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. The centre channel is encoded by placing it equally in the left and right channels; the rear channel is encoded using phase shift techniques, typically an out of phase stereo mixdown. A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered^[1]) mono rear channel—while systems lacking the decoder play back the audio as standard stereo.

A Pro-Logic decoder also uses 'Steering Logic', which drives amplifiers, to raise or lower the output volume of each channel based on the current dominant sound direction. For example, while a mono signal is played, the strong correlation to the center channel triggers the output volume of the left, right and surround channels to be lowered. This increases the channel separation achievable, to around 30 decibels between channels. By careful tuning of the response of the amplifiers, the total amount of signal energy remains constant and is unaffected by the operation of the channel steering. Additionally the response time of the system to changes in sound direction is important as too fast a response results in a twitchy feel, while too slow a response leaves sounds coming from an inappropriate direction. ^[2].

In addition to 5db of noise processing, the surround channel is slightly delayed, so that any front channel sounds that leak into the surround channel arrive at the listener after the front channels, providing an illusion of greater separation.

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.

Dolby Pro Logic II

In 2000, Dolby introduced Dolby Pro Logic II (DPL II), an improved implementation of Dolby Pro Logic created by Jim Fosgate. DPL II processes any high quality stereo signal source into five separate full frequency channels (right front, center, left front, right rear and left rear). 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.

Because of the limited nature of the original DPL, many consumer electronics manufacturers 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. The extra channel content is extracted using the difference between the spatial audio content between two individual channels of stereo tracks or Dolby Digital encoded 5.1 channel tracks and outputs it appropriately. In addition to five full range playback channels, Pro Logic II introduced a Music mode which includes optimized channel delays, and adds user controls to - for example - adjust apparent front sound stage width.

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 such as Dolby Digital, or DTS.

Dolby Pro Logic IIx

A newer Dolby Pro Logic IIx system is also now available, which can take two-channel 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.

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.^[3] The channels it adds are matrixed, not discrete.

Software encoding/decoding

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.

Minnetonka Audio's SurCode for Dolby Pro Logic II is capable of encoding LPCM files into Dolby Pro Logic I & II for surround playback.

Pro Logic vs. Dolby Surround

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.

Dolby Surround at a glance

Decoder	Encoder	Year	Description	Channels
	Dolby Stereo	1975	Cinema use with optical technology. Uses Dolby A for noise reduction. Upmix stereo to Surround 4.0	FL FR with C and MonoSurround 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. Uses Dolby SR for noise reduction.	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	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	non-discrete 6.1 or 7.1 (5.1 with Center Rear matrixed onto SL & SR)	FL FR C SL SR (with matrixed RearMono) SUB [non-discrete 7.1: BackLeft and BackRight]
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 Left Back and Right Back
	Dolby Digital Plus		High bitrate version of Dolby Digital EX. Currently uses 7.1 channels with support for up to 13.	FL FR C SL SR SUB Left Back and Right Back
	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 (Left Back), Rrs (Right Back), LFE, Lvh and Rvh

Dolby encoding matrices

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

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

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	$1$	$0$	$\sqrt{\frac{1}{2}}$	$-j \sqrt \frac{19}{25}$	$-j \sqrt \frac{6}{25}$
Right Total	$0$	$1$	$\sqrt{\frac{1}{2}}$	$j \sqrt \frac{6}{25}$	$j \sqrt \frac{19}{25}$