolete.
D3 uses ½"
tape instead of the 19mm (3/4") tape used in D1 and D2. It also uses a different error correction system, a different modulation codingsystem
(for recording), and a low-tension,
low-friction transport. D3 is plug-in compatible with D2 - it's a composite format. In addition, D3 uses a flying
erase head for insert editing - D2, in contrast, just records over previous
material, which, in fact, works out because of the nature of digital recording.
This machine is now obsolete.D4 does not exist - it’s said that the Japanese consider the number 4 to be unlucky, so this digital VTR "version" never appeared.
D5 is D3 playback compatible, but offers 10-bit processing (which some say improves the quality of the recording), and is a composite and component VTR. This machine is now largely obsolete.
D6 is a digital HDTV recorder that records 64 minutes of 1.2 GBytes/sec video on a D1 large-shell cassette. Luminance sampling is 72 MHz and chroma sampling is 36 MHz. Sampling happens on an 8-bit level. Notice that this format specifies its recording ability not as "NTSC" or "HDTV" per se, but as a capability of recording so many "bits per second."
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D7 is
the standard for Panasonic and DVCPRO,
which is a 4:1:1 component machine with 8-bit recording and 5:1 compression
using DCT (direct cosine transform). We will go into DCT later on, but for now,
notice that this is the first "D" series machine to use
compression. This format is also
compatible with the many MiniDV camcorders out there.
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D9 is the proposed standard for Digital-S, the format from JVC
that is to replace S-VHS. Digital-S uses full 4:2:2 sampling, with 8-bit
recording and a 3.3:1 DCT compression scheme, in a ½" tape in a shell that
looks like an S-VHS cassette. The specification calls for four channels of
audio, even though only two are used in current machines. Its video inputs accept
composite, component, digital component, digital composite, and S-video. The
machine will play back your existing S-VHS library.
D10: MPEG IMX. A digital component system, which will
record and play back MPEG encoded video. It will also play back all Betacam
formats (digital or analog) except for HDCAM, making it a possibly excellent
replacement for existing "workhorse" VTR playback and record
applications such as master control, satellite feedroom recordings and so on.
D11: HDCAM, below.D12: DVCPRO 100, below.
2.14 Digital Videotape Recorders: The Others
Betacam SX (Sony) is a 4:2:2 component VTR with 8-bit resolution
and MPEG-2 compression of 10:1, recording on ½" metal particle tape with
four channels of 16-bit audio. This VTR plays back everyone's vast library of
Betacam SP tapes.
DCT 700d (Ampex) uses 2:1 compression on a proprietary tape
format, 3/4" wide. It's an 8-bit format, taking composite, component, and
digital component inputs and outputs. It’s now obsolete.
Digital Betacam (Sony) is a 4:2:2 component VTR with 10-bit resolution
and a 2.34:1 compression ratio on a 1/2" metal particle tape. This VTR
also plays back everyone's vast library of Betacam SP tapes.
DVCAM (Sony) is a 4:1:1 component system, 8-bits resolution,
with 5:1 DCT intraframe compression. It's similar
to D7 (DVCPRO) in many respects, but incompatible. It will play back Sony's consumer DV tapes,
making it possible to record on inexpensive digital camcorders for
"disposable camera" shooting by broadcasters.
DVCPRO 50 is a close cousin of DVCPro (see D7, above), but is a
4:2:2 component recording system, instead of 4:1:1. Its compression rate is a
little less, too, at 3.3:1. It will take composite input as well as component.
D-VHS is the replacement for consumer VHS. It will have no
analog inputs or outputs - the VCR will be fed from digital off air or digital
cable or satellite dish signals, and its output will be sent directly to
digital televisions (DTV.) It will be capable of recording any of the proposed
DTV formats (of which we will study later in this text), and will play back
your existing library of VHS videotape.
HDCAM (D11) is a format that uses ½" metal particle tape
(unlike D6, the other HDTV format, which uses 3/4" tape.) The idea behind
this is to enable
handheld HDTV recording, because of the smaller transport. There are some
tricks to this, though: HDTV is set up as 1,920 pixels across - HDCAM reduces
this to 1,440(4:3 subsampling). This can be represented in the shorthand
of the day as 3:1:1.
DVCPRO 100
(D12) is the HD version of regular DVCPro. A very new format
so details were sketchy at time of publication of this textbook (summer, 2003).
Digital VTRs Record and Play Back
The digital
systems vary somewhat in their processes and track layouts on the videotape, but
the principles are the same for them all.
In some systems,
the video enters the recorder as digital information or component video; in
others, the video is in composite form. If
the video is in anything other than digital form, it is first converted to digital
via A/D converters. Once the video is in digital form, it is "pre-coded," a kind of
organized scrambling. This effectively breaks up the video into a
pseudo-random pattern; originally
adjacent pixels are no longer next to each other.
Similar procedures
are performed on the four audio channels.
Because of all the
shuffling and coding that's taken place, it's
possible that a very low frequency signal has been created (one with not a lot
of fast changes in the "0" and "1" pattern). This
signal cannot be recorded to tape - you need fluctuations to make a recording.
The solution has its direction in mathematical and physical theory that states
that random noise has no very low frequency (DC) component. Therefore, the shuffled data stream
is modulated (digitally, this time) with a pseudo-random sequence of
"1"s and "0"s, called non-return-to-zero-transition-on-I
(NRZI) code. More on this code later when we go into digital video in
general, in that chapter.
In the D2 system,
it's modulated with a pattern of "1"s and "0"s known as
Miller-squared code. The intriguing thing about this particular pattern is that
it is insensitive to amplitude variations and confines its signal energy into a
relatively narrow bandwidth, compared with NRZ code. The D2 system has no guard
bands between adjacent tracks written on the tape (there's an azimuth offset
between the two recording heads, like some other helical scan formats). The
restricted bandwidth of Miller-squared code reduces crosstalk between D2 video
tracks, and also has another valuable characteristic in that it can overwrite its own previous
recordings. This means a
flying erase head is not required in D2.
D3 and D5 use
something called 8-14 modulation coding, and use a flying erase head. But the
principles and the point of it all is the same - don't have any low frequencies
which we have to try to record on the videotape.
