The BTSC Format
This system, selected by the EIA's Broadcast Television Committee, was adopted by the FCC as the standard for stereo television transmission in 1984. Sometimes known as MTS for Multichannel Television Sound, it is similar in format to FM stereo but has the ability to carry 2 additional audio channels. This is the Secondary Audio Program channel (SAP), sometimes used to carry bilingual audio and a low fidelity audio channel for engineering purposes.
Left plus right mono information is transmitted the same way as in stereo FM, maintaining compatibility with monaural, analog television receivers. A 15.734 kHz pilot is used, instead of 19 kHz, which allows it to be phase-locked to the horizontal line frequency. Also like FM stereo, a double sideband-supressed carrier at twice the frequency of the pilot transmits the left minus right stereo information. The stereo information is dbx encoded to aid in noise reduction. The SAP channel is located at 5 times the pilot frequency. This FM subcarrier carries mono; dbx encoded audio used for a second language or independent program source. A professional audio channel may be added at 6.5 times the pilot frequency. This is a low fidelity FM subcarrier used for the transmission of voice or data.
AND my question is
there is eny exist mts modulators with 4 ch inputs
thanks