From: schmidt@auvax1.adelphi.edu
Subject: Re: SCA radio (was Re: Taiwan news via shortwave radio)
Organization: Adelphi University, Garden City NY
Distribution: usa
Date: Fri, 31 Jul 1992 05:06:12 GMT

In article <1992Jul30.191939.20347@bnr.ca>, mwandel@bnr.ca (Markus Wandel) writes:
> In article <1992Jul27.142923.10298@cbnewsc.cb.att.com> rats@cbnewsc.cb.att.com (RS PRO-2006) writes:
>>
>>In <1992Jul26.211229.4578@ecl.psu.edu> David Perrussel states:
>>
>>|> The subcarrier is typically in the 60-100khz range and modulated via
>>|> AM.
>>
>>I have fed the output from the headphone jack of a stereo directly
>>into the antenna input of an ICOM IC-R70 receiver and detected the
>>SCA signal.
>>...
>>You will get a noisy signal, albiet a usable signal such that you could
>>listen to SCA stations for the blind and such...
> 
> Is the SCA subcarrier really usually AM?  I have never heard one but I read a
> Philips application note for the NE565 PLL that actually showed how to use it
> as an SCA demodulator, and that was definitely for FM modulation.  Do SCA stations
> attempt to "hide" their signals from competing receivers by varying the subcarrier
> frequency and modulation method?
> 
> Markus Wandel

Most SCA program subcarriers are FM.  Data, etc may use other schemes.  The FCC
allows "any form of modulation"  Subcarrier signals, including significant
sidebands must be between 53 khz and 99 khz.  Mono stations must be between 20
khz and 99 khz. Common frequencies are 67 and 92 khz, with a few mono stations
using 41 khz.  Normal deviation is either 6 or 4 khz for "100 %" deviation of
the subcarrier.  The allowable injection depends on the number of subcarriers
and whether you are running stereo, but is often 10 or 20 percent.  These
figures may be different if one or more data subcarriers are also used.  

Stations tend to use the same frequencies, as they want to use readily
available equipment for SCA generation, monitoring, and receiving.

It is difficult to get much SCA signal out the mono output jack of most tuners, 
due to the 75 microsecond deemphasis in the baseband mono demodulator, which
attenuates the signals 6db per octave.  If you can find an old mono FM receiver
with a "stereo adapter" jack on the back, that will have an undeemphasized
(sp?) baseband signal.  This can be run into a communications receiver with
long wave capability down to 40 khz, which also has a FM demodulator,
preferably of somewhere near the correct bandwidth.  AM "slope" detection, will
work to a degree, at least to identify the signal, if you don't mind the
distortion.  Another way to listen is to hetrodyne the baseband output up to
a frequency receivable by a scanner, and use the scanner medium bandwidth FM
mode to demodulate the signals.  The oscillator for hetrodyning the signal up
must be rather clean, or you will have problems.

I once built a (vacuum tube) SCA demodulator by building what amounted to an AM
radio, except the local oscillator kicked the subcarrier UP to 455 khz, and I
used a 455 khz discriminator transformer instead of an envelope detector.  It
worked well enough to listen to, but there was a lot of crosstalk from the main
channel, probably due to nonlinearities in the IF's.

Be aware it is now illegal to listen to SCA's without permission.

-- 
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John H. Schmidt, P.E.              |Internet: schmidt@auvax1.adelphi.edu
Technical Director, WBAU           |Phone--Days     (212)456-4218
Adelphi University                 |       Evenings (516)877-6400
Garden City, New York 11530        |Fax-------------(212)456-2424
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