Recording On The Tape
Finally, it's time for the video (and audio tracks) to be recorded to tape.D1
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The design of D1 is to capitalize on performance
characteristics available with digital signals. D1 records the video
information at both ends of the recorded track, with the audio information in
the centre, to keep it away from the more damage
prone edges of the tape. Audio data is written twice. When doing audio insert
edits, one copy of the original audio data is retained, in case a different
approach to the edit is preferred later on. There are additional longitudinal
tracks - cue, control, and time-code. The rotating heads' azimuths are set at
0 degrees. There is a guard band between the tracks.
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Fig (18): D1 track layout
D2 track
 D2
answers the need for a more economical format in applications where the
digital output can be appreciated, but component or fully digital technology
is not required. It records the video
information in the centre of the recorded track, with the audio information
recorded redundantly at each end of the track.
There are the same group of longitudinal tracks as D1. The rotating heads'
azimuths are set at +15 and -15 degrees for a total offset of 30 degrees;
there is no guard band between adjacent tracks. |
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Fig (19): D2 track layout
D3 and D5
D3 and D5 are
similar. D3 records its audio
on the ends of the tracks, and D5 records the audio in the middle of the tracks.
The interesting thing about the D5 machines is that they can be configured to
play back D3 recordings!
DVCPro
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DVCPRO is based on
the consumer DV format (which was assembled by a consortium of more than ten
manufacturers.) The PRO format, however, uses a wider helical track and a
higher tape speed (33.8 mm/s vs. 18.8 mm/s). An interesting track arrangement
has been designed into DVCPRO: an
area called "ITI" (holding such information as time code) starts
the track, followed by audio, then video, and then finally an ending segment
called "subcode". Also, each frame of video is recorded
as ten tracks,
so there isn’t the "one track per field" arrangement as in other
recorders.
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2.15 Damage Control
Suppose that you
lose 20 pieces out of a 1000 piece picture puzzle before it is assembled. The
result is not total destruction - when the puzzle is assembled, the error is
spread out. If you view the assembled picture, you can
still see it, although you may need to interpolate some spots. That's the
concept behind processing in the digital formats. In the digital
recording and playback system, mapping transforms are used a lot. Not only is a
field broken into segments and sectors, but also the segments, sectors and
individual lines undergo shuffling. As a result, tape damage, instead of destroying portions of the picture, may appear as
little faults spread over the picture.
Error Correction
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Here's an example of
how error correction can work. Suppose you have the following values of three
pixels: 3, 7, and 10. If you add the values together, you get 20. This total
is called a checksum.
You can make an additional checksum by "weighting" the values of
the pixels and adding them up again. If we multiply pixel #1 by 1 we get 3;
multiplying pixel #2 by 2 gives us 14; multiplying pixel #3 by 3 gives us 30.
If we add these three new numbers up we get 3 + 14 + 30 = 47. This is our
second checksum. Notice how the weighting factor is dependent upon the
position of the pixel.
Suppose that now we read our three pixels
off tape, and we get the erroneous values of 3, 11, and 10 (the second pixel
is wrong). The addition of these values comes to 24; the addition of the
weighted off-tape values (3, 22, 30) comes to 55.
Using our original checksums, and the off-tape
incorrect checksums, we see that we're high by four (24-20) in our values some place,
and it is in pixel number two(55-47
÷ 24-20). Sure enough, it's the "7" pixel that got read out as a
"11". We can correct it, and replace it in the picture with the
proper value.
The pixel is off by four in its actual value (24-20), and the
actual pixel that's wrong is pixel number two [(55-47) ÷ (24-20)].
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Error Concealment
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The other
process used to suppress problems is error concealment, which is
interpolation. In dropout compensation in an analog VTR, missing information
is replaced from the previous line's video. In
error concealment, values of a group of adjacent pixels are applied to a
mathematical formula to determine the probable value of the missing pixel.
These algorithms can be quite complex. If minute details involved many
single, separated pixels of vastly different luminance and chrominance, and a
pixel was missing somewhere in this complicated video detail, error
concealment might not regenerate the lost data properly. Television, however,
seldom has this type of detail, especially over a period of several TV
fields.
Imagine a group of nine adjacent pixels, with
all of the values known except for the centre one:
The simplest interpolation of a situation like
this is to just add up all the outer pixels and divide by 8:
7 + 8 + 7 + 6 + 7 + 5 + 6 + 5 = 51 ÷ 8 = 6.375
The missing pixel has a value of about 6. This
is a simple algorithm, but it illustrates the principle.
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Track Size
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With the yearly
advancement in digital and analog videotape recording, there has been a
parallel reduction in the track width.
The moral of this story is simple: keep your
videotapes and videotape machines scrupulously clean, and always store
videotape in a protected environment, away from extremes of heat, cold,
humidity, magnetism, and dust.
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Digital Versatile Disk (DVD)
While engineers keep trying to perfect a broadcast quality re-writable optical disk, another format has come into being that is of the write-once-read-many (WORM) variety - DVD.A later evolution of the compact disc (CD), the DVD has a storage capacity that is seven times higher than its older relation. The recording "pits" are less than half the diameter of a CD's, and the track can sometimes exceed eleven kilometres in length. All of this adds up to 4.7 gigabytes of storage - typically room for 135 minutes of widescreen (16:9) video at MPEG-2 quality accompanied by multiple audio and subtitle channels. This is called the DVD-5 format. Two-sided versions are available, too - a flippable format not seen since the days of vinyl LPs - the DVD-10. With this capability, along with multi-layer versions of the disk, storage capacity of up to 17 gigabytes is available - the DVD-18.
While not strictly a broadcast video recording format (it’s a write-once technology, remember) it has its uses as a more or less permanent storage medium where applications require many short duration elements for live or edited production, quick changes in live to air elements, or several versions of interstitial elements. In addition, this format is now readily accepted into the home due to its relatively low cost and high quality. We can expect to see DVD players in broadcast applications at consumer prices.
