ComPol, Inc. did not invent SCA, but we have done a lot to perfect it. On this page is a simple explanation, so that people with no technical background can understand how it works.
When the FCC established the FM broadcast system, they assigned each station a channel with a width of 200 kHz. For example, the station assigned to 88.1 MHz which is really 88,100 kHz is authorized to use frequencies from 88,000 to 88,200 kHz. When a tone of a certain frequency is transmitted over FM, the bandwidth of the transmitted signal is a minimum of twice that frequency. Thus, frequencies up to 100 kHz can be transmitted on an FM broadcast station. When you consider that the average person can only hear frequencies up to 15 kHz, this leaves a lot of space on the FM transmitter for other signals.
The frequencies from 19 to 53 kHz are used for transmission of stereo. Thus, the frequencies from 53 to 100 kHz are not usable for the transmission of programming. SCA takes advantage of this extra space by converting the special programs into frequency bands that center on 67 and 92 kHz through frequency modulation of those frequencies. Because they are well beyond human hearing range, they have no effect upon reception of normal programming. At the receiver, the subcarriers are converted back down to their original frequencies.
The diagram above shows the assignment of frequencies used in an FM transmitter. The main channel is the program heard with a monaural FM radio. These frequencies extend from below 25 Hz to 15 kHz. At 19 kHz, is the stereo pilot. The pilot is used to extract the stereo programming, which exists from 23 to 53 kHz. At 57 kHz is the RDS subcarrier used for data to carry such information as the station's callsign, and the name of the selection currently playing. From 60 to 74 kHz is the 67 kHz subcarrier and from 85 to 99 kHz is the 92 kHz subcarrier. Notice that the amplitude of the subcarriers is very low compared to the main and stereo. This prevents interference to stations on adjacent channels.
The technical nomenclature for SCA transmissions is: FM subcarrier on FM carrier, frequency division multiplexing. SCA is a legal term which is short for Subsidiary Communications Authorization.