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From: duncan@bolero.ati.com (James R. Duncan)
Newsgroups: sci.electronics,rec.radio.amateur.misc
Subject: Re: FM SCA Subcarrier Demodulation
Message-ID: <29955@bolero.ati.com>
Date: 21 Apr 91 17:02:58 GMT
References: <1991Apr19.050618.23977@elroy.jpl.nasa.gov>
Reply-To: duncan@bolero.ati.com (James R. Duncan)
Organization: KUFX 94.5 FM Stereo, San Jose, CA "The Fox"
Lines: 36

In article <1991Apr19.050618.23977@elroy.jpl.nasa.gov> szeto@kilroy.jpl.nasa.gov (James T. Szeto) writes:
>After reading the FCC Rules and Regulations concerning the engineering standard
>for subsidiary communications (SCA), I was wondering if someone would give a 
>clearer understanding on how SCA is added to the FM signal and how it is 
>recovered from the signal.  Also, I'm looking for a reference where I may find
>an example circuit which recovers SCA.
>
There's no big technical mystery to "SCA" operation.  This is simply a
plain vanilla subcarrier addition process.  The total stereo program
material, which also includes a DSB subcarrier at 38 kHz, is
considered to theoretically use up the first 53 kHz of the roughly
speaking 100 kHz bandwidth available in the usual FM broadcast carrier.
In the remaining space it is common to add one or more subcarriers
which can carry data, Muzak, paging, stock market quotes, or whatever.

Typically, a broadcaster may elect to add a 67 kHz subcarrier which has
been modulated by whatever "SCA" material.  That's all there is to it
basically; it's just added in with a relatively simple network.  

Recovery is also quite easy.  In this case, one would simply take an FM
detector circuit tuned to 67 kHz and attach it to the receiver's
detector circuit.  

The mulitplex or subcarrier process is discussed in many books,
probably some ham publications as well.  The most convenient FM
detector for subcarriers is the PLL, so you would need to find
applications notes for these from National Semiconductor, Exar, or
especially Signetics.  The NS literature is readily available; it even
used to be carried by Radio Shack (and maybe still is).  e


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