"Radium" wrote in message
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On Jun 12, 10:57 am, Rich Grise wrote:
On Fri, 08 Jun 2007 20:58:04 +0000, Radium wrote:
Why not carry the luminance-signal on FM and the audio-signal on AM?

Because it would be abysmally stupid to try to do so. You'd need about
10 times as much bandwidth if you used FM for the video, and using AM
for the audio would make it susceptible to all manner of noise.

Isn't the interference on AM video as much of a concern as it would be
on AM audio?

Nope, not really. The creators of the analog TV standard
did something very clever and yet very simple - in choosing
VSB-AM for the video modulation, they also set it up with
"negative modulation," meaning that the peaks of the
modulated signal correspond to the "blackest" portions of
the video (luminance) waveform - which happen to be the
sync pulses. This means that, in the presence of noise, you
are still very likely to maintain a stable picture; the picture
itself may degrade and get a bit "snowy," but you can still
see it. And it really takes quite a bit of noise to make visible
"snow" show up in the picture. For one thing, video has tons
of redundancy due to the repeated raster-refresh scanning,
so you're very unlikely to see the occasional noise spike.
But also, due to this choice re the modulation, low-level
noise is most likely to show up affecting the brighter parts
of the image, where it is a good deal more difficult to see.
VSB-AM was really the ONLY practical choice for analog
video in the broadcast world. And, as has already been noted,
the situation re the modulation schemes used with "digital"
television are considerably different/more complicated.

Bob M.