Second audio program

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Second[ary] audio program[ming] (SAP) is an auxiliary audio channel for television that can be broadcast or transmitted both over the air and by cable TV. It is often used for an alternate language (hence giving the facetious "Spanish audio program" expansion to the acronym), or for the Descriptive Video Service (DVS) offered in the U.S. by PBS, along with broadcasting the NOAA's Weatheradio service or a local National Public Radio station at times where translation or DVS is not needed. SAP is used for broadcasting Canada's Parliamentary television channel CPAC in both English and French.

SAP is part of MTS, the multichannel television sound standard set by the National Television Systems Committee (NTSC) in 1984 in the U.S. The NTSC video format and MTS are also used in Canada and Mexico, and Japan.

MTS features, including stereo and SAP, travel on subcarriers of the audio carrier, much like stereo information for an FM radio broadcast. The SAP subcarrier is located at 78.670 kHz, which is five times the 15.734 kHz MTS pilot signal. In turn, the MTS pilot is locked to the horizontal sync frequency of the video carrier for stability. The SAP channel contains mono audio which has been dbx-encoded for noise reduction, to improve the signal-to-noise ratio. The SAP audio goes from 60 Hz to 12 kHz, which is less than the "regular" audio channel accepting from 50 Hz to 15 kHz.

Though not technically a SAP channel, television stations can also broadcast a "PRO" (professional) audio subcarrier which is used to communicate with station personnel. This one-way audio channel allows individuals at the television station to send messages to people located away from the station and is frequently employed during newscasts. This channel is located at 6.5 times the pilot, and is also part of the MTS standard.

Because of its 251 kHz bandwidth, the audio carrier used in the NTSC television system can easily handle multiple audio channels.