Anthony Fremont said:
That doesn't sound like it would do anything usefull at all. How will
that make the L-R information relocate itself down to human audible
frequencies from the 38kHz carrier? It's Seems to me that you'd at
least have to demodulate the L-R stuff to convert it back to its
original form and then mix that with the L+R stuff. I presume that the
switching is phase locked to the 19kHz pilot tone which you'd have to
filter off as well. If that's all the case, then it sure doesn't sound
all that much easier.
Consider how DSBSC is recovered in the first place, the 19 kHz pilot
tone's role in that - and then realize that "sampling" a signal at 38 kHz
and amplitude modulation of a fixed-amplitude 38 kHz carrier are to
a great degree exactly the same thing. That should make both the
encoding and decoding processes much clearer for all concerned.
There seems to be a huge argument going on here about two apparently
different things which really do not differ at all. "Sampling left and
right
channels alternately at 38 kHz" results in PRECISELY the same signal
as the original "L+R at baseband, plus L-R DSBSC around 38 kHz" -
they're just essentially two ways of doing the same thing. Either way,
you take the result and use it (with a 19 kHz pilot tone added in) to FM
the VHF carrier. At the receiving end, the output of the FM demodulator
is as it always was - L+R at baseband, and L-R DSBSC up around
38 kHz. Sampling THAT whole shebang at 38 kHz equates to "demodulate
the DSCSC L-R signal and combine the recovered signal with L+R to get
L and R separately." (Note that sampling the total signal at only 38 kHz
is actually UNDERsampling it, per Nyquist - which you are counting on,
since the aliasing of the various components is what's going to
bring those together and properly combine them, mathematically,
and so result in L and R separated and at the correct audible frequencies.)
Bob M.