On Wed, 4 Nov 1998 23:23:47 +1000, Paul B. Webster VK2BZC wrote:

>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.

Yes, the sound carrier part is rubbish!

My memory of classes I took 14 years ago
is somewqhat foggy.  Now I remember that the audio carrier is 4.5Mhz *above* the
main carrier.  The part about using vestigial side band is to keep the bandwidth
used by the channel to a minimum.  The total spacing between most channels is 6M
hz.
If both sidebands were transitted, you would wipe out the audio of the channel b
elow
and probably most of the luminance as well.

Well, now, I am reminded once again why I went into software instead of hardware
!

Sorry for the misinformation.  I should have pulled out my books before speaking
, but
unfortunately they are all packed at the moment, in anticipation of my upcoming
move.

>
>  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.
>


Dan