Editing Dolby Pro Logic (section)

{{short description|Surround sound processing technology developed by Dolby Labs}}
{{Redirect-distinguish|Pro Logic|Logic Pro}}
{{infobox brand
|name           = Dolby Surround/Dolby Pro Logic/Dolby Pro Logic II
|logo           = 
|logo_size      = 120px
|image          = 
|type           = Surround sound
|currentowner   = [[Dolby Laboratories]]
|origin         = United States
|introduced     = 1982
|markets        = Worldwide
|website        = [https://www.dolby.com/ Dolby official website]
|related        = [[Dolby Pro Logic#Dolby Surround|Dolby Surround]]<br>[[Dolby Pro Logic#Dolby Pro Logic|Dolby Pro Logic]]<br>[[Dolby Pro Logic#Dolby Pro Logic II|Dolby Pro Logic II]]<br>[[Dolby Pro Logic#Dolby Pro Logic IIx|Dolby Pro Logic IIx]]<br>[[Dolby Pro Logic#Dolby Pro Logic IIz|Dolby Pro Logic IIz]]<br>[[Dolby Pro Logic#Dolby Surround (2014)|Dolby Surround]]<br>[[Dolby Digital]]<br>[[Dolby Stereo]]<br>[[LtRt]]
}}
'''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.<ref name=":0" /> 

'''Dolby Stereo'''—also known as '''Dolby MP''' (''Motion Picture'') or '''Dolby SVA''' (''stereo variable-area'')—was developed by Dolby in 1976 for analog [[movie theater|cinema]] sound systems. The format was adapted for home use in 1982 as '''Dolby Surround''' when [[VCR#Stereo Sound and HiFi|HiFi]] capable consumer [[VCR]]s 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 channels|Surround]]. The surround channel was limited to 7&nbsp;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&nbsp;dB) 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 that 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 [[stereophonic sound|stereo]] (two-channel) soundtrack. The center channel is reduced in level by 3&nbsp;dB and summed to the left and right channels; the surround channel is attenuated by 3&nbsp;dB, passes through a [[band-pass filter]] (cutting frequencies under 100&nbsp;Hz and above 7&nbsp;kHz), passes through [[Dolby noise-reduction system#Dolby B|Dolby B]] noise reduction and is encoded on the left and right channels with opposite polarity (this is achieved by applying a +90-degree [[phase shift]] to the left channel and a −90-degree phase shift to the right channel).<ref>{{Cite web |last=Robjohns |first=Hugh |date=September 2001 |title=Surround Sound Explained: Part 2 |url=https://www.soundonsound.com/techniques/surround-sound-explained-part-2 |access-date=2024-01-06 |website=[[Sound on Sound]]}}</ref> 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 channel|center]], and a single limited frequency-range (7&nbsp;kHz [[low-pass filter]]ed)<ref name="dplii">{{cite web |last=Dressler |first=Roger |title=Dolby Surround Pro Logic Decoder – Principles of Operation |url=http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/208_Dolby_Surround_Pro_Logic_Decoder.pdf |url-status=dead |archive-url=https://web.archive.org/web/20140326110501/http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/208_Dolby_Surround_Pro_Logic_Decoder.pdf |archive-date=March 26, 2014 |access-date=February 8, 2019 |work=[[Dolby Laboratories]]}}</ref> mono rear channel&mdash;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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