Subsidiary communications authority
||This article needs additional citations for verification. (March 2009)|
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.1
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 authorizedclarification needed under other radio and telecommunications acts, and may require a fee.2
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:
- For talking book/radio reading service for the blind:1 usually carried on NPR-affiliated public radio stations in the United States.citation needed In Canada, this is usually provided by VoicePrint over television second audio programs instead.4
- As a transmitter/studio link, sending telemetry data from a broadcast FM station's transmitter to the studio for monitoring the condition of the transmitter
- For commercial paging service: sent via FM subcarrier to the subscriber with a compatible pager
- For data broadcasting: Microsoft's DirectBand service (used by MSN Direct) relies on SCA FM subcarriers; Data Broadcasting Corporation's Signal service used SCA 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.
- For the transmission of Muzak's background music service to its subscribers (i.e., supermarkets, restaurants, etc.), which was also relayed by leased phone lines: these transmission methods have been phased out with Muzak now transmitting its programming via satellite.
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 as Norver, ComPol, 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 is usually 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.
It's possible to also receive the SCA/SCMO signal using a radio that is able to tune and demodulate signals below 100 kHz in FM mode. In that case, the baseband audio (recovered from the discriminator of the FM receiver before multiplex decoding or filtering) is fed into the receiver's RF (antenna) input and the radio tuned to the subcarrier frequency (typically between 53 and 94 kHz, 67 and 92 are common) to extract the signal's contents. Alternatively, the baseband audio before deemphasis can be heterodyned with a suitable IF (intermediate frequency) (that is in the range of the receiver's ability to demodulate FM) and fed into the receiver (tuning it to the appropriate frequency in that case) or even fed directly into the demodulating stage (provided one has the technical knowledge required).
- Bruce Elving, author and advocate of SCA broadcasting
- Radio Data System — widespread program information standard; sometimes considered a type of SCA
- Second audio program (SAP) — a similar service on television signals
- "Radio Subcarriers / SCAs / Subsidiary Communications Authority". Federal Communications Commission Audio Division. Retrieved 2009-03-28.
- "SP-1452 — Spectrum Policy Provisions to Permit the Use of Digital Radio Broadcasting Installations to Provide Non-Broadcasting Services". Industry Canada, Spectrum Management and Telecom. September 1997. Retrieved 2009-03-28.
- "Appendix 2: Emission Standard for the Australian Frequency Modulation Sound Broadcasting Service" (PDF). Broadcasting Services (Technical Planning) Guidelines 2007. Australian Communications and Media Authority. 2007-07-06. Retrieved 2009-03-28.
- "Where to find VoicePrint". Toronto: National Broadcast Reading Service. Retrieved 2009-03-28.