Multichannel Television Sound

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 (BTSC), a parallel to color television's National Television System Committee, which developed the NTSC television standard. It was approved in 1984 and was widely deployed in the next two years.

Source: Wikipedia — Multichannel Television Sound (CC BY-SA 4.0)