SCA.2
Next, along came stereo.  In order to avoid the wrath of all the owners
of monaural FM sets, the FCC in its wisdom decreed that a "compatible"
system would be necessary before they would approve FM stereo.  The
engineers quickly noted that the A+B signal from two microphones gives a
passable monaural signal (especially if "one-point" miking is used).
Now the problem was how to get A and B out of A+B.  Well, as your high
school algebra teacher probably taught you, (A+B)+(A-B)=A and
(A+B)-(A-B)=B.  So all they had to do is send (A-B) in some clever way
and the receiver could reconstruct A and B by "matrixing".  The method
adopted was to "multiplex" this (A-B) signal onto the main carrier by
using it to modulate a SUBcarrier located at 38 KHz. Double-Sideband
Suppressed Carrier (DSBSC) modulation was chosen.  This gave a "lower
sideband" extending downward from 38KHz (less 20Hz or so) to 23KHz
(because the highs were cut off at 15 KHz.)  A "pilot carrier" was put
at 19 KHz which allows the receiver to recover the precise frequency
phase of the 38 KHz carrier so that recovery of the (A-B) signal could
proceed.  In fact it turns out that this carrier can be used in an even
more clever way to recover A and B signals directly from the "composite"
signal ( A+B plus pilot carrier plus A-B ).
CONTINUED IN SCA.3