Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo 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 Pro-Logic decoding system in 1987.
The Dolby MP Matrix was the professional system that encoded 4 channels of film sound into 2. 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 4 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 3 channel decoder : L/R and Mono Surround. The surround was limited to 7kHz. 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 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 4 channel decoding as the Dolby Stereo theater processor with active steering logic and much better channel separation (up to 30dB) as well as including a dedicated center channel output for the first time. Many standalone Pro Logic decoders also included a phantom center option for compatibility with earlier non-Pro Logic Dolby Surround equipped home theaters to split the center channel signal to the L/R speakers for legacy phantom center reproduction.
Dolby Surround Pro Logic is the full name which refers to the matrix surround format and decoding system in one. When a Dolby Surround soundtrack is created in post production [Dolby MP Matrix], four channels of sound are matrix-encoded into an ordinary stereo (two channel) sound track. The center channel is encoded by placing it equally in the left and right channels minus 3dB; and the surround channel is encoded using phase shift techniques for out of phase information (L-R). The surround channel was often used for ambient background sounds in the original recording, music scores and effects.
A Dolby Pro Logic decoder/processor "unfolds" the soundtrack back into its original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered) mono rear channel—while systems lacking the decoder play back the audio as standard stereo.
Although Dolby Surround was introduced as an analog format, all Dolby Digital decoders incorporate a digitally implemented Dolby Surround Pro Logic decoder for digital stereo signals that carry matrix-encoded Dolby Surround. One of the first was the MSP400 surround sound receiver and amplifier by RCA for their high-end Dimensia brand. It was released in 1987 for the Digital Command Component System.
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