So that's all the stuff you can hear. Here's everything else. But, let me warn you now, that under Electronic Communications Privacy Act of 1986, it is illegal for you to monitor these transmissions. I have no idea why. It's not really that exciting and they can't tell you're doing it anyway. Unenforcible laws are just silly. So here's what you can't hear.
Stereo Beacon or Reference Signal: This unmodulated subcarrier is transmitted at 19 kHz. This is lets the FM stereo receiver know that the broadcast is in stereo. The receiver doubles the frequency of the pilot tone and uses it as a phase reference to demodulate the stereo information.
A subcarrier is totally different than a reference tone. The SCA can carry data. This separate signal is carried on the main radio transmission. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth. This is in some ways similar to the multiplexing that's the big excitment (ho-hum) in HD Radio. I've talked about that before as well.
But there's all kinds of data in an SCA. That Horrible easy listening music you hear in the evevator or dentist office, Reading services for the blind, Foreign language news services, and theres a growing variety of data transmissions.
A typical FM broadcast may contain anywhere from 2 to 12 subcarrier voice channels in addition to the main signal, and subcarriers may havesubcarriers of their own. Sounds wildly complicated right? A lot of surveilance devices use these frequencies as well http://tscm.com/bugfrqVLF.html
I also caught an interesting note about Fireworks in Los Angeles using a 92 kHz SCA:
"a man was just reciting numbers, mostly one-at-a-time, but sometimes two." I'm not from KXLU, nor was I involved in that particular fireworks show, but as a fireworks pyrotechnician (one of my sidelines), that sure sounds like firing queues to me. It's becoming far more common to coordinate fireworks shows with music from radio stations and using sub-carriers is a common way to do that" I cut the identifiers out of that because it's not legal to have heard it. oy.