brainbaby said:
The IF section of both monochrome and colour receivers must include rejection filters to suppress adjacent channel interference...
The problem with real RF front-end mixers is that any asymmetry in construction will cross-modulate the biggest RF signals present, then sum the unwanted cross-mods into the target IF signal. If you put a 3dB RF attenuator before the mixer you will get a 6db reduction in cross-mods, with only a 3dB reduction in the wanted signal.
Analogue TV has a huge carrier with big synchronisation sidebands, a few MHz away is a quadrature colour carrier. There are two FM carriers for sound, one FM modulated by the L+R audio sum, the other smaller, with the L–R difference. In between the main carrier and the audio carriers are a few MHz of analogue picture brightness information. Any cross-mods that find their way into the picture part of the channel spectrum will appear as diagonal stripes on the screen. Delayed multi-path signals will appear as sideways shifted ghosts.
The frequency separation of the adjacent channel VSB AM and FM carriers, and the separation of co-channel colour and audio sub-carriers were very well defined, so a trap could be implemented at a fixed frequency in the IF if there were any problems.
For analogue TV, the immediately adjacent channel was not co-located at the same transmit site. To allocate four TV services to three transmitter sites, every third channel could be used at each site. At site 'A' channels 30, 33, 36 and 39. At site 'B' channels 31, 34, 37 and 40, then at site 'C', 29, 32, 35 and 38. Those channel groups all fall in the bandwidth of one fixed antenna. The antenna directional beamwidth will then provide the first level of adjacent channel rejection from other sites. The separation of carriers is then three times more, so adjacent channel interference is not such a problem. VSB AM TV was all a big compromise, frozen by immature standards, in a Freudian suspended development.
Cable TV used one cable for all signals so it gave much greater control over the amplitude of adjacent channels. There was no big hostile signal immediately adjacent to a weak target signal, the channels could be packed closer in the cable spectrum, separate from the rest of the hostile universe.
Analogue VSB AM TV was a disaster, mainly because it used AM for the vision. If it had used FM, then digital TV would not have needed to be implemented as soon. Nowadays we can use an image rejection mixer to down-convert QPSK modulation to baseband in one step, then digitally demodulate the several kilobit wide data stream. QPSK is spread spectrum with a suppressed carrier, so there is no huge carrier to cause cross-mods.
@brainbaby, I have to wonder why you are studying the intricate detail of a past failure, namely AM TV, while the rest of the world is spreading the spectrum and moving digital information reliably at unprecedented rates.
