SCA
Additional Thoughts on SCA Reception: 

As I recall, the setup described by Bob Parnass (I think that was who it
was) was to hook a VLF receiver to the output of an ordinary FM receiver
to pick up SCCA (sic - I always call it that, Sports Car Club of
America, when it should really be SCA - Subsidiary Communications
Authorization or something!) transmissions multiplexed on ordinary FM
broadcasts.

To understand what is going on, you need to know what the FM station
actually transmits.

Let's do it in "top down" fashion.  All you computer jocks out there
should relate nicely to that.

First of all, the FM station has a "composite audio" input - this is
just the input on which SOMETHING (consider it a stub subroutine to
written later) is fed in to the modulator.

From the FM modulator's point of view, what you put on this input is
just the MODULATING SIGNAL which you want to Frequency Modulate (FM) the
station's carrier.  Intuitively, you can think of the station as putting
out a "pure" carrier at frequency F when this modulating signal is zero.
When it is NONZERO, however, the instantaneous frequency of the
transmitter is changed.  Say the modulating signal value, in volts, or
whatever, is M.  Then the transmitter output frequency is set to F + K*M
where K is a sensitivity constant which is unimportant except that
whatever maximum value of M is applied to the FM modulator input should
result in a maximum "frequency deviation" K*M which is just about the
maximum that the FCC allows.
CONTINUED IN FILE SCA.1