2.16 EditingWe do post production editing to combine diverse elements, to trim existing elements, to correct mistakesand to build up shows from smaller segments...The simplest editing is when you combine program portions by simply cutting the various videotaped pieces together into the proper sequence. The more care that was taken during the pre-production, the less work you have to do in the post production stage. Many editing assignments involve trimming the available material to make the final videotape fit a given time slot or to cut all extraneous material. This occurs in ENG editing, where you may have 10 minutes' worth of exciting fire footage, but only 20 seconds to tell the story. Editing is often done to correct mistakes, by cutting out the bad parts, and/or replacing them with good ones. This can be quite simple and may only involve cutting out a few seconds during which the talent made a mistake. It also can become quite challenging, especially if the retakes do not quite fit the rest of the recording, as to colour temperature, background sounds, continuity, or field of view. The most difficult, but most satisfying editing assignments are those in which you must build a show from a great many takes. In this case, the edit is the major production phase. This is especially true in EFP post production, when all takes are shot with a single camera to be combined later. 2.17 On-Line and Off-LineOff-line editing produces a work print, a preliminary and usually lower-quality tape dubbed from the higher quality master. On-line editing produces that master copy that is used on the air or for dubbing off copies. The terms off-line and on-line don't refer so much to the tape format used, but rather the intent of the edited product.The major advantage of off-line editing is that you can take time for reviewing the unedited material and deciding where to cut, without tying up expensive equipment. With burn-in dubs (those with a window featuring time code numbers "burned into" the bottom of the frame), you can identify the exact spot where you'll cut, and note these decisions on an editing shot list. Later, you can proceed with the actual editing. Too many times, however, people start editing without having properly thought about the editing sequence. This can sometimes help to save time, but more often than not you will get lost in a maze of detail. In all but the most routine editing jobs, you will need to do an editing outline, a list of the desired event sequences and the necessary transitions. |
2.18 Analog Video Editing
Assemble and Insert Modes on Analog VTRs
One's first thought is that, if you wanted to assemble a series of shots back to back, you'd use the assemble edit mode, and when you wanted to insert a shot somewhere to replace existing video, you'd use the insert mode. This is not exactly true.When in the assemble mode, you dub onto the record tape so you always add new, fresh control track. The recording VTR is supposed to make perfectly continuous control track in this mode. Unfortunately, even the best machines sometimes fail in this regard. As a result, some assemble edits experience sync roll or momentary tearing.
When editing in insert mode, on the other hand, you do not transfer fresh control track to the record videotape. Instead, you use it as a guide and position reference for laying down the inserted video on the tape. Therefore, to do insert edits, This takes time, but you gain roll-free edits.
2.19 Time Code
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Time code is a way of representing time and
position information about a tape in either an audio or visual form. To
identify and mark where all segments occur, various address code systems have
been developed. The two most common are the control
track or pulse-count system, and SMPTE
time code.
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Control Track Counter
The control track counter takes advantage of the control track pulses on the tape. These are counted, one by one, as elapsed time - one pulse per frame. The advantage of this system is that no special code needs to be recorded onto any videotape - either during the original shoot, or on the master edited reel. The disadvantage of control track counting systems is that they are not always frame accurate. You may lose one or two frames over the course of stopping, starting, and shuttling the machine.SMPTE Time Code
SMPTE time code, on the other hand, is an electronic signal that provides an address for each frame of video. This address is recorded on the time code track of the videotape.If you look at the audio time code signal as an audio wave, it appears as a sort of square wave at a frequency of somewhere between 2400 Hz and 4800 Hz. This signal, upon closer inspection, is actually at least 80 transitions (cycles) about every 1/30 second. The code itself is made up of 80 binary digits (bits) of information:
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Time Code Bits
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Bit
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What It Does
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0-3:
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Frames units
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4-7:
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[Assignable bits]
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8-9:
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Frames tens
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10:
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Drop Frame bit
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11:
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[Unassigned bit]
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12-15:
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[Assignable bits]
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16-19:
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Seconds units
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20-23:
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[Assignable bits]
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24-26:
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Seconds tens
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27:
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[Unassigned bit]
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28-31:
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[Assignable bits]
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32-35:
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Minutes units
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36-39:
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[Assignable bits]
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40-42:
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Minutes tens
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43:
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[Unassigned bit]
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44-47:
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[Assignable bits]
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48-51:
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Hours units
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52-55:
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[Assignable bits]
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56-57:
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Hours tens
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58-59:
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[Unassigned bits]
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60-63:
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[Assignable bits]
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64-79:
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Synchronizing word
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As you can see
there are several
groups of "assignable" bits for reel and show IDs and the like, as well as some future
expansion room in the form of "unassigned"
bits. These single digits can be any hex (base 16) value (0-9 and A-F).
These can be used as a date, a "scene and take" number, or even as a
source identifier.
The remainder make
up the "hours: minutes: seconds: frames" we're familiar with, and a synchronizing word that always contains the same information
to provide time code readers with a clue to when
each time code word begins and ends and
in which direction the tape is
moving. The sync word of the time code frame must correspond exactly with
the vertical interval of the matching TV frame.
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The figure on the
left shows the basic signal when no specific information is being generated -
actually, it's sending constant zeros. Keep in mind that the time code signal
always changes polarity at the end of each bit interval, regardless of
whether changing information is being sent. Sending these continuous signals
- on, off, on, off - with no other aberrations sends a series of zeros to the
time code reader. Changing
the polarity within one of these half-cycles sends a one.
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To see how a typical number (17:33:49:25) is sent, look at this illustration:
How the time code number 17:33:49:25 is generated
The time code reader, in its simplest form, is a box that takes the audio or VITC representation of time code, and converts it into a displayed set of numbers. It can display on an LED or LCD readout or on a "burn-in" within a video signal.
Frequently, there is a time code generator built into the same box as the reader. It can generate fresh code from numbers manually dialled in, or can frequently be "jammed" with time code from another source. This means that it reads in the time code, and locks its generator to that incoming source, creating fresh code.
Why generate fresh code? Time code is a square wave. It's read back most reliably when that square shape is preserved. Electronic devices will decay that shape, rounding off the sharp corners of the wave, making reading unreliable. Jam sync code helps prevent the problem by making new sharp square waves, identical to the incoming set. Also, if the incoming time code is interrupted, the jam synced generator will continue to create fresh code.
Drop Frame Time Code
Regular time code counts 30 frames per second. Colour television has, in fact, 29.97 frames per second.
In drop frame
time code, frames 00 and 01 are dropped from the counter every minute, except
multiples of 10 minutes (10, 20, 30, 40, 50, 00).
Therefore, 108 frame numbers are dropped
every hour (3.6 seconds),
or one frame about every 33.3 seconds. Notice that it's digits from the
numbering system that are dropped, not actual frames of video. You still have
all of your video information with drop frame time code.Rule of Thumb When Spinning Shows
"Spinning a show" is the act of quickly fast-forwarding the program, to get the final running time, instead of watching the tape in real time.To calculate the real duration of a show that has been spun using non-drop-frame control track pulse counters, multiply the result on the non-drop frame counter by .999. If you've spun a show using a non-drop frame control track pulse counter by accident, subtract 3.6 seconds per hour, or 1 second for every 16.67 minutes. This is a ball park estimate; your mileage may vary.
