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.The system was developed by an industry group known as the Broadcast Television Systems Committee, a parallel to color television's National Television System Committee, which developed the NTSC television standard.
MTS works by adding additional audio signals in otherwise empty portions of the television signal, and allows up to a total of four audio channels, with two producing the left and right stereo channels. An additional second audio program (SAP) is used to broadcast other languages or television radio services, including weather radio that could be accessed by the user, typically through a button on their remote control. The fourth channel was a professional audio channel used for internal purposes by broadcasters and is indecipherable with a common consumer receiver.