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From: bill@thd.tv.tek.com (William K. McFadden)
Newsgroups: sci.electronics,rec.radio.amateur.misc
Subject: Re: FM SCA Subcarrier Demodulation
Message-ID: <1991Apr19.232932.20114@tvnews.tv.tek.com>
Date: 19 Apr 91 23:29:32 GMT
References: <1991Apr19.050618.23977@elroy.jpl.nasa.gov>
Sender: news@tvnews.tv.tek.com (news user)
Organization: Tektronix TV Products
Lines: 23

The SCA carrier is an FM signal (usually 67 kHz) that is added to the
audio of an FM radio station before it is fed to the transmitter.  If
the station is stereo, there will also be a 19 kHz pilot tone and a 38
kHz AM suppressed carrier signal that carries the stereo difference
information.  All these signals plus the mono audio are mixed together
and fed to the FM transmitter.

To decode these, you first detect the FM signal.  The portion from 0 to
15 kHz is lowpass filtered to recover the L+R mono audio.  If a 19 kHz
tone is present, a stereo receiver will turn on the 38 kHz DSB detector
to recover L-R, which is combined with L+R in a matrix to decode L and
R.  If the receiver has an SCA detector and a 67 kHz carrier is
present, another FM detector demodulates it.  Think of an SCA receiver
as having two FM detectors in series.

If you need a better explanation, send email.

For a circuit, try looking at the data sheet for the 565 PLL made by
Signetics, Motorola, National Semiconductor, and others.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
bill@videovax.tv.tek.com,     {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920                 "SCUD: Shoots Crooked, Usually Destroyed"