If your show runs shorter than it was supposed to, you possibly did your timings using non-drop-frame on the control track pulse counter.
If your show runs longer than it was supposed to, you possibly have your show material time-coded with non-drop-frame time code.
An 24-hour broadcast day can be out by about a minute and a half in one direction or the other, if all control track pulse counters or time code generators are in the non-drop-frame mode by accident.
2.20 Analog Editing Systems
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When videotape was
first introduced, tape was actually cut with a razor blade and spliced
together like audio tape. It was important to cut between the individual
tracks of video, otherwise jumping or rolling of the video picture would
occur. You can see a video editing suite in the picture, circa early 1960s.
The microscope thing is what was called a Smith
splicer, and allowed precision physical edits to be made. To make an edit in
these days, you as the editor would push the stop button where you wanted the
edit to occur. You’d mark the tape with a grease pencil, carefully unthread
the tape from the quad machine, then put it in the Smith splicer.
You can’t see video tracks, so you would then
wipe a "developer fluid" (essentially very fine iron filings
suspended in alcohol) over the videotape which would magically reveal the
tracks. You’d find the nearest control track pulse, and gingerly make your
edit with the hinged blade. The second piece of tape (the continuation of
your show) would be stopped, marked, developed and hacked in a similar
fashion, and then the two pieces would be put together with adhesive splicing
tape.
If you were good, the picture wouldn’t roll or
break up (or the join fall apart) as each splice went through the high speed
rotating heads with a distinctive "zinging" noise.
With today's helical scan VTRs, this procedure
is impractical if not impossible. Thankfully.
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Despite the array
of available equipment, videotape editing comes down to two basic systems:
single-source editing, and multiple-source editing.
Developed sync pulses, along with video tracks
Developing the video tracks
Single-Source Editing
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Single-source editing
simply involves a
playback VTR and a recording VTR. The in point
of the record VTR's edit switches the record VTR from playback to record
mode; the out point
cues the machine to go back to play mode. This is a "straight cuts
only" system, since there is not a second source to which you may
perform transitions.
The two machines are operated via an editing
control unit that allows: selection of precise edit points; control of VTR
rolls; control of play and record modes (assemble, insert) and editing of
audio and video tracks separately or together. Most units also allow you to:
see and hear the tape at other than normal speeds; run a trial edit so you
can rehearse it before actually performing it; trim the editing point frame
by frame; perform split edits (edit video and audio separately without their
affecting each other); and review the edit once it's been performed.
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Fig (20): Typical straight cut suite
2.21 Multiple-Source Editing
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The multiple-source
editing system consists of two
or more sources (VTRs,
character generator, stillstore, etc.) generally labelled with letters (A, B,
C, and so forth), and a record VTR. Most of the time, multiple-source systems
are also interfaced with
a switcher, audio console, audio tape machine, special effects and signal
processing equipment. Most multiple-source editing
systems use SMPTE time code.
The key element of these systems is the computerized editing control unit,
capable of storing and performing many different editing functions
automatically. In addition to the functions performed by a single-source
editing unit, it can: locate any frame on either the source or record VTRs;
preroll and run all VTRs in sync, simultaneously or staggered; preview and
perform a variety of switcher-type transitions; do audio crossfades and other
transitions; store
many editing decisions (in an "edit decision list," or EDL);
shift any one edit point, with all the others moving accordingly; print a
hard copy of the EDL; interface with a wide variety of VTRs and other
production equipment; use the "user bits" in SMPTE time code for
scene numbers or videotape reels. Because of this increased complexity, when
working with multiple source editing units, you have to learn a few more controls
and procedures than when using single source units.
Fig
(21): Typecal A/B suite
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2.22 Non-Linear Editing
Now that you've studied a whole chapter on analog videotape machines, the future of video recording and editing may not be based on tape at all.
In the beginning,
to get moving pictures recorded, there was motion picture film. Editing film
meant finding the right shot on the reels (via a Movieola or flat bed editing
machine); cutting on the individual frame lines; and splicing using tape, film cement,
or a hot splicer. To keep track of all of these shots, the ribbons of cut film
would be hung on a rail with a box below it (to catch the tail ends) - this was
called a film bin. The
process was slow and tedious, but very direct, and the editor felt in control.
There's nothing quite like touching the actual frames of the shot. You could
actually see how long a shot was, by looking at the film strip. The process was
not perfect, however. Film would get shredded or sometimes lost somewhere in
the bottom of the bin (or on the floor, whereupon the cat would eat it.)
For television
people, when videotape came along, it was a whole new ball game. There was no
negative or work print developing - the shots were simply reproduced on a
second VTR. However, once you
had laid down 50 shots or so, it was very difficult to insert another piece
between, say, shots 27 and 28. Your
choice was to insert the extra shot, and overwrite the rest of your hard-earned
editing session, or make a dub of the end of the edit, insert the shot, and dub
back the remaining program again (losing two generations in the process.)
Somehow, something had been lost in the editing process, which can be so much a
matter of cutting and re-cutting, inserting new material, and experimentation.
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Now, the phrase nonlinear
editing is the buzz word in
the electronic editing field. Nonlinear editing systems digitize and store
analog footage onto computer hard disk drives, providing random access from
that digital storage. The editing process takes place on a computer, running
appropriate software to perform various functions. The video and audio
information is stored on large computer hard drives, where it can be viewed,
modified, and eventually played back in real time from the system. The
concept of having quick access to video information is a very powerful one.
This allows the editor to arrange and re-arrange material to his or her
heart's content, when the work can then be output directly to air or to more
traditional videotape.
Despite the initial high cost of using this
type of technology, specialized off-line applications have been using it for
years. Today, these systems have on-line quality output and user-friendly
interfaces. Recently, prices have decreased, making nonlinear video editing
cost effective for an even wider range of applications, and for consumer use
as well.
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In many systems,
shots are displayed on the computer screen as being stored in a list,
contained within a bin -
the old film term comes back. This list can be displayed and sorted according
to name, length, or any of several other categories. To view any shot, you
just click on it with a mouse or trackball interface, and drag it onto the
source screen where it's available for viewing. With nonlinear, there is no
waiting for tapes to spool to preview a segment.
