Alternate language audio, better known as secondary audio programming or "SAP," was created as a method to add a low bandwidth, low quality audio channel alongside the main audio carrier of an analog NTSC television signal. The provision for an SAP carrier was added at the same time the provision for the carrier for BTSC stereo was added. Unfortunately the performance of the SAP carrier is severely limited in quality. With a bandwidth barely exceeding 7.5 kHz and noise performance that is not so great, the SAP channel does not compare very favorably to the performance of the main carrier.
With the advent of digital television, the plight of alternate language programming has taken a major turn for the better. Thanks to ATSC specifications, multiple audio programs can be carried simultaneously. This includes audio for alternate languages as well as descriptive audio (DVS) for visually impaired viewers. As all ATSC audio programs are Dolby Digital (AC-3) encoded, they can be mono, stereo, or even 5.1 channels. This means that a television network can, in theory, broadcast a video program with 5.1 channel English, 5.1 channel Spanish, and stereo or even 5.1 channel DVS audio versions. The only tough part of this scenario is how to get this many channels through the entire signal chain and to the audio encoders for delivery to consumers.
