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!

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.

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.

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.htmlMore 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