One of the best things about nonlinear editing,
is that the edit is instant - no splicing and waiting for the glue to set as
in film, and no having to actually play back the entire shot for its full
duration, as in videotape. One mouse click and it's done - on to the next
edit. And the shot can, of course, be placed absolutely anywhere, even in
between two frames of a previously laid down shot.
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Fig (23): Non-linear edit suite flow
diagram
Another advantage of nonlinear editing can be
its reduced VTR cost. Obviously, to have instant access to all of your shots,
you have to shoot on a non-digital camera and VTR, and dump the raw footage
into the nonlinear editing system, but this requires only one VTR (you can use
the same videotape machine to record the finished product afterwards, too.) The
down side of this, however, is the cost of hard drive storage - many editors
only dub over the final takes of their shoot, not all of their raw footage.As well, television stations over the last few decades have poured millions of dollars into analog VTRs and linear editing systems. Economically speaking, they’re going to want to get their full money’s worth out of this equipment, and won’t simply discard it because it’s not the latest and greatest gear - especially if it is getting the job done already, quickly and effectively.
Consider another editing situation. Suppose you have a multi-hour live television special that was recorded on videotape and now has to be cut down into a one-hour special, involving a handful of edits. Would it be economical (in terms of time and image degradation) to digitize all that footage into a computer, make your few edits, and then play out the finished product back onto another piece of tape? It would be far quicker to simply assemble the finished show from the original record tapes straight onto the master tape - you’d be done in a little over an hour. In a nonlinear environment, it would take you that long just to digitize the original footage! So, in some cases, linear editing is far better than nonlinear.
2.23 Security
Digital video
recorders configured for physical
security applications record
video signals from closed circuit
television cameras for detection
and documentation purposes. Many are designed to record audio as well. DVRs
have evolved into devices that are feature rich and provide services that
exceed the simple recording of video images that was previously done through
VCRs. A DVR CCTV system provides a multitude of advanced functions over VCR
technology including video searches by event, time, date and camera. There is
also much more control over quality and frame rate allowing disk space usage to
be optimized and the DVR can also be set to overwrite the oldest security
footage should the disk become full. In some DVR security systems remote access
to security footage using a PC can also be achieved by connecting the DVR to a
LAN network or the internet. Video NEXT also makes a NVR surveillance
application for the Mac OS X. Some of the latest professional digital video
recorders include video analytics firmware, to enable functionality such as
'virtual tripwire' or even the detection of abandoned objects on the scene.
Security DVRs may
be categorized as being either PC based or embedded.
A PC based DVR’s architecture is a classical personal computer with video capture cards designed to capture video images. An
embedded type DVR is specifically designed as a digital video recorder with its operating system and application
software contained in firmware or read
only memory.
Chapter
3
Signal Satellite Broadcasting TV
3.1 General Standards
3.1.1 The vertical blanking
interval may be used for the transmission of ancillary signals.
3.1.2 Subject to certain
constraints, the ancillary signals shall be inserted in the interval beginning
with line 10 and continuing to line 21.
3.1.3 Any type of ancillary
signal, whether program related or not, may be carried in the blanking interval
provided the operation is implemented on a non-interfering basis
to regular picture transmission.
3.2 Line Allotment Standards
3.2.1 Lines 10 through 20
may be used for the transmission of any ancillary signal.
3.2.2 The level of VBI
transmissions should not exceed +70 IRE on lines 10-15, and +80IRE on lines 16-20.
3.2.3 Line 19 shall be used
only for the transmission of the Ghost Cancelling Reference (GCR)
signal developed by Philips Laboratories.
3.2.4 Line 21 may be used
for the transmission of "closed captioning" a program related data
signal which, when decoded, presents visual information representative of the
audio signal of the program.
3.2.5 When line 21 is not
being used for the transmission of the "closed captioning" signal,
other types of ancillary signals may be transmitted.
3.2.6 Line 21 will also
provide rating data to the program blocking circuitry (commonly known as V-Chip). The
underlying technology needed for the implementation of the program blocking
system is the same as the one for close-captioning. The tentative plan for implementing the
system is to add the program rating information to line 21 of field no. 2,
along with the closed-captioning information.
Line 21 is also being used for newer "extended data services" (XDS)
which provides scheduling information and station call letters to the viewers.
The complexity of the content rating system, which is presently under
development, will impact on the three data signals that are to be fitted in
line 21.
3.3 Standards for the Ghost Cancelling Reference Signal
3.3.1 The Ghost Cancelling
Reference (GCR) signal provides a reference signal with a
group delay linearly related to frequency. This signal is transmitted in an eight
field sequence of changing polarity (+,-,+,-,-,+,-,+). The GCR starts 12 fs after the leading edge of
the horizontal sync and ends 35.5 μs later (refer to Figure 9). It allows
to cancel ghosts from -3 μs to +45 μs relative to the main signal.
3.3.2 The GCR signal is placed on line 19 and
the corresponding line in the following field. To avoid constraining the
performance of the ghost cancelling device, the lines immediately preceding and
following line 19 should not contain any time varying information such as
teletext. Also, it is highly recommended not to have any type of eight field
sequence pattern in the lines adjacent to line 19. This type of sequence will
greatly affect the performance of the system.
The above constraints on the
content of the lines immediately following and preceding line 19 do not
apply when the GCR signal is not inserted on
line 19.
4.3.3 Waveforms of the GCR signal are shown in Figure 9 and
represent line A and line B respectively. Line A and line B have the
same pedestal height V1=30 IRE,
but GCR polarity is inverted from line A
to line B. Numerical values of the reference signal as a function of time can
be calculated from the following equation:
W(ω) is the window function:
Where the constants: A, b, Ω, c
and Ω1 are given in the table below.
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Parameters
for GCR
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A
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3.592 x 10-7 Volts
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b
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0.53656 x 10-12 sec2/radian
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Ω
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2π x 4.3 x 106 radian/sec
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c
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0.917998 x 106 radian/sec
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Ω1
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2π x 4.15 x 106 radian/sec
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3.4 Line 21 Data Signal
3.4.1 The program related data
signal shall conform to the format described in Figure 10a.
3.4.2 A reference pulse for a
decoder associated adaptive multipath equalizer filter may replace the data
signal every eighth frame. The reference pulse shall conform to the format
described in Figure 10b.
3.4.3 A decoder test signal
consisting of data representing a repeated series of alphanumeric characters
may be transmitted at times when no program related data is being transmitted.
