Second audio program
|This broadcasting article does not cite any references or sources. (April 2007)|
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 TV. SAP is part of the Multichannel television sound (MTS), the standard 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.
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. on some TV stations or cable networks (primarily the Big Three television networks 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 Canada's 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 TV. It is not carried on the audio carrier like stereo 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 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 an 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.
Other media 
Second audio programs are also available in other media. On analog big-dish satellite TV 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 TV systems, selection is done through a menu as with analog TV, 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 TV stations.