Dolby Pro Logic

Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. The terms Dolby Stereo and LtRt (Left Total, Right Total) are also used to describe soundtracks that are encoded using this technique. Dolby Stereo—also known as Dolby MP (Motion Picture) or Dolby SVA (stereo variable-area)—was developed by Dolby in 1976 for analog cinema sound systems. The format was adapted for home use in 1982 as Dolby Surround when HiFi capable consumer VCRs were introduced. It was further improved with the Dolby Pro Logic decoding system after 1987. The Dolby MP Matrix was the professional system that encoded four channels of film sound into two. This track used by the Dolby Stereo theater system on a 35mm optical stereo print and decoded back to the original 4.0 Surround. The same four-channel encoded stereo track was largely left unchanged and made available to consumers as "Dolby Surround" on home video. However, the original Dolby Surround decoders in 1982 were a simple passive matrix three-channel decoder: L/R and mono Surround. The surround channel was limited to 7 kHz. It also had Dolby Noise Reduction and an adjustable delay, for improved channel separation and to prevent dialog leaking and arriving to listeners' ears first. The front center channel was equally split between the left and right channels for phantom center reproduction. This differed from the Cinema Dolby Stereo system which used active steering and other processing to decode a center channel for dialog and center focused on-screen action. Later on in 1987, the Pro Logic decoding system was released to consumers. It featured virtually the same type of four-channel decoding as the Dolby Stereo theater processor with active steering logic and much better channel separation (up to 30 dB) as well as including...