It is not as important today but at one time receiving stereo audio over the (analog) TV airwaves was a big deal. When Sony started home video recording color television was still fairly new and receiving stereo was just in the development stages. The exact system for stereo reception had yet to be decided. This is why some early Betas have a MPX jack in the back. This was put there so that when stereo did come along a separate stereo decoder could be added later. It would allow the VCR to receive stereo and record in Betahi-fi, depending on the model. Later all stereo Betas incorporated the new decoding system and were branded as having Stereocast reception capability (MTS* analog). A switch allowed you select from the following sound options: Mono: Select for channels broadcasting in mono or if there is difficulty receiving a quality stereo audio signal. Stereo: Normally selected for TV channels that are broadcasting in stereo. SAP (Secondary Audio Program): Select this position to receive or record a Secondary Audio Program. This could be a foreign language translation or added commentary pertaining to the recording. *MTS means Mutli-Channel Television Sound. ©Misterßetamax go to homepage