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Author Topic: FM SCA Listening experiences of the past  (Read 260 times)

WD4MTW

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FM SCA Listening experiences of the past
« on: May 05, 2022, 10:24:07 AM »

I did a cursory search and didn't see anything on this topic. While I spent a good part of my hamming as a swl/rtty/digital listener/digital sleuth in a passive capacity due to time and family constraints from the early 70's to the early 90's, FM SCAs were something I found fascinating over the years during the evenings. There were many unique services there that were never meant for casual listening or decoding. Have any of you here pursued this over the years? Forget Musak/Elevator music, there was a very interesting world here under most folks noses that could have been experienced. What equipment did you use or build?  Tuners,decoders,special equipment for broadcast professionals? What were the odd things you heard? Telemetry? Special digital services? Blind services? Issues beyond simple decoders such as filtering from main channel "hash"? I'd love to hear your experiences as this, like early satcom, shortwave vft's were a hidden world from most despite the resources they owned.
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W9FR

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Re: FM SCA Listening experiences of the past
« Reply #1 on: May 05, 2022, 12:13:29 PM »

I never listened to the various “secret” services available, but when I worked at Regency Electronics they made a handheld unit called the quotrek that gave instantaneous stock market quotes on a large LCD display and it operated off the SCA channels from specifically authorized FM stations primarily in large markets.  I used to keep one at my desk watching their stock since it reportedly did not have the 15 minute delay.  The service no longer exists since HD radio took up the sub channel.  Noel, W9FR
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W1VT

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Re: FM SCA Listening experiences of the past
« Reply #2 on: May 05, 2022, 12:19:57 PM »

I recall the local police coming over the SCA channel at a drug store!

Zak W1VT
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SWMAN

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Re: FM SCA Listening experiences of the past
« Reply #3 on: May 05, 2022, 01:54:44 PM »

 When I was a kid a long time ago I always wondered how SCA worked. I could never hear anything on my radio, didn’t have an SCA receiver, that must be why. I still don’t totally understand it. Do they even use it anymore ?
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KC6RWI

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Re: FM SCA Listening experiences of the past
« Reply #4 on: May 05, 2022, 02:19:30 PM »

I bought a kit probably off an advertisement in monitoring times or other magazine, the kit or circuit board used a pilot light to let you
know that you were on frequency. You had to solder it into an fm radio. There was book reading for the blind if my memory is right.
I have something else that I was never able to find the answer. I discovered a text news channel when the tv signals were analog. I was
using one of the text channels off the main channel. You would know it a T1 or T2.
So I was curious if other channels might be sending out text, I emailed the fcc to ask if there is any general guide to what stations have
a text service, but the question came back as not so technical consumer answer, as if it wasn't read correctly.
The reason I asked is that it takes a few secs to access a text channel, it would be tedious to search.
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NA6O

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Re: FM SCA Listening experiences of the past
« Reply #5 on: May 05, 2022, 03:05:48 PM »

When I worked at WILL radio (PBS) in the 70s, we offered reading for the blind on our SCA. I never had a receiver but recall a Signetics PLL app note that showed how to build one. My HF hearing was good enough that I could hear the 19 kHz SCA pilot tone that leaked thru cheap FM receivers. So I could tell which stations transmitted SCA.

Gary NA6O
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KD2BD

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Re: FM SCA Listening experiences of the past
« Reply #6 on: May 05, 2022, 04:29:47 PM »

I learned an incredible amount about radio while experimenting with SCA decoders as a teenager, several years before becoming a ham.  My first decoder used an LM565 PLL purchased at Radio Shack that included an SCA decoder schematic on the back of the package.

After experimenting with it for a while, I soon realized I could improve on the design.  I added a 2-stage active filter ahead of the PLL to improve the SNR.  Each stage employed a small-signal NPN transistor with a Twin-Tee negative feedback network between the base and collector tuned to 67 kHz.  I developed enough SCA carrier with those stages that I could rectify it, apply it to a Schmitt Trigger circuit (from the 1979 ARRL Handbook), light an LED, and forward bias an audio squelch circuit to silence the decoder between music selections (or squelch the noise while tuning the host FM receiver).

