Dolby Pro Logic

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Dolby Pro Logic logo

Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Surround Stereo was originally 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 then replaced by the newer and improved Pro Logic system in 1987. However, the term "Dolby Surround" is still used to describe the encoding technology or matrix-encoded soundtrack, whereas Pro Logic refers to the decoding technology/processor. It is the domestic equivalent of the theatrical Dolby Stereo technology used in movie cinemas in the 1970s and '80s. The two technologies are mostly identical but a change in marketing was needed so as not to confuse cinema stereo which is at least four channels of audio with home stereo which is only two. Thus Dolby Pro-Logic is the consumer version of theatrical Dolby stereo.

[edit] Dolby Pro Logic

Dolby Surround/Pro Logic is based on basic matrix technology. When a Dolby Surround soundtrack is created, four channels of sound are matrix-encoded into an ordinary stereo (two channel) sound track by using phase shift techniques. A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range 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.

[edit] Dolby Pro Logic II

Dolby Pro Logic II logo

In 2000, Dolby introduced Dolby Pro Logic II (DPL II), an improved implementation of Dolby Pro Logic. DPL II processes any high quality stereo signal source into five separate full frequency channels (left, center, right, left surround and right surround). Dolby Pro Logic II also decodes 5 channels from stereo signals encoded in traditional four-channel Dolby Surround. DPL II implements greatly enhanced steering compared to DPL, and as a result, offers an exceptionally stable sound field that simulates 5 channel surround sound to a much more accurate degree than the original Pro Logic.

Because of the limited nature of the original DPL, many consumer electronics manufactures introduced their own processing circuitry, such as the "Jazz", "Hall", and "Stadium" modes found on most common home audio receivers. DPL II forgoes this type of processing and replaces it with simple servo (negative feedback) circuits used to derive five channels. In addition to five full range playback channels, Pro Logic II introduced a Music mode which would not add any processing to the left and right channels, but will still extract a center channel and two surround channels, providing a net effect of a wider center channel.

Pro Logic II system also features a mode designed specifically for video gaming, and it is frequently used in game titles for Sony's PlayStation 2, Nintendo's GameCube and Wii as an alternative to digital surround formats like Dolby Digital, or DTS.

[edit] Dolby Pro Logic IIx

Dolby Pro Logic IIx logo

A newer Dolby Pro Logic IIx system is also now available, which can take stereo, Dolby Surround (sometimes called Dolby Stereo Surround) and Dolby Digital 5.1 source material and up-convert it to 6.1, or 7.1 channel surround sound. Dolby Pro Logic IIx also takes signals intended for Dolby Pro Logic II, and up-converts them to a 6.1 or 7.1 channel surround sound.

[edit] Dolby Pro Logic IIz

Dolby Pro Logic IIz expands on Pro Logic IIx with the addition of a height component, creating front height channels above the front left and right speakers, expanding a 5.1 or 7.1 system to 7.1 Height or 9.1. It identifies spatial cues in low-level, uncorrelated information, such as ambience and effects like rain or wind, and directs it to the front height speakers.^[1]

[edit] Software encoding

The liba52 decoder library for AC3 and A52 digital sound optionally exports stereo sound compatible with Dolby Surround and Pro Logic.

Handbrake is capable of downmixing Dolby Digital AC-3 5.1 to Stereo for Dolby Pro Logic I & II for surround playback.

[edit] Pro Logic vs. Dolby Surround

Dolby Surround logo

Dolby Surround is the encoding counterpart to Dolby Pro Logic's decoding technology, but early home implementations of Dolby Surround decoding went by the name Dolby Surround which can cause some confusion. Dolby Surround and Dolby Pro Logic decoders are similar in principle, as both use matrix technology to extract extra channels from stereo-encoded audio. However, Dolby Surround only decodes the rear surround channel, while Dolby Pro Logic also decodes the center channel. Pro Logic uses advanced algorithms, superior to the earlier home Dolby Surround system and similar to the original cinema Dolby Stereo processors, in order to not only extract the extra channels, but to also improve steering and discreteness between the channels.

[edit] Dolby Surround at a glance

Decoder	Encoder	Year	Description	Channels
	Dolby Stereo	1975	Cinema use.	FL FR with C and RearMonoSurround matrixed
Dolby Surround	Dolby Surround	1982	First Home use. Analog. Upmix stereo to Surround 3.0	FL FR and MonoSurround matrixed
	Dolby Stereo SR	1986	Cinema use.	FL FR with C and MonoSurround matrixed
Dolby Pro Logic		1987	Improved Dolby Surround. Upmix Stereo to Surround 4.0.	FL FR with C and MonoSurround matrixed
Dolby Digital	Adaptive Transform Coder 3 (AC3)	1992 Film 1995 Laser Disc	Discrete channel encoder/decoder. Pro Logic Decoder can be used for downmixed stereo inputs.	FL FR C SL SR SUB
Dolby Digital EX/Dolby Digital Surround EX		1999	6.1 or 7.1 (5.1 with Center Rear matrixed onto SL & SR)	FL FR C SL SR SUB RearMono
Dolby Pro Logic II		2000	Improved Dolby Pro Logic. Upmix Stereo to Surround 5.1 in either Movie, Music, or Game mode.	FL FR C SL SR SUB
Dolby Pro Logic IIx		2002	Upmix Stereo or Surround 5.1 to 6.1 or 7.1 in either Movie, Music, or Game mode.	FL FR C SL SR SUB RearLeft and RearRight
	Dolby Digital Plus		High bitrate version of Dolby Digital EX. Currently uses 7.1 channels with support for more.
	Dolby TrueHD		Lossless encoder for High Definition Video Sound. Eight full-range channels of 24-bit/96 kHz. Higher Bitrate than DD-Plus.
Dolby Pro Logic IIz		2009	Upmix Stereo or Surround 5.1/7.1 to 7.1 Height or 9.1 with the addition of front height channels. (Based on Dolby Pro Logic IIx.)	L, C, R, Ls, Rs, Lrs, Rrs, LFE, Lvh and Rvh

[edit] Dolby encoding matrices

Note that j represents a 90° (^π⁄₂ radians) phase shift.

Dolby Surround	Left	Right	Center	Surround
Left Total	$1$	$0$	$\sqrt{\frac{1}{2}}$	$j \sqrt{\frac{1}{2}}$
Right Total	0	1	$\sqrt{\frac{1}{2}}$	$-j \sqrt{\frac{1}{2}}$

Dolby Pro Logic	Left	Right	Center	Rear
Left Total	$1$	$0$	$\sqrt{\frac{1}{2}}$	$j \sqrt{\frac{1}{2}}$
Right Total	$0$	$1$	$\sqrt{\frac{1}{2}}$	$-j \sqrt{\frac{1}{2}}$

Dolby Pro Logic II	Left	Right	Center	Rear Left	Rear Right
Left Total	$1$	$0$	$\sqrt{\frac{1}{2}}$	$j \sqrt \frac{2}{3}$	$j \sqrt \frac{1}{3}$
Right Total	$0$	$1$	$\sqrt{\frac{1}{2}}$	$-j \sqrt \frac{1}{3}$	$-j \sqrt \frac{2}{3}$