Dolby Pro Logic

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"Pro Logic" redirects here. It is not to be confused with Logic Pro.
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 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 and 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.

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[edit]

Dolby Pro Logic II logo

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.

Older logo, before Dolby updated their overall logo design.

Pro Logic II system also features a mode designed specifically for video gaming, and was 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[edit]

Dolby Pro Logic IIx logo

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 IIz[edit]

Dolby Pro Logic IIz logo.

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 in the side and rear surround channels, and directs it to the front height speakers.[3] The channels it adds are matrixed, not discrete.

Software encoding/decoding[edit]

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

HandBrake and FFmpeg are capable of downmixing Dolby Digital AC-3 5.1 to Stereo for Dolby Pro Logic I & II for surround playback.

Pro Logic vs. Dolby Surround[edit]

Dolby Surround logo

Dolby Stereo 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[edit]

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 Dolby Media Encoder 2005 Lossy compression codec; 48 kHz sampling frequency, 20-bit word length; supports data rates of 32 kbit/s – 6 Mbit/s, scalable, including 768 kbit/s – 1.5 Mbit/s on high-definition optical discs, typically, and 256 kbit/s for broadcast and online. 1.0- to 7.1-channel support for current media applications; extensible to 16 channels; discrete. Backward compatibile with Dolby Digital through S/PDIF connection up to 640 kbit/s. Supports Dolby Metadata. FL FR C SL SR SUB Left Back and Right Back
Dolby TrueHD Dolby Media Encoder 2005 Lossless compression codec; supports 44.1 kHz to 192 kHz sampling frequency up to 24-bit word length; supports variable data rate up to 18 Mbit/s; maximum channel support is 16 channels as presently deployed. Higher bitrate than Dolby Digital Plus. Blu-ray Disc channel support up to eight channels of 96 kHz/24-bit audio; six channels (5.1) up to 192 kHz/24-bit; and two- to six-channel support up to 192 kHz/24-bit maximum bit rate up to the maximum of 18 Mbit/s.
Dolby Pro Logic IIz Dolby Laboratories 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[edit]

Note that j represents a 90° (π2 radians) phase shift function (theoretically a Hilbert transform).

Dolby Surround Left Right Surround
Left Total
Right Total
Dolby Pro Logic Left Right Center Rear
Left Total
Right Total
Dolby Pro Logic II Left Right Center Rear Left Rear Right
Left Total
Right Total

See also[edit]


  1. ^ "Dolby Surround Pro Logic II Decoder Principles of Operation" (PDF). Dolby Laboratories. Retrieved 2009-12-04. 
  2. ^ "Dolby Surround Pro Logic Decoder Principles of Operations" (PDF). Dolby Laboratories. 
  3. ^ "Dolby Pro Logic IIz". Dolby Laboratories. Retrieved 2009-01-22. 

External links[edit]