SCA.3
Note that all the new junk -- pilot at 19KHz and A-B from 23 to 38 K
-- are so high in frequency that most people wouldn't hear them, and
most older monaural FM sets and loudspeakers won't reproduce them
audibly anyway.

Well, as if this wasn't bad enough, then along came the SCCA (I warned
you about my warped sense of humor) and asked for authorization to put
MUZAK on the air.  The obvious thing to do was to put on yet another
subcarrier, this time at a frequency far enough above the audio so that
it wouldn't interfere with stereo broadcasting.  67 KHz was chosen as
the magic frequency.  But this time, FM modulation was chosen.
Remember, all of this stuff is being stacked up in frequency above the
normal monaural FM broadcast audio in such a way that a normal FM
receiver won't be affected (much) by it.  So before we even go into
"composite audio" input of our simple FM transmitter, we have a VERY
"composite" signal indeed.  One might diagram it like this:


CONTINUED IN SCA.4