newton2@ucbtopaz.CC.Berkeley.ARPA (10/08/84)
To: stever@tektronix.UUCP
Subject: Re: Interpreting SONY's SL2710 Manual
References: <3788@tektronix.UUCP>
"Multiplex filters" are notch filters or sharp elliptic low-pass filter
to remove the "pilot-tone" which accompanies FM stereo transmissions
(similarly for TV stereo, which uses essentially the same Zenith
transmission system). FM stereo sends the left-minus-right signal
as a double-sideband suppressed carrier AM centered at twice the
pilot-tone frequency; the pilot-tone is used as a phase reference to
reinsert the carrier for demodulation (also offers an automatic
sitching feature for stereo/mono). FM pilot tone is 19 KHz; TV is
the horizontal line freq.
It's desirable to remove pilot tone because it's present at a high
level (10% modulation in FM) and is audible, can saturate the electronics,
blow up tweeters, paralyze noise-reduction systems on tape recorders and
beat with the bias oscillators of tape recorders. It can do these last
two terrible things even at quite low residual levels, particularly
when Dolby noise reduction is employed (especially Dolby C).
Awesomely effective and incredibly cheap sophisticated active elliptic
(redundant?) filters are available as modules ("filter blocks") for
FM multiplex filters; presumably the same will shortly be true for
TV stereo versions. Some multiplex decoder ICs use a clever servo'd
cancellation loop (augmentation to the phase-locked loop multiplex
demodulator) to subtract a properly phased and magnituded replica of
the pilot tone from the composite signal, thus allowing much less
restrictive (or no) subsequent pilot-tone filtering and therefore a
more extended bandwidth (beyond the 16 kHz or so that the filter-only
scheme imposes).