The standard aspect ratio (width/height) of a standard TV picture is 4:3. Since we're transmitting 625 lines every 25th of a second each line scan must take 64 microseconds. To obtain the same picture resolution in the vertical and horizontal directions we would want to able to distinguish the brightnesses of (4/3) × 625 = 833 distinct pixels (or pels = picture elements) along each line. The highest signal frequencies will therefore be required if we alternate, light-dark-light-dark-light-..., pixels along the line. Each light-dark alternation is essentially one cycle of the signal, so the highest signal frequency we'd require to get this horizontal (along-the-line) resolution is 833/2 cycles in 64 microseconds, i.e. 6·5 MHz. We therefore need a 6·5 MHz video bandwidth for the picture signal to get the same detail resolution in the vertical and horizontal directions.
In practice, the UK TV system only allocates a video signal bandwidth of 5·5 MHz. This means that the level of detail horizontally is slightly less than vertically, but this usually goes unnoticed when we watch “Thunderbirds” from the other side of the room! In practice the video signal doesn't just contain the line-by-line brightness signal as described above. It also contains some extra patterns which are designed to help the TV receiver synchronise its display correctly with the transmitted pattern. To do this we have to modify the video signal to include a synch level pattern.
In the UK this is done as illustrated in figure 21.3. Each line of video information is preceded by a line blanking period. During this period the video signal carries ‘extra’ information to help the TV receiver synchronise its operation correctly. The main synch activity is the presence of a Line Synch Pulse which defines the start of each scan line. UK TV transmissions use Negative Amplitude Modulation to send the Luminance (i.e. brightness) information. A high modulation level corresponds to a dark portion of the picture, not a light portion. The synch pulses are distinguished from luminance because they're ‘blacker than black’. The receiver can look for these regular pulses and use them to align its line scanning. A set of all-synch-level lines is used to indicate to the TV when a half-frame is about to start. This use of a special level to indicate timing and synchronisation information is an example of the synchronisation method we mentioned in an earlier lecture when considering how time division multiplexing can be made to work.
The synch level corresponds to maximum carrier amplitude (i.e. 100%). The nominal black level is 77% of this, white is 18% of the maximum level. Using sync levels which are always the maximum means that, at the start of every line, the TV set can use this level to calibrate the amplitude which corresponds to black and to white. The ‘upside-down’ nature of the signal means that we shouldn't find the picture brightness and contrast depend upon how far away we are from the transmitter! If we'd used ‘normal’ AM the TV would have trouble deciding just how bright the picture should be. Another practical advantage of negative AM is that low-level interference tends to produce small dark spots/bands which are less noticeable than white defects on the picture.
To save on bandspace, UK TV uses a Vestigial Sideband system. This is similar to AM, but one of the sidebands (the lower one) is filtered down to avoid duplicating all the picture information on both sides of the carrier. This allows us to fit the entire TV signal — including colour and sound information — into an 8 MHz transmission bandwidth whilst maintaining a 5·5 MHz video luminance information bandwidth.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.