Dolby's Technologies

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Dolby A-type, introduced in 1965 was designed for use by professional recording studios to make quiet master tape recordings. In the early to mid 1970's its use was extended to film recording studios and motion picture release prints in order to make films sound better.

Dolby B-type noise reduction, providing about 10 dB of noise reduction at high frequencies, was a simplification of A-type. It extended the use of Dolby technology into the consumer environment, giving consumer electronic companies the ability to make cassette tapes and players which gave the consumer quiet recordings.

Dolby C-type noise reduction, introduced in 1981, was Dolby's second generation consumer system -- basically doubling the amount of noise reduction that B-type provided, and adding other technological features as well (spectral skewing, anti-saturation, etc.).

Dolby SR (spectral recording) was introduced in 1986 as Dolby's second generation professional recording system. Not only was it designed to provide more noise reduction but it also provides a number of other technological innovations that extend the recording's dynamic range and gives the user a master recording that is indistinguishable from the live sound. Thus SR is referred to as a signal processing system rather than just a noise reduction system.

Dolby S-type was derived from Dolby SR, and shares with it such developments as combining both fixed and sliding bands, anti-saturation, spectral skewing and modulation control. Dolby S-type gives the home consumer the ability to make cassette tapes that sound like CDs.

Dolby HX Pro, introduced in the early 1980s, provides high frequency headroom extension for improved tape recording by dynamically adjusting recording bias level.

Dolby Stereo. After introducing the use of A-type noise reduction to the film industry, Dolby's next major contribution was Dolby Stereo. This contribution allowed movie makers to put 4 channels of sound information (left, right, center, surround) on motion picture release prints using matrix technology, and gave theaters the ability to replay this 4-channel format for the movie going public.

Dolby Surround is the home embodiment of Dolby Stereo.

Dolby Pro Logic is Dolby's second generation licensed home surround system. A major advantage of Dolby Pro Logic is the use of an active center channel with its own speaker.

AC-1 was Dolby's first digital audio coding scheme. First adopted by systems providers in 1984 when bit rate reduction was in its infancy, AC-1 is a refined form of adaptive delta modulation (ADM), whereby changes in the signal amplitude from moment to moment are transmitted, rather than absolute values.

AC-2 is a perceptually based adaptive transform coding algorithm that combines very high audio quality with a low bit rate, thus substantially reducing the data capacity required in such applications as satellite and terrestrial links and digital audio storage media.

Dolby Digital (AC-3) is an advanced perceptual coding technology for transmission and storage of up to five full-range channels, plus a supplemental bass-only effects channel (referred to as a .1 channel due to the smaller number of bits needed for the information), in less space than is required for one linear PCM coded channel on a compact disc.

Dolby E is an audio coding technology that allows a single AES/EBU audio pair, or a single pair of digital VTR audio tracks, to carry up to eight channels of broadcast-quality audio for post-production and distribution purposes.

Further in 70mm reading:

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Processors
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In 70mm Dolby

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Dolby Laboratories