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Multichannel television sound, better known as MTS (often still as BTSC, for the Broadcast Television Systems Committee that created it), is the method of encoding three additional channels of audio into an NTSC-format audio carrier.

It was adopted by the FCC as the U.S. standard for stereo television transmission in 1984. Sporadic network transmission of stereo audio began on NBC on July 26, 1984, with the The Tonight Show Starring Johnny Carson, although at the time, only the NBC station in New York City had stereo broadcast capability;^[1] regular stereo transmission of programs began in 1985.

It has also been adopted by

• Canada for NTSC
• Mexico for NTSC
• Brazil for PAL-M (television)
• Argentina for PAL-N
• Taiwan for NTSC

The first channel is the stereo difference (left minus right), used to add stereophonic sound to the existing monophonic (the left plus right stereo sum) audio track.

In other words the normal mono television audio consists of L+R information. A second signal (MTS) rides on top of this mono carrier wave. This MTS signal consists of L minus R.

• When the two audio channels are added together, or summed (L+R plus L-R), the left channel is derived.
• When the second audio channel is subtracted from the first by a phase reversal (L+R minus L-R), the right channel is derived.

• In ideal circumstances MTS Stereo is about 1.5 db better in performance than standard VHF FM Stereo.
• Usually with MTS, as with VHF FM Stereo, a certain amount of crosstalk is encountered, limiting stereo separation.

The stereo information is dbx-encoded to increase the signal-to-noise ratio (at low levels), to aid in noise reduction. Because of the use of dbx companding, every TV device that uses analog circuitry to decode MTS costs somewhat more because a royalty must be paid to dbx, Inc., until its patent runs out. These patents do not apply to some digital circuits, however.

The second audio program (SAP) also is part of the standard, providing another language, a video description service like DVS, or a completely separate service like a campus radio station or weatheradio.

A third PRO (professional) channel is provided for internal use by the station, and may handle audio or data. The PRO channel is normally used with electronic news gathering during news broadcasts to talk to the remote location (such as a reporter on-location), which can then talk back through the remote link to the TV station.

• Specialized Receivers for the PRO channel are generally only sold to broadcast professionals.
• NICAM has a similar pro-channel feature that has only been implemented in the UK (http://tallyho.bc.nu/~steve/nicam.html#SPARE).

MTS signals are indicated to the television receiver by adding a 15.734 kHz pilot tone to the signal.

• NICAM
• Zweikanalton