Take a look at my GNURadio SCA Receiver. It’s a neat way to use your RTL-SDR to explore hidden signals in your community.
In the history of radio and telecommunications, we have a tradition of stacking signals within signals. It’s called multiplexing. FM radio uses frequency multiplexing to make best use of its allocated bandwidth.
Subsidiary Communications Authority (SCA) also known by other names has been around since the mid 1950’s. Engineers figured out how to add secondary, hidden channels in sub-carriers. Initially, these were used for background music but can perform a variety of narrow-band FM services. Shortly thereafter, sub-carriers were again used to send stereo information to create FM Stereo in the 1960’s. Most recently, an additional data sub-carrier has been added for Radio Data System.
It’s easy to build your own GNURadio SCA receiver. The flow diagram is shown above; just make a copy and expand the graphic to see all the parts and how to wire them together. Also, see this project in action on my short video. Here is the GNURadio Companion file if you want to use it.
SCA is used less often these days. There are many other ways to distribute specialized programming including data lines and wireless, including satellite. But most major markets have some SCA services. Just demodulate the FM signal to baseband and look for sub-carriers in the ultra-sonic range at 67 or 92 kHz.
In Calgary, three FM stations use SCA sub-carriers. Two of these are dedicated to specialty Asian music. The third simulcasts a popular AM radio station. In addition to music and programming, you might find other stations using SCA to send telemetry back to the studios.
GNURadio SCA Receiver – Double Decoding
You start the project by building a normal wideband FM receiver. Once you have this demodulated to baseband, you then translate the sub-carrier to another baseband receiver with narrow band FM demodulation. The SCA channel will be monophonic and sound much the same as an AM receiver, with audio bandwidth of 5 to 7 kHz. While normal FM broadcasting uses a frequency deviation of ±75 kHz, SCA uses only ±7.5 kHz.
Since SCA is a signal within another signal, you need some pretty sharp filtering to make decoding work properly.
Next year, I will figure out how to decode those mysterious Radio Data System RDS signals used to provide program and traffic information. They work similar to SCA, but use phase shift keying instead of audio.