3.4.4 A framing code to be used
by the data decoder may be transmitted during the first half of Line 21
Field 2 when data reference pulse and test signals are present. The format
of the framing code is shown in Figure 10c.
3.4.5 The data signal shall be
coded using a non-return-to-zero (NRZ) format and
shall employ standard ASCII 7 bit plus parity character
codes.
3.4.6 The signals on
field 1 and 2 shall be distinct data streams, for example, to supply
caption in different languages.
3.5 Television Broadcast Videotext Data Signal
3.5.1 The digital data signal
representing text and pictorial information shall be in accordance with the
parameters outlined in Broadcast Specification No. 14.
3.5.2 The data shall be non-return-to-zero (NRZ) binary
encoded.
3.5.3 The transmission bit rate shall be
5 727 272 ± 16 bits per second. The data signal shall be
phase locked to the colour subcarrier when inserted into a colour television
transmission and to the horizontal line scanning rate when inserted into a
monochrome television transmission.
Chapter
4
Composite video signal
4.1 Composite video
Composite video is the format of an analog television (picture only) signal before it is
combined with a sound signal and modulated onto
an RF carrier. In contrast to component video (YPbPr)
it contains all required video information, including colors in a single
line-level signal. Like component video, composite-video cables do not carry
audio and are often paired with audio cables (see RCA connector).
Composite video is often designated by the CVBS initialize, meaning "Composite
Video, Blanking, and Sync."
4.1.1 Signal
components
A composite video signal combines on one wire the video
information required to recreate a color picture, as well as line and frame
synchronization pulses. The color video signal is a linear combination of the luminance of the picture, and a modulated
subcarrier carries the chrominance or color information, a combination of hue and saturation. Details of the encoding
process vary between the NTSC, PAL and SECAM systems.
The frequency spectrum of the modulated color signal overlaps that of the baseband signal, and separation relies on the fact that frequency components of the baseband signal tend to be near harmonics of the horizontal scanning rate, while the color carrier is selected to be an odd multiple of half the horizontal scanning rate; this produces a modulated color signal that consists mainly of harmonic frequencies that fall between the harmonics in the baseband luma signal, rather than both being in separate continuous frequency bands alongside each other in the frequency domain. In other words, the combination of luma and chroma is indeed a frequency-division technique, but it is much more complex than typical frequency division multiplexing systems like the one used to multiplex analog radio stations on both the AM and FM bands.
Color burst is
a composite analog video signal generated by a video-signal generator used to gunlock, keep the chrominance subcarrier synchronized in television studios for color
television.
4.1.2 Signal Modulation
Composite video can easily be directed to any broadcast
channel simply by modulating the proper RF carrier frequency with it. Most home analog video equipment record a signal in (roughly)
composite format: Laser Discs store a true composite signal, while
consumer videotape formats (including VHS and Beta
max) and lesser commercial and industrial tape formats (including U-Matic) use modified composite
signals (generally known as "color-under"). On playback, these devices often give
the user the option to outputting the baseband signal or to modulating it onto
a VHF or UHF frequency compatible with a TV tuner (i.e. appearing on a selected TV
channel). The professional television
production uncompressed digital video videocassette format known as D-2 (video), directly recorded and reproduced standard NTSC composite
video signals, using PCM encoding of the analog
signal on the magnetic tape.
4.1.3 Standard
Connectors
In home applications, the composite video signal is
typically connected using an RCA
jack (phono plug), normally
yellow. It is often accompanied with red and white connectors for right and
left audio channels respectively. BNC
connectors and higher quality coaxial cable are often used in professional television studios and post-production applications. BNC Connectors were also used for composite video
connections on early home VCRs,
often accompanied by either phono
connector(s) or a 5-pin DIN
connector for audio.
In Europe, SCART connections are often used instead of
RCA jacks (and to a lesser extent, S-Video),
so where available, RGB is used instead of composite video
with computers, video game consoles, and DVD players.
4.1.4
Modulators
Some devices that connect to a TV, such as VCRs, older video game consoles and home
computers of the 1980s, output a
composite signal. This may then be converted to RF with an external box known
as an RF modulator that generates the proper carrier
(often for channel 3 or 4 in North
America, channel 36 in Europe).
Sometimes this modulator was built into the product (such as video game
consoles, VCRs, or the Atari, Commodore 64, or TRS-80 CoCo home-computers) and sometimes it
was an external unit powered by the computer (in the case of the TI-99 or some Apple modulators) or with an
independent power supply. In the USA, using an external RF modulator frees the
manufacturer from obtaining FCC approval for each variation of a device.
Through the early-1980s, electronics that output a television channel signal
were required to meet the same shielding requirements as broadcast television
equipment, thus forcing manufacturers such as Apple to omit an RF modulator,
and Texas Instruments to have their RF modulator as an external unit, which
they had certified by the FCC without mentioning they were planning to sell it
with a computer. In Europe, while most countries used the same broadcast
standard, there were different modulation standards (PAL-G versus PAL-I, for
example), and using an external modulator allowed manufacturers to make a
single product and easily sell it to different countries by changing the
modulator.
4.1.5
Demodulation loss
The process of modulating RF with the original video
signal, and then demodulating the original signal again in the TV, introduces
several losses. RF is also "noisy" because of all of the video and
radio signals already being broadcast, so this conversion also typically adds
noise or interference to the signal as well. For these reasons, it is typically
best to use composite connections instead of RF connections if possible. Almost
all modern video equipment has at least composite connectors, so this typically
isn't a problem; however, older video equipment and some very low-end modern
televisions have only RF input (essentially the antenna jack); while RF
modulators are no longer common, they are still widely available to translate
baseband signals for older equipment.
However, just as the modulation and demodulation of RF
loses quality, the mixing of the various signals into the original composite
signal does the same, causing a checkerboard video artifact known as dot crawl. Dot crawl is an infamous
defect that results from crosstalk due to the inter modulation of the
chrominance and luminance components of the signal. This is usually seen when
chrominance is transmitted with a high bandwidth, and its spectrum reaches into
the band of the luminance frequencies. This has led to a proliferation of
systems such as S-Video and component
video to maintain the signals
separately. Comb filters are also commonly used to separate
signals, and eliminate artifacts, from composite sources.
4.1.6 Aspect Ratio in Composite Signal
When
used for connecting a video source to a video display that supports both 4:3
and 16:9 display formats, the PAL television standard provides for signaling
pulses that will automatically switch the display from one format to the other.
