Second audio program
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Second audio program (SAP), also known as secondary audio programming, is an auxiliary audio channel for analog television that can be broadcast or transmitted both over-the-air and by cable television. SAP is part of the multichannel television sound (MTS) standard originally set by the National Television Systems Committee (NTSC) in 1984 in the United States. The NTSC video format and MTS are also used in Canada and Mexico.
SAP is often used to provide audio tracks in languages other than the native language included in the program (as a result, the facetious term "Spanish audio program" is used as an expansion to the acronym), or for the Descriptive Video Service (DVS) offered in the United States on some television stations or cable networks (primarily ABC, CBS, NBC, and PBS), along with broadcasting the local NOAA Weather Radio services or a local National Public Radio station (on many PBS stations) at times where translation or DVS is not needed. SAP is used for broadcasting the Canadian Parliamentary television channel CPAC in both English and French. SAP is also used for student radio stations that cannot otherwise get on the air due to crowded FM broadcasting bands.
MTS features, including stereo and SAP, travel on subcarriers of the video carrier, much like color for television. It is not carried on the audio carrier in the manner of stereo sound for an FM radio broadcast, however, as it only has a frequency deviation of ±25 kHz, whereas regular FM broadcasting has a deviation of ±75 kHz. The SAP subcarrier is located at 78.670 kHz, which is five times the 15.734 kHz corresponding to the 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 has a bandpass from 60 Hz to 12 kHz, which is less than the "regular" audio channel which runs 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, particularly those engaged in electronic news gathering. 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 on-location newscasts as the foldback channel to reporters and cameramen. This channel is located at 6.5 times the pilot (102.271 kHz), and is also part of the MTS standard.
Second audio programs are also available in other media. On analog television receive-only satellite television systems, audio programs are manually tuned by their subcarrier frequency, commonly around 6 MHz, often as low as 5.8 or as high as 7.2. These travel the same way as the discrete left and right main audio channels, and ATIS station ID, which is heard as Morse code. On FM radio, radio reading services and other audio programs can be heard on subcarriers; however, these are generally prohibited from public listening. On digital television systems, selection is done through a menu as with analog television, though some provide easier access rather than having to dig down to find the option. Stations may also choose to transmit audio-only programs by assigning them to separate digital subchannels with an optional screen of title and artist information; however, this prevents the viewer from watching the main video in the case of television stations.