Second audio program

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 (the Public Broadcasting Service).

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

MTS features, including stereo and SAP, travel on subcarriers of the video carrier, much like stereo information for an FM radio broadcast. (The video is used since the audio carrier is too narrowband to fit any subcarriers.) The SAP subcarrier is located at 78.670kHz, which is five times the 15.734kHz 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.

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.

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