The Composite video connection supports this operation. However the NTSC
television standard has no such provision, and thus the display must be
manually switched.
4.1.7
Extensions to The Composite Video Standard
Since TV screens hide the vertical
blanking interval of a composite
video signal and even crop the
edges of the picture, extensions have been implemented by taking advantage of
these unseen parts of the signal. Examples of these extensions include teletext, closed captioning, digital information
regarding the show title, transmitting a set of reference colors that allows TV
sets to automatically correct the hue mal-adjustments common with the NTSC
color encoding system, etc.
Other extensions to the standard include S-video; S-video is an extension to
the standard because it uses 2 parallel signals, one for luminance and one for chrominance (color).
4.2 Frequency Modulation
The basic communications system has:
Transmitter: The
sub-system that takes the information signal and processes it prior to
transmission. The transmitter modulates the information onto a carrier signal,
amplifies the signal and broadcasts it over the channel
Channel: The
medium which transports the modulated signal to the receiver. Air acts as the
channel for broadcasts like radio. May also be a wiring system like cable TV or
the Internet.
Receiver: The
sub-system that takes in the transmitted signal from the channel and processes
it to retrieve the information signal. The receiver must be able to
discriminate the signal from other signals which may using the same channel
(called tuning), amplify the signal for processing and demodulate (remove the
carrier) to retrieve the information. It also then processes the information
for reception (for example, broadcast on a loudspeaker).
The information signal can rarely be
transmitted as is, it must be processed. In order to use electromagnetic
transmission, it must first be converted from audio into an electric signal.
The conversion is accomplished by a transducer. After conversion it
is used to modulate a carrier signal.
A carrier signal is used for two
reasons:
- To reduce the wavelength for efficient transmission and reception (the optimum antenna size is ½ or ¼ of a wavelength). A typical audio frequency of 3000 Hz will have a wavelength of 100 km and would need an effective antenna length of 25 km! By comparison, a typical carrier for FM is 100 MHz, with a wavelength of 3 m, and could use an antenna only 80 cm long.
- To allow simultaneous use of the same channel, called multiplexing. Each unique signal can be assigned a different carrier frequency (like radio stations) and still share the same channel. The phone company actually invented modulation to allow phone conversations to be transmitted over common lines.
The process of modulation means to
systematically use the information signal (what you want to transmit) to vary
some parameter of the carrier signal. The carrier signal is usually just a
simple, single-frequency sinusoid (varies in time like a sine wave).
The basic sine wave goes like V(t) = Vo sin
(2 p f t + f) where the parameters are defined below:
V(t) the voltage of the signal as a function of time.
Vo the amplitude of the signal (represents the
maximum value achieved each cycle)
f the frequency of oscillation, the number of cycles per
second (also known as Hertz = 1 cycle per second)
f the phase of the signal, representing the starting point
of the cycle.
To modulate the signal just means to
systematically vary one of the three parameters of the signal: amplitude,
frequency or phase. Therefore, the type of modulation may be categorized as
either
AM: amplitude modulation
FM: frequency modulation or
PM: phase modulation
Frequency modulation uses the
information signal, Vm(t) to vary the carrier frequency within some
small range about its original value. Here are the three signals in
mathematical form:
- Information: Vm(t)
- Carrier: Vc(t) = Vco sin ( 2 p fc t + f )
- FM: VFM (t) = Vco sin (2 p [fc + (Df/Vmo) Vm (t) ] t + f)
We have replaced the carrier frequency
term, with a time-varying frequency. We have also introduced a new term: Df, the
peak frequency deviation. In this form, you should be able to see that the
carrier frequency term: fc + (Df/Vmo) Vm (t)
now varies between the extremes of fc - Df and fc + Df.
The interpretation of Df becomes clear: it is the farthest away from the
original frequency that the FM signal can be. Sometimes it is referred to as
the "swing" in the frequency.
We can also define a modulation index
for FM, analogous to AM:
b = Df/fm ,
where fm is the maximum modulating frequency used.
The simplest interpretation of the
modulation index, b, is as a measure of the peak frequency deviation, Df.
In other words, b represents a way to express the peak deviation
frequency as a multiple of the maximum modulating frequency, fm,
i.e. Df = b fm.
Example: suppose in FM radio that the
audio signal to be transmitted ranges from 20 to 15,000 Hz (it does). If the FM
system used a maximum modulating index, b, of 5.0, then the frequency
would "swing" by a maximum of 5 x 15 kHz = 75 kHz above and below the
carrier frequency.
4.2.2
Simple FM signal:
Here, the carrier is at 30 Hz, and the
modulating frequency is 5 Hz. The modulation index is about 3, making the peak
frequency deviation about 15 Hz. That means the frequency will vary somewhere
between 15 and 45 Hz. How fast the cycle is completed is a function of the
modulating frequency.
4.2.3 FM Spectrum
A spectrum represents the relative amounts of different frequency
components in any signal. Its like the display on the graphic-equalizer in your
stereo which has leds showing the relative amounts of bass, midrange and
treble. These correspond directly to increasing frequencies (treble being the
high frequency components). It is a well-know fact of mathematics, that any
function (signal) can be decomposed into purely sinusoidal components (with a
few pathological exceptions) . In technical terms, the sines and cosines form a
complete set of functions, also known as a basis in the infinite-dimensional
vector space of real-valued functions (gag reflex). Given that any signal can
be thought to be made up of sinusoidal signals, the spectrum then represents
the "recipe card" of how to make the signal from sinusoids. Like: 1
part of 50 Hz and 2 parts of 200 Hz. Pure sinusoids have the simplest spectrum
of all, just one component:
In this example, the carrier has 8 Hz
and so the spectrum has a single component with value 1.0 at 8 Hz
The FM spectrum is considerably more
complicated. The spectrum of a simple FM signal looks like:
The carrier is now 65 Hz, the modulating
signal is a pure 5 Hz tone, and the modulation index is 2. What we see are
multiple side-bands (spikes at other than the carrier frequency) separated by
the modulating frequency, 5 Hz. There are roughly 3 side-bands on either side
of the carrier. The shape of the spectrum may be explained using a simple
heterodyne argument: when you mix the three frequencies (fc, fm and Df)
together you get the sum and difference frequencies. The largest combination is
fc + fm + Df, and the smallest is fc -
fm - Df. Since Df = b fm, the
frequency varies (b + 1) fm above and below the carrier.
