Second audio program (SAP), also known assecondary audio programming, is an auxiliaryaudio channel foranalog television that can bebroadcast or transmitted bothover-the-air and bycable television. Used mostly foraudio description or other languages, SAP is part of themultichannel television sound (MTS) standard originally set by the National Television Systems Committee (NTSC) in 1984 in the United States. The NTSCvideo format and MTS are also used inCanada andMexico.
SAP is often used to provide audio tracks in languages other than the native language included in the program. In the United States, this is sometimes used forSpanish-language audio (especially during sports telecasts), often leading to the function being referred to facetiously as the "Spanish audio program". Likewise, some Spanish-language programs may offer English on SAP, seen withsoccer networks includingbeIN Sports (which features the same setup on its English and Spanish channels, and more rarely with general channels. Some stations may relayNOAA Weather Radio services, or, particularly in the case ofPBS stations, a localNational Public Radio (NPR) sister station, on the audio channel when SAP is not being used, and even commercial radio stations which are either sister operations orbroker their programming onto the SAP channel. In Canada, parliamentary and public affairs channelCPAC similarly uses SAP to carry both English andFrench-language audio.
SAP is also a means of distribution foraudio description of programs for thevisually impaired. Under theTwenty-First Century Communications and Video Accessibility Act of 2010, top U.S. television networks and cable networks have been gradually required to broadcast quotas of audio described programming per-quarter,[1][2][3][4] Since May 26, 2015, broadcasters have been required under the Act to provide dictations on SAP of any "emergency information" displayed in a textual format outside of theEmergency Alert System and newscasts.[5][6][7]
MTS features, includingstereo and SAP, travel onsubcarriers of thevideocarrier, much like color for television. It isnot carried on the audio carrier in the manner of stereo sound for anFM radio broadcast, however, as it only has afrequency deviation of ±25 kHz, whereas regular FM broadcasting has a deviation of ±75 kHz. The SAP subcarrier is located at 78.670kHz, which is five times the 15.734 kHz corresponding to theMTSpilot signal. In turn, the MTS pilot is locked to thehorizontal syncfrequency of the video carrier for stability. The SAP channel containsmono audio which has beendbx-encoded fornoise reduction, to improve thesignal-to-noise ratio. The SAP audio has abandpass 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 inelectronic 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 duringon-location newscasts as thefoldback channel toreporters andcameramen. 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 analogtelevision receive-onlysatellite 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 thediscrete left and right main audio channels, andATISstation ID, which is heard asMorse code. On FM radio,radio reading services and other audio programs can be heard on subcarriers throughsubsidiary communications authority; however, these are generally prohibited from public listening without special equipment.HD Radio subchannels, somewhat more widely available, operate on a similar premise but generally carry totally unrelated programming to the main feed.
Ondigital 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 separatedigital 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.