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Dolby Digital is a series of audio encoding and decoding technologies developed by Dolby Laboratories. It is used in cinemas, home entertainment systems, PCs, automobiles and some computer games. This technology includes variants such as the Dolby TrueHD, the Dolby Digital Plus, the Dolby Digital EX, the Dolby Digital Surround EX and the Dolby Digital Live.
Dolby Digital technology uses six channels of sound to produce high quality audio output. It can transmit mono, stereo or 5.1-channel surround sound. The common version contains five channels for normal range speakers (right front, centre, left front, right rear and left rear) and one channel for the low frequency effects driven by the subwoofer. Due to its coding efficiency and its ability to deliver high quality, discrete multi-channel audio without compromising video quality, Dolby Digital has become the designated audio standard for DVDs worldwide.
In a 5.1-channel system, sound information contained in each of the six available channels is distinct and independent. There are five full-bandwidth channels with a 3Hz to 20KHz frequency range for front left and right, centre, and surround, plus one "Low Frequency Effects" (LFE) subwoofer channel devoted to frequencies from 3Hz to 120Hz. The “.1” in 5.1 refers to the LFE channel.
Other variations have been developed for different requirements and developments in new generations of cinema and video technology. For example Dolby TrueHD is an advanced lossless audio codec and supports 24-bit, 96KHz audio channels at up to 18 Mbits over 14 channels. However, High Definition (HD) DVD and Blu-ray Disc standards currently limit the maximum number of audio channels to eight. True HD also supports extensive metadata, including dialogue normalization and Dynamic Range Control.
Dolby Digital decoders have the capability of distributing encoded channels to available speