Subcarrier

For AM broadcasting, different analog (AM stereo) and digital (HD Radio) methods are used to produce stereophonic audio.

In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking the remainder.

For the audio part, MTS uses subcarriers on the video that 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 or broadcast engineer at a remote transmitter site to talk back to the studio), or any other use a TV station might see fit.

In RF-transmitted composite video, subcarriers remain in the baseband signal after main carrier demodulation to be separated in the receiver.

Services like these and others on broadcast FM subcarriers are referred to as a Subsidiary Communications Authority (SCA) service by the FCC in the United States, and as Subsidiary Communications Multiplex Operations (SCMO) by the Canadian Radio-television and Telecommunications Commission (CRTC) in Canada.

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.

FMeXtra on FM uses dozens of small COFDM subcarriers to transmit digital radio in a fully in-band on-channel manner.

Removing other analog subcarriers (such as stereo) increases either the audio quality or channels available, the latter making it possible to send non-audio metadata along with it, such as album covers, song lyrics, artist info, concert data, and more.

Many stations use subcarriers 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.

This is now mostly superseded by digital TV (usually DVB-S, DVB-S2 or another MPEG-2-based system), where audio and video data are packaged together (multiplexed) in a single MPEG transport stream.