🚀 go-pugleaf

mmm@cup.portal.com (Mark Robert Thorson) writes:

>I seem to recall that Muzak is transmitted in the FM band using some kind
>of funny encoding.  Can someone describe how this works, and what would
>be needed to build a transmitter to break into these broadcasts (purely
>for informational purposes, of course).


One way that Musak(TM)-like (and some other subscriber services, 
e.g., reading for the blind, simultaneous translation into a foreign 
language, minority language programs, stock info, traffic warnings, etc) 
signals are transmitted is through the SCA channels that piggy-back the 
standard FM broadcast signals.  

These are essentially FM-on-FM signals.  I.e., after the usual FM 
demodulation process, there is yet another FM carrier above the L+R, 
19 kHz stereo pilot and the L-R signals.  

The SCA signals appear between about 57 kHz and 91 kHz - if memory
serves, there are three standard carriers, 57 kHz, 67 kHz and 91 kHz 
in use.

If you take this FM signal and demodulate it once more (at the SCA 
frequency, not the FM receiver's usual 10.7 MHz IF), you get the
desired output.  "Audio" bandwidth is quite limited, although digital
data of around 5 kbits/s is possible.

Get a spectrum analyser and it can be quite educational to look at, 
especially if you are taking a beginning communications theory course.
Remember to tap your FM receiver before the stereo decoder and the
audio deemphasis circuitry.  Eons ago, I used to have lots of fun
looking at them.  And when the Signetics 560-series PLLs first appeared 
in the market (1969?), had built passable demodulators.

I believe I read somewhere that Musak (TM) has also gone to satellite
distribution in addition to using SCA.


Regards,

Kok Chen			kchen@apple.COM, kk6dp
Apple Computer, Inc.