Dolby Processes
History
Dolby Labs was founded by Ray Dolby in England in 1965. He moved the company
to the United States in 1976. The first product he made was Dolby Type A Noise
Reduction, a simple compander. One of the things that set Dolby's compander
apart was that it treated only the quiet sounds that would be masked by tape
noise. Dolby marketed the product to record companies.
Dolby was persuaded by Henry Kloss of KLH to manufacture a consumer version of his noise reduction. Dolby worked more on companding systems and introduced B-type in 1968.
Dolby did not manufacture consumer products outright; it licensed the technologies to consumer electronics manufacturers.
Dolby also sought to improve film sound, which at the time was optical and monaural. In 1975 Dolby released Dolby Stereo, which included a noise reduction system in addition to more audio channels (Dolby Stereo could actually contain additional center and surround channels). Dolby reworked the system slightly and introduced Dolby Surround and Dolby Pro Logic.
Dolby developed a digital surround sound compression scheme for the cinema. Dolby Digital was first used on the 1992 film Batman Returns. The format was later adopted as part of the DVD specification.
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 phase shift, called Dolby Surround or Lt/Rt. The Pro Logic system split the sound between four channels: left and right, center, and a limited mono rear channel which was split again between two speakers.
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 an upgraded version of Pro Logic, adding the subwoofer and a second rear channel to the encoded information, giving it control over the full 5.1 channels used by surround systems nowadays. It is in particular quite frequently used by the current generation of videogames for ps2, xbox and gc, even though they also have digital outputs.
Dolby also defined Dolby Pro Logic IIx, which extends a 5.1 sound source to 6.1- or 7.1-channel output.
Dolby Pro Logic has been superseded by Dolby Pro Logic II. It
is considered to have comparatively poor channel separation and suffer from
the bandwidth limits of the rear channel. Likewise, Pro Logic II has been superseded
by digital 5.1, 6.1 and 7.1 channel systems (such as Dolby Digital and DTS)
offering full digital channel separation. However, it still has some advantages
over the latter, for example that you don't need digital connections (coaxial
or optical) between your equipment and it is more suitable for certain distribution
and broadcast media.
Dolby Pro Logic vs. Dolby Surround
The difference between Dolby Surround and Dolby Pro Logic is that one describes
the sound source, and the other describes the sound processor (usually an amplifier).
A DVD or CD can never be said to have 'Dolby Pro Logic' sound, and the other
way around an amplifier can also never be said to have 'Dolby Surround'.
Surround sound
Surround Sound is the concept of expanding the spatial imaging of audio playback from 1 dimension (mono/Left-Right) to 2D or 3D.
This is often performed for a more realistic audio environment, actively implemented in cinema sound systems, technical theatre, home entertainment, video arcades, computer gaming, and a growing number of other applications.
Many popular surround sound formats have evolved over the years. They include ambisonics, quadraphonic, Dolby 5.1 Surround Sound, and DTS.
There are methods of using 1-dimensional (a.k.a. stereo) output to emulate a surround environment.
Surround sound is not limited to placement of speakers along a flat (2-dimensional) plane. Vertically-located audio sources can be considered.
Five full-range audio channels:
Three for speakers at the front (stereo left and right, plus centre mono)
Two for surround speakers at the side or rear (stereo left and right)
A sixth, low-frequency effects (LFE) channel carries supporting deep bass sound
effects, ranging from 10 Hz to 120 Hz, which can for example be used by a subwoofer.
5.1 EX includes a third surround channel that can be decoded at the listener's
option for playback over additional surround speakers placed behind the viewers
(rear speakers). The additional center rear information is split between the
left and right back surround channels.
7.1 would use two additional speakers, although no consumer home cinema applications currently exist for it. Some computers are capable of outputting a discrete 7.1 signal.
Note: A distinction is made between the number of discrete channels encoded in the original signal, and the number of channels that are reproduced for playback; these can be added using matrix decoding. A distinction is also made between the number of channels reproduced for playback, and the number of speakers over which these channels are played.
A 5.1 EX signal, for example, contains 5.1 discrete channels of information. Parts of the signal are then passed through a matrix decoder to create a 6.1 channel playback signal. The 6.1 channels are then properly played over 7.1 channels (i.e. 7 speakers and a subwoofer).
This notation, e.g. '5.1', reflects the number of full range, discrete channels;
including a ".1" to reflect the limited range of the LFE channel.
e.g. 5 full-range channels + 1 LFE channel = 5.1
It can also be expressed as the number of full-range channels in front of the listener, separated by a slash from the number of full-range channels beside or behind the listener, separated by a decimal point from the number of limited-range LFE channels.
e.g. 3 front channels + 2 side channels + an LFE channel = 3/2.1
This notation can then be expanded to include the notation of Matrix Decoders. Dolby Digital EX, for example, has a sixth full-range channel incorporated into the two rear channels with a matrix. This would be expressed:
3 front channels + 2 rear channels + 3 channels reproduced in the rear in total + 1 LFE channel = 3/2:3.1
Note: The term stereo, although popularised in reference to two channel audio, can also be properly used to refer to surround sound.
Link to Dolby http://www.dolby.com/
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