Second audio programSecond[ary] audio program[ming] (SAP)
is an auxiliary sound 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, 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.
SAP (and other MTS features such as stereo and back-to-studio) travels on a subcarrier of the video signal itself rather than its radio carrier, so it stays with the video even onto videotape.