Posted by Lawrence D'Oliveiro on 10/27/06 11:05

In message <1160458985.368061.254500@m7g2000cwm.googlegroups.com>, Radium
wrote:

> In the old days when only B&W TV existed, was anythere any such thing
> as a "subcarrier" in the video signal?

Nope, quite the opposite. If I recall rightly, the monochrome video carrier
is suppressed, along with most of one sideband. The monochrome video signal
is amplitude-modulated, just like radio used to be in the days before FM.
But with an AM signal with carrier frequency Fc, where the signal has
frequency components from 0Hz up to frequency Fs (assuming Fc is greater
than Fs), the modulated signal will occupy frequencies ranging from Fc - Fs
to Fc + Fs. With traditional video, Fs is something like 6MHz, so a single
TV channel takes up quite a lot of bandwidth.

However, the frequencies greater than Fc (the upper sideband) contain an
exact mirror image of the information carried by the frequencies less than
Fc (the lower sideband). And of course the frequency Fc itself carries no
information at all. So in theory you should be able to get rid of one
sideband and the carrier, and still be able to reconstruct the original
signal. If you do this, you end up with an SSB (Single Side Band) signal,
which in the analog days was the most energy-efficient known means of
transmitting information via a radio signal.

However, decoding an SSB signal involved a lot of complicated analog
circuitry to reconstruct the missing sideband and carrier before the result
could be demodulated as a normal AM signal. So analog TV, instead of
completely getting rid of the second sideband, simply cuts it down to
a "vestigial" sideband. Apparently this is enough for conventional AM
demodulation circuitry to work, while still managing to cut the amount of
bandwidth needed for an analog TV channel by nearly half.

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