From northern New Jersey, I used to hear a myriad of commercial-free music broadcasts, reading services for the blind, and foreign language programming.  The "Physician's Radio Network" ran an hour-long loop of news items of interest to medical doctors paid for by the pharmaceutical companies.  On Christmas, they suspended their news broadcasts and played Christmas music, instead.

There were various data transmissions as well, but I had no way of decoding them.  One station would suspend their data transmissions to broadcast a religious program called, "The LDS Family Hour".  I can also recall a "Dow Jones Radio 2" broadcast that included audio clips of recent news reports, each preceded by a sequence of audio tones (possibly DTMF).

This was really cool stuff!  Here I was privy to a whole world of "hidden" radio broadcasts that no one else knew about!   ;D

I remember the Muzak broadcasts would pause on the hour and half hour to transmit a sub-audible tone for maybe 10 seconds.  When I used to go to stores or a local burger joint and it was close to one of those times, I used to say to my friends, "The music is going to pause momentarily".  And they could never understand how I knew that would happen.  ;D

Some FM stations that also ran AM stations would sometimes broadcast their AM programming on their FM SCA subcarrier.

Then several years later while I was experimenting with a homebrew LF converter, I decided to connect it to the ratio detector of my FM receiver and discovered it received SCA signals far better than my 565-based decoder.

By this time, some stations started using 92 kHz as an additional SCA subcarrier.  So, my next decoder employed a 2-stage OP-AMP active filter, switchable between 67, 78, and 92 kHz.  That fed an LM1496 doubly balanced mixer with an LM566 VCO serving as a local oscillator.  The mixer upconverted the subcarriers to a fixed IF of 455 kHz.  The mixer fed a Murata 455 kHz ceramic filter into a quadrature FM detector chip (another Radio Shack item).  The VCO was switchable so I could receive 67 kHz, 92 kHz, or TV SAP broadcasts (which were at around 78.6 kHz and virtually identical to FM SCA).  I also used the DC output from the quad detector as an AFC voltage for the LM566.  I also developed a squelch system that used a combination of carrier level and noise detection that was completely bullet-proof.

Years went by, and I discovered that Exar's popular XR2211 PLL-based AFSK decoder could be configured as a linear FM detector, and its carrier detection circuitry could serve as a squelch.  So, out came the soldering iron, and yet another SCA decoder was born.  ;D

Today, FM SCA is nothing like it was when I first discovered it.  The music, data, and news services are long gone.  The only thing left is a scattering of foreign language broadcasts.  This list illustrates what's left in the New York City area:

         http://www.n2nov.net/NYCareaFM_SCA.html

More recently, I have discovered that a PC running Linrad (SDR software) does an excellent job of demodulating SCA subcarriers.
 
My HF hearing was good enough that I could hear the 19 kHz SCA pilot tone that leaked thru cheap FM receivers. So I could tell which stations transmitted SCA.

Gary NA6O

19 kHz is the stereo pilot carrier.  Any station not transmitting in stereo can transmit SCA material at 41 kHz in addition to 67 and 92 kHz.

73 de John, KD2BD
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KC6RWI

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Re: FM SCA Listening experiences of the past
« Reply #7 on: May 06, 2022, 07:40:49 AM »

Great post, John,
Hidden signals, I was always  curious of how the muzak worked, way back before the net, I had guessed that small antennas might be picking up a broadcast, and of course I would have loved to find the frequency just to sample that odd music.
Currently at the dentist, I noticed the easy listening music no commercials, i didn't ask but I am sure its streaming from some dedicated modem.
Thats good all the building you did with sca stuff, I just did a little of that myself.\
The tv also had some sca or services hidden, but I never found away to see it, before the net.
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KD2BD

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Re: FM SCA Listening experiences of the past
« Reply #8 on: May 06, 2022, 11:54:00 AM »

Great post, John,
Thanks!  ;)

I never knew what the term "Muzak" meant back then, and I thought the music being played on SCA subcarriers was of local origin.  Imagine my surprise when I discovered the exact same music on a New York City radio station being played on a station in Philadelphia!  So, the content was likely distributed to radio stations nationwide via satellite.

I can remember many years ago when a local supermarket that played Muzak had their rooftop FM yagi replaced by a small dish.  It wouldn't surprise me if the content was distributed via the 'net, today.

73 de John, KD2BD
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