From: utzoo!decvax!cca!hplabs!menlo70!sytek!zehntel!teklabs!keithe@sri-unix
Newsgroups: net.music
Title: On commercial-free FM
Article-I.D.: teklabs.1299
Posted: Tue Jul 27 12:35:10 1982
Received: Thu Jul 29 04:33:59 1982

Commercial-free doesn't mean they have no source of income.  There are
the SCA channels (for Subsidiary Carrier Authorization, I think) which
can be carried on standard FM signals which send other signals around;
one of their (ab)uses is the Muzak (tm?) you hear in grocery stores,
elevators and wherever.  I think some paging-systems signals for
doctors, computer technicians and other healers are also transmitted on
these signals. Southwest Technical Products will sell you a kit to
receive them - at least they used to - and it wasn't too expensive,
either.  But the musical quality of the signal is nothing to write home
about, since it has to be bandlimited to prevent over-modulation
problems. And it won't be in stereo. You can find stations broadcasting
SCA in your area by using a spectrum analyzer (a Tektronix 7L12 or 7L13
will do just fine; so will some others but I don't know their model
numbers) and observing the sidebands on the signals in the FM band.
With some practice you'll be able to differentiate the L-R sidebands
(which are at carrier +/- 38kHz) from SCA signals which will be located
further away from the carrier.  These sidebands are more easily
distinguishable during periods of silence on the main channel.

keith ericson at tektronix