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Dolby Pro Logic is an analog surround sound system developed by Dolby Laboratories, Inc. in 1986 for domestic use, based on their 1976 cinema system, Dolby Stereo. It decodes surround information from stereo sound, if present.
Suitable source programs encode four channels of sound on the ordinary stereo sound track by using phase shift techniques, and calling them Dolby Surround or Lt/Rt. The Pro Logic system splits the sound between four channels: left and right, center, and a limited-range mono rear channel which is split again between two speakers to enhance ambience.
In 2000, Dolby introduced Dolby Pro Logic II, which has a 'Music' and a 'Movie' profile (latter also suitable for games). Pro Logic II is basically a redesigned version of Pro Logic, adding the low frequency effects (.1) channel and stereo surround channels to the encoded information, giving it full-range control (as opposed to Pro Logic's limited range) over the 5.1 channels used by most home theatre systems nowadays. It is in particular quite frequently used by the current generation of videogames for Sony PlayStation 2, Microsoft Xbox and Nintendo GameCube, as their digital ouputs (Which can output dts interactive, Dolby Digital or PCM Stereo) introduce a slight delay required for digital encoding.
Dolby Technologies have also created Dolby Pro Logic IIx, which extends a stereo sound source to a 6.1- or 7.1-channel output.
Dolby Pro Logic has been superseded by Dolby Pro Logic II. The former is considered to have comparatively poor channel separation and suffer from the bandwidth limits of the rear channel. Pro Logic II has been superseded by digital 5.1, 6.1 and 7.1 channel systems such as Dolby Digital (5.1), Dolby Digital EX (6.1 Matrix), DTS (5.1) and (6.1 Discrete) offering a much richer audio experience. However, Pro Logic II still has some advantages over the latter, particularly in providing 5.1 channels from two-channel stereo sources--broadcast media, for example--without requiring digital coax or optical connections.
Dolby Surround vs. Pro Logic
The difference between Dolby Surround and Dolby Pro Logic is that one is a rather straight phase and frequency-based method of extracting a surround channel, whilst leaving the left and right channels intact, whilst Pro Logic is named for using advanced algorithms in order to not only extract a center channel, but also improve steering and discreteness between the front channels.
Dolby Decoding Matrices
| Dolby Surround | Left | Right | Surround |
| LeftTotal | 1.000 | 0.000 | -1.000 |
| RightTotal | 0.000 | 1.000 | 1.000 |
| Dolby Pro Logic | Left | Right | Center | Surround |
| LeftTotal | 1.000 | 0.000 | 0.707 | -1.000 |
| RightTotal | 0.000 | 1.000 | 0.707 | 1.000 |
| Dolby Pro Logic II | Left | Right | Center | Rear Left | Rear Right |
| LeftTotal | 1.000 | 0.000 | 0.707 | -0.866 | -0.500 |
| RightTotal | 0.000 | 1.000 | 0.707 | 0.500 | 0.866 |
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