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Dolby® Pro Logic® II is an advanced matrixing technology that delivers convincing, realistic surround sound from stereo content.

Provides compelling five-channel surround sound from any stereo program source, including TV broadcasts, movies, video games, and music

Delivers natural, realistic surround derived from spatial cues naturally occurring in stereo content; no artificial added effects

Complements Dolby Digital 5.1 decoding to ensure a surround experience on all content

Dolby Pro Logic II was developed to allow a surround listening experience from stereo content. It delivers five full-range channels (including two surround channels) derived from spatial cues that occur naturally in stereo recordings, soundtracks, and broadcasts. The result is a compelling yet entirely natural and realistic surround effect that is incorporated in virtually all home theater products, including audio/video receivers (AVRs) and home-theater-in-a-box (HTIB) systems.

Dolby Pro Logic II technology is also used by broadcasters to encode their stereo signals specifically for Pro Logic II playback. This makes it possible for them to deliver specific surround and localization effects over stereo services that are more like discrete 5.1 transmissions, without compromising regular stereo and mono playback.

Later variations of Pro Logic II derive even more (up to 9.1) channels from stereo content. Pro Logic IIx adds back surround channels to the standard 5.1-channel configuration for 7.1 surround, while Pro Logic IIz adds two front height channels that can be used in addition to or instead of the back surrounds in a regular 7.1 setup.

In a 9.1-capable receiver or processor, Dolby Pro Logic IIz can expand channel lineup to provide a 9.1 listening environment that includes back surround and front height speakers.

Why Dolby

When DVDs with Dolby Digital 5.1 soundtracks were first introduced, consumers experienced the thrill of discrete multichannel surround in their homes for the first time. The conventional stereo sound from other sources paled by comparison, leading to the development of Dolby Pro Logic II to help close the gap. While consumers now enjoy discrete Dolby Digital 5.1 audio from more sources (including HDTV), stereo content remains common, sustaining the need for the enhancement provided by Dolby Pro Logic II.

All versions of Dolby Pro Logic II are available under license to consumer electronics manufacturers to include in home theater products such as AVRs, HTIBs, sound bar products, and television sets. The technology is available in a wide choice of DSP and IC solutions that also typically implement a variety of surround options including Dolby Digital 5.1.

Dolby Pro Logic II is a major step forward from first-generation surround decoding. In earlier decoding systems, a control circuit looks at the relative level and phase between the input signals. This information is sent to the variable output matrix stage to adjust VCAs controlling the level of antiphase signals. The antiphase signals cancel the unwanted crosstalk signals, resulting in improved channel separation. This is called a feed-forward design.

Dolby Pro Logic II looks at the same input signals and employs a servo loop to match their levels. These matched audio signals are sent directly to the matrix stages to derive the various output channels. Because the same audio signals that feed the output matrix are themselves used to control the servo loop, it is called a feedback logic design. Incorporating global feedback around the logic steering process improves steering accuracy and dynamic behavior.

Major advantages of this advanced design include independent rear surround channels with full-range frequency response. First-generation systems provide only a monaural surround channel, with frequency response limited to avoid noise and crosstalk.

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