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Subsidiary Communications Authorization (SCA) in the United States, andSubsidiary Communications Multiplex Operation (SCMO) in Canada, is asubcarrier on aradio station, allowing the station to broadcast additional services as part of its signal.
Subsidiary Communications Authorization is the United StatesFederal 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 ascommon carrier orLand Mobile Radio Service transmissions.[1] 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 toAM radio broadcasting. By extension, the already limited bandwidth of AM means that it is impossible to multiplex any secondary audio service on an AM signal; any SCA usage would be limited to text.
TheCanadian Radio-television and Telecommunications Commission (CRTC) calls this serviceSubsidiary 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[clarification needed] under other radio and telecommunications acts, and may require a fee.[2]
InAustralia, the service is calledAncillary Communications Service (ACS) and theRadio Data System is specifically addressed by government guidelines.[3]
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:
Venture Technologies, which owns a large number ofanalog low-power television stations with audio subcarriers that operate as FM radio stations, proposed using SCA rules to continue transmitting those analog audio services along anATSC 3.0 signal.[6]
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.[citation needed]
Companies in the past such asNorver,ComPol,Mani National Corporation, McMartin, & Dayton Industrial; and current companies such asMetrosonix 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.[citation needed]
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