Multichannel Television Sound (MTS) is the method of encoding three additional audio channels into analog 4.5 MHz audio carriers on System M and System N. It was developed by the Broadcast Television Systems Committee, an industry group, and sometimes known as BTSC as a result.
MTS worked by adding additional audio signals in otherwise empty portions of the television signal. MTS allowed up to a total of four audio channels. Normally two were broadcast to produce the left and right stereo channels. An additional second audio program (SAP), could be used to broadcast other languages or entirely different audio like weather alerts that could be accessed by the user, typically through a button on their remote control. The fourth channel, PRO, was only used by the broadcasters.