From: mkeitz@bev.net (Mike Keitz)
Newsgroups: rec.radio.amateur.homebrew,rec.radio.amateur.digital.misc
Subject: Re: What is S.C.A receiver ?
Date: 9 Jun 1995 23:55:41 GMT
Organization: TSE Systems
NNTP-Posting-Host: mkeitz.bevb.blacksburg.va.us
X-Newsreader: News Xpress Version 1.0 Beta #3

In article <RAHUL.95Jun9132152@passat.cities.lehman.com>,
   rahul@passat.cities.lehman.com (Rahul Vipparthi) wrote:
>
>
>There is a FM station in New York which can be
>cannot be heard through a conventional FM recevier.
>
>They sell "special" radios which are capable of receiving
>this station. The sticker on the back says Manufactured by
>XYZ specialists in S.C.A receivers.
>
>Can this is received through a SONY ICF-SW7600G ?
>
>I would appreciate any help. If this is a wrong newsgroup, I 
>apoligize and please direct me to the correct newsgroup.
>
>
>Thanks
>
>-rahul-

Well, this isn't really a digital question (although it could be), or really a 
purely amateur question, but it is definitely a radio question, and something 
I know a little about.  Perhaps any further followup should be conducted in 
rec.radio.homebrew, where the topic has appeared before, ironically in a 
digtal context.

The SCA "radio stations" are actually signals that are combined with the main 
signal of a broadcast radio station.  A conventional FM radio only plays the 
main station, ignoring the SCA.  It's kind of like the process of transmitting 
stereo FM, this is done in a manner that a mono radio still receives the 
signal properly in mono.
The "special" radios, therefore, start as conventional FM radios with the 
tuning fixed to the station that has the SCA signal added.  But rather than 
play the "raw" output, they process the signal additionally to extract the SCA 
 audio.

Uses for this technology are varied, although I don't think it ever really 
took off because for various reasons the signal quality is not very high, and 
(except in big cities), there aren't enough potential users to justify all the 
equipment.  Also, it can degrade the main signal slightly, causing a potential 
loss of listeners.  Public radio stations here broadcast a rather amateurish 
"reading" session of people reading various newspapers and magazines for the 
benefit of those who are blind or otherwise can't read newspapers and 
magazines.  In the commercial segement of the band (>92 MHz), programming is 
likely to be "background music"  or digital services of various sorts like 
paging or stock quotes.  In Europe, there were some experiments to link data 
transmitted via SCA into the "Smart Road" taxpayer-milking scheme.  It is 
probably not legal to publicly receive these without paying a license fee to 
whoever is providing the service, that license fee is most likely included in 
the price of the special radio they sell or lease to you.

Into the more technical stuff:  The SCA is transmitted as a "subcarrier" on 
the baseband audio.  Looking at the spectrum coming out of the FM demodulator 
in a conventional radio, before it is de-empahsized, etc:
{Mono audio} P {Stereo Information}  {SCA}  (P= Stereo Pilot 19.0 KHz tone)
0           19K         38K           67K   (Hz)
The SCA carrier, which is usually more than 60 KHz (57 KHz for Europe, usually 
67 KHz in the USA, although it can be even higher, up to 100 KHz, especially 
for narrowband data), will be filtered out by a conventional radio.  A good FM 
receiver will often include a "SCA Trap" tuned to 67 KHz to get rid of this 
signal entirely so it won't interfere with the stereo audio. 

The SCA carrier is FM modulated with about 7 KHz deviation for an audio 
channel.  This gives a frequency response to about 5 KHz in the baseband, so 
it sounds like AM radio.  Actually, everything I've heard on SCA sounds 
*worse* than a nearby AM radio station.  This is due to the other major 
problem: various nonlinearities in the FM transmitter, the radio channel 
(multipath), the IF filters, and the discriminator in the radio will cause the 
main channel audio and stereo information to either (1) produce harmonics in 
the SCA band and/or (2) cross-modulate the SCA carrier.  In either case, the 
result is that the main channel signal bleeds over into the SCA signal, 
causing what sounds like static cracks and pops in the SCA when peaks are hit 
on the main channel, especially in stereo programming.  This is not helped by 
the fact that the SCA signal is only about 10% of the total modulation of the 
radio station, so it will not take too much power away from the main channel 
that most receivers are listening to.

Adapters to extract the SCA signal from a conventional receiver used to be a 
popular magazine project, as it can theoretically be fitted to any receiver 
that receives commercial FM. I have built a few SCA adapters to experiment 
with, both magazine designs and my own, and have not been too satisfied.  I 
also tried one commercially-produced SCA receiver and got about the same 
results, so I don't think my circuitry was too much at fault.  I would suggest 
listening closely to any SCA receiver before you buy it to be sure you can 
live with the quality.

-Mike KD4QDM