Sean Breheny wrote, quoting Dan:

>> TV carriers use a vestigial side band.  Only the center + the uppper
>> sideband are transmitted.  There is a small lower side band where the
>> FM audio hangs out.

> Dan, are you sure?
> I thought I had seen Amateur TV transmitters which use full double
> side band video.  In fact, I believe that the reason why vestigial
> side band is used instead of SSB is that some vestige of the other
> sideband (and the full carrier) is required for downward compatibility
> with DSB receivers (not sure of this last assertion).

  Well, Dan may be sure, but I'm sure it isn't!  The part about audio
is, I'm afraid, rubbish.

  TV is vestigial sideband.  This means one sideband, presumably as
suggested, the lower, is attenuated as far as is possible without
distortion.  This is much more difficult than for an audio signal, so
you end up with a small amount of sideband, thus vestigial.  If you are
running a basic ATV system, it may be a lot easier to do without the
tricky filters and just transmit DSB or asymmetric SB, but the unwanted
one is mostly removed by the receiver IF anyway.  You don't need
compatibility for DSB receivers because - there aren't any.

  Sound is "intercarrier".  What this means is that instead of
modulating sound onto a subcarrier as part of the video signal (which is
what you do if you use a simple modulator as was proposed, and what most
Amateur TV systems do), you generate a separate carrier the required
(frequency) distance away from the video carrier, and frequency modulate
it.

  When this is received, it appears as a subcarrier in the IF and is
demodulated accordingly.  The elegance is that transmitted in that
fashion, there is *no* image on the other side of the video carrier *at
all*.  Note that the sound carrier is on the other side of the video
sideband to the main (video) carrier, *not* the low side.  The two
carriers neatly delineate the video channel.  Nowadays, most
broadcasters produce additional sound carriers for stereo, subtitle,
multi-language and so on.
--
  Cheers,
        Paul B.