Subsidiary Communications Multiplex Operation

Subsidiary Communications Authorization (SCA) in the United States, and Subsidiary Communications Multiplex Operation (SCMO) in Canada, is a subcarrier on a radio station, allowing the station to broadcast additional services as part of its signal.

"Subsidiary Communications Authorization" is the United States Federal Communications Commission's official designation for this type of service. SCA was deregulated in 1983; since then, both AM and FM licensed broadcast stations have been allowed to use subcarriers in the United States in general without requiring separate authority; authorization is only required for some uses which are still otherwise regulated, such as common carrier or Land Mobile Radio Service transmissions. The fidelity (bandwidth) of SCA channels on FM is generally quite limited compared to that of the main program material, resulting in audio quality similar to AM radio broadcasting.

The Canadian Radio-television and Telecommunications Commission (CRTC) calls this service "Subsidiary Communications Multiplex Operation" (SCMO). SCMO for internal uses by the radio station, such as internal monitoring and cue control, are allowed under the normal broadcasting certificate. Non-broadcasting uses are allowed when the type of use is authorized under other radio and telecommunications acts, and may require a fee.

In Australia, the service is called "Ancillary Communications Service" (ACS) and the Radio Data System is specifically addressed by government guidelines.

Subcarrier channels falling under the description of SCA are usually on FM at 67 kHz and 92 kHz from the main carrier, and 67 kHz is the most common. Major uses of SCA include:

Most programming transmitted by SCA/SCMO is usually pay/subscription-based, making unauthorized reception of such programming illegal, but programming which is not commercial in nature, such as reading services, can be received legally.

Companies in the past such as Norver, ComPol, Mani National Corporation and McMartin, and current companies such as Dayton Industrial, and Metrosonix, as well as electronic kit companies such as Ramsey Electronics and North Country Radio make radios and adapters for receiving SCA/SCMO channels. It can be difficult to keep the main channel's FM stereo difference subcarrier from interfering with such SCA decoders, as the stereo signal is a much stronger signal, and distortions due to multipath can also cause problems.

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