[BC] AM Carriers

[Richard Fry]

Jim T wrote about the FM stereo subchannel/subcarrier:
>The signal which may be used to generate that subchannel signal,
>a "carrier" at 38 kHz,  may very well be referred to as the subcarrier
> itself.  But it never appears in the final product.
__________________

It is true that no L-R spectrum at exactly 38 kHz will exist in a baseband 
FM stereo signal.

However in the case of the NTSC and PAL chrominance signal, the exact 
frequency of the suppressed color subcarrier can appear in the composite 
baseband signal.  This is the result of the quadrature modulation of the 
color subcarrier by two video signals (I&Q in NTSC, U&V in PAL).  The 
sideband pairs from each video signal combine at the encoder output to 
produce an amplitude and phase modulated waveform at the (suppressed) 
subcarrier frequency.  Variation of amplitude conveys color saturation, and 
of phase -- hue.

The color subchannel spectrum needed to convey unchanging color(s) is very 
narrow, and centered on the color burst frequency (which is the same 
frequency as the suppressed color subcarrier).   The only time that the 
color subchannel signal has significant bandwidth is during the transition 
time(s) from one color to another.

But in any case, the original color subcarrier modulated by I&Q or U&V _is_ 
suppressed (not transmitted).

RF