mmm@cup.portal.com (Mark Robert Thorson) (01/09/90)
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).
kchen@Apple.COM (Kok Chen) (01/10/90)
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.
markz@ssc.UUCP (Mark Zenier) (01/12/90)
In article <25740@cup.portal.com>, 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). These signals are FM subcarriers above 53 Khz, added in with the baseband audio, and the other stuff that makes up a FM stereo signal. All of this frequency modulates the main carrier. It's all in a package, so you can't override one part with out mucking up the rest. A good way to listen to this stuff is to hook up a VLF receiver to the output of the detector of an FM tuner. Interesting what you find, but the @#$%^& Electronic Communications and Privacy Act says you shouldn't. Most of elevator music is by subscription and money talks. markz@ssc.uucp