A subcarrier is separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth. This is a method of multiplexing.
Stereo is a subcarrier of an FM radio station, which takes the left channel and "subtracts" the right channel from it — essentially by hooking up the right-channel wires backward and then joining left and right. The result is modulated with AM onto a 38kHz radio frequency, which is joined with the mono left+right audio (which ranges 50Hz~15kHz). A 19kHz pilot "tone" is also added to trigger radios to decode the stereo subcarrier, and voilá, you have FM stereo, fully compatible with mono. Once the receiver has the L+R and L-R signals demodulated, it adds the two ( L+R + L-R = 2L ) to get the left channel and subtracts ( L+R - L-R = 2R) to get the right.
Likewise, TV signals are transmitted with the black and white luminance part as the main signal, and the color chrominance as the subcarriers. A black and white TV simply ignores the extra information, as it has no decoder for it. Subcarriers on the video can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another for second audio programs (such as descriptive video service for the vision-impaired, and bilingual programs), and yet a third hidden one for the studio to communicate with reporters or technicians in the field (or for a technician at a remote transmitter site to talk back to the studio), or any other use a TV station might see fit.
- Many noncommercial stations in the US (especially NPR) broadcast a reading service for the blind, which reads articles in local newspapers and sometimes magazines. The vision-impaired can request a special radio, permanently tuned to hear audio on a particular subcarrier frequency (usually 67kHz or 92kHz), from a particular FM station.
- The RDS/RBDS subcarrier (57kHz) allows FM radios to display what station they are on, pick another frequency on the same network or with the same format, scroll brief messages like station slogans, news, weather, or traffic -- even activate pagers or remote billboards. It can also broadcast EAS messages, and has a station "format" name ALERT to automatically trigger radios to tune in for emergency info, even if a CD is playing. While it never really caught on in North America, European stations rely on it quite a bit. An upgraded version is built into digital radio.
- Many stations use it for internal purposes, such as getting telemetry back from a remote transmitter, often located in a difficult-to-access area at the top of a mountain. A station's engineer can carry a decoder around with him and know anything that's wrong, as long as the station is on the air and he is within range. (See also STL and TSL.)
- IBOC digital radio on FM uses dozens of small COFDM subcarriers to transmit the data. Removing other analog subcarriers (such as stereo) increases the audio quality, and other non-audio data that can be sent along with it such as album covers, song lyrics, artist info, concert data, and more.
- Before satellite, Muzak was transmitted to department stores on FM subcarriers. The FCC also allowed betting parlor s in New York state to get horse-racing results from the state gaming commission via the same technology.
Note that higher frequencies drop off much more rapidly, which explains why FM stereo gets noisy at a distance, while switching to mono is still perfectly clear and easy to listen to.