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Subsidiary Communications Authority (SCA) is the United States Federal Communications Commission's (FCC) official designation for subcarrier channels transmitted by a broadcast FM radio station along with its main carrier. In Canada,the service is labeled by the Canadian Radio-television and Telecommunications Commission (CRTC) as "Subsidiary Communications Multiplex Operation" (SCMO).
Subcarrier channels falling under the description of SCA are usually at 67 kHz and 92 kHz from the main carrier (67 kHz is the most used). SCA subcarrier broadcasting is usually utilized in the following ways:
- For talking book/reading services for the blind (these are usually carried on NPR-affiliated public radio stations in the United States, and by VoicePrint in Canada for carriage on cable FM and on SAP channels),
- For sending transmitter telemetry data from a broadcast FM station's transmitter to the studio for monitoring the condition of the transmitter (this makes troubleshooting impossible when the transmitter fails),
- For commercial paging service, pager data is sent via FM subcarrier to the subscriber with a special pager,
- For data broadcasting, Microsoft's DirectBand service (used by MSN Direct) relies on SCA FM subcarriers, and formerly for Data Broadcasting Corporation's Signal service for sending real-time stock quotes,
- For closed-circuit or specialized radio programming aimed at certain markets or professions, such as the Physician's Radio Network, agricultural commodity & futures information (via voice or data), or ethnic foreign-language radio programming,
- and formerly for the transmission of Muzak, which was also relayed by leased phone lines. This practice has been phased out with Muzak now transmitting its programming via satellite.
In Canada, SCA subcarrier transmission is referred to by the CRTC as SCMO (Subsidiary Communications Multiplex Operations).
In Australia, the service is (or used to be) called "Ancillary Communications Service" (ACS). It is unclear if the service is still in operation.
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 and McMartin, and current companies such as ComPol, 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.
