ABSTRACT
Background Mono vinyls utilized lateral (horizontal) groove excursions to encode amplitude variations. When stereo records were introduced in 1958 there needed to be a way to encode both the left and right channels. One proposal was to encode the left channel as lateral excursions and the right channel as vertical ones. The problem was that, had a stereo record been played on a mono player, only the left channel would have been heard (due to the infancy of pan pots back then, this could have meant losing the drums, vocals or bass). As part of his stereo invention, Alan Blumlein taught us that any left and right information can be converted into middle and side and vice versa. Based on this, EMI cut the first stereo record encoding the middle as lateral groove excursions and the side as vertical. This way, even if a stereo record was played on a mono player, the player would only decode the lateral motion, which represented the mid (the mono sum of left and right). A stereo player would decode both lateral and vertical motions (mid and side) and would convert these into left and right. This mid-side system is used in vinyl pressing to this day. Stereo transmissions of FM radio and television are also encoded in MS.