A more realistic example is to use an
audio spectrum to provide the modulation:
In this example, the information signal
varies between 1 and 11 Hz. The carrier is at 65 Hz and the modulation index is
2. The individual side-band spikes are replaced by a more-or-less continuous
spectrum. However, the extent of the side-bands is limited (approximately) to
(b + 1) fm above and below. Here, that would be 33 Hz
above and below, making the bandwidth about 66 Hz. We see the side-bands extend
from 35 to 90 Hz, so out observed bandwidth is 65 Hz.
You may have wondered why we ignored the
smooth humps at the extreme ends of the spectrum. The truth is that they are in
fact a by-product of frequency modulation (there is no random noise in this
example). However, they may be safely ignored because they are have only a
minute fraction of the total power. In practice, the random noise would obscure
them anyway.
4.3 Uplink / Down Link Frequency:
The term uplink chain is used to refer to the series of
pieces of equipment that are used to produce a radio frequency signal for
sending out data. The description provided here is imprecise as the exact
configuration can vary widely. The downlink chain is
built using nearly the same equipment in reverse order.
Take a look at the drawing below and note the path of
devices on the left hand side with all the up arrows.
4.3.1 Digital Satellite Uplink Chain
1.
Digital data is sent to the modulator which takes the data and
converts it into a modulated signal in the Intermediate Frequency range
(70-140 MHz). The modulators use standards such as Digital
Video Broadcast to
organize communication over the microwave link.
2.
The Intermediate Frequency is
piped to an "up converter" (usually via shielded coaxial cable) which
mixes the intermediate frequency with a higher frequency to
produce a final frequency which carries the modulated data.
3.
Noise is removed from the signal via either a band pass filter
or other means and then it is amplified in a Klystron, Travelling Wave Tube or
Solid State amplifier.
4.
The final cleaned signal is transmitted down the wave guide to
the dish.
5.
The feed horn at
the focal point of the dish emits the high frequency radio transmission, which
the dish focuses into a directional transmission at the satellite.
Computer
data is sent through a serial cable to a modulator. The modulator takes the
data and produces a radio frequency from it. This frequency is usually in what
is called the 'L-band' range (70-140 MHz). The modulator passes the information
over coaxial cable to an 'up converter', which converts the radio frequency from 'L-band' up to microwave frequencies
in the C, S, X, Ka, and Ku band ranges (frequencies above 1,000 MHz). Once the
final signal has been produced, it's amplified to increase its total effective
output power. The signal is then sent out a dish via the feed horn.
4.3.2 Downlink Chain
1.
The satellite transmits a signal containing data
2.
The signal is received at the satellite dish
3.
The signal is amplified and fed to the Down Converter
4.
The Down Converter down mixes the signal to create an intermediate
frequency
5.
The intermediate frequency is fed to the demodulator and
converted into a data signal
6.
The data stream is forwarded into the network via a router.
Chapter
5
Conclusion
Conclusion
This
last chapter will conclude everything about this thesis. It summarized the
advantages and the weakness of this project. Also some advice for development
and future work is including. In this project, the total television
broadcasting methods is not fulfilled. But it is done for future works in
television broadcasting mythology and signaling. And we try to fulfill our work
and success.
Future scopes
In
the future, it is suggested that using the upcoming digital system and machine
it should be maintain easily and signal transmission become smooth than today.
So it is recommended for the authority to have a look to this project and make
a sense by proving its reality.
REFERENCE
[1]: Reimers, Ulrich (ed.): Digital Fernsehtechnik
–Datenkompression und Ubertragung fur DVB.2sn Edition. Berlin, Heidelberg, New
York: Springer_Verlag, 1997.
[2]: Reimers, Ulrich (ed.):A Guideline for the Use
of DVB Specifications and Standards,
DVB-Document TM 1694,rev.1 September 1996.
[3]: Radio Engineering By G.K Mithal.
[4]: Antenna Engineering by T.K. Bandhopdhyay.
[5]: Electronics Communication By S. Gupta.
[6]: Electronics & Communications by D.C
Agarwal.
[7]:
Television Engineering by B.Grod
[8]:
Basic television and Video Systems by Bernard Grob
LIST OF ABBREVIATION
1.
A
2.
ADC-Analog
to Digital Converter
3.
ALSA-Advanced
Linux Sound Architecture
4.
APL-Application
Programming Interface.
5.
B
6.
BPP
–Bit Per Pixel.
7.
C
8.
CATV-
Community Antenna Television
9.
CRT-
cathode ray tube
10. CA-Conditional
Access
11. CD-Compact Disc.
12.
D
13. DVB-Digital
Video Broadcasting.
14. DTA-Digital
Audio Tape
15. DAC- Digital to
Analog Converter
16. DVD- Digital
Video Disc.
17. D-SNG- Digital
News Gathering
18.
E
19. EDTV-Enhanced Definition
Television.
20. ETR-European Telecommunications Report.
21.
F
22. FEC-Forward
Error Correction.
23. FCC-Federal
Communication Commission
24.
G
25. GOP-Group of
Picture.
26.
H
27. HDTV-High
Definition Television.
28. HPA-High Power
Amplifier.
29.
I
30. IPR-Intellectual
Property Rights.
31. ISDN-Integrated
Services Digital network
32. IRD-Integrated
Receiver Decorder.
33.
M
34. MC- Music
Cassetter.
35. MTF- Modulation
Transfer Function.
36. MPEG-Moving Picture Experts Group.
37.
MMDS- Multichannel Multipoint
Distribution Service.
38. N
39. NTSC- National Technical Systems Committee.
40.
O
41. OS- Operating
System .
42.
P
43. PCM-Pulse Code
Modulation.
44. PSU-Power Supply
Unit.
45. PAL-Phase
Alternating Line.
46. PAL DL- Phase
Alternating Line Delay Line.
47. PSTN- public switched telephone
network.
48.
Q
49. QEF-Quasi Error
Free.
50.
S
51. SECAM-Sequential
Couleur Avec Memoire French For Sequential Color With Memory.
52. SCART-Syndicat
des Constructeursd Appareils
Radiorecepteurs Televisions.
53. SDTV- Standard definition television.
54. SMS- Subscriber
Management System.
55. SAS-Subscriber
Authorization System .
56. SFN-Single
frequency Network .
57.
U
58. UHF-Ultra-High Frequency.
59.
V
60. VHS-Video Home
Frequency.
61. VCRS-Video
Cassette Recorders.
62. VTR-Video tape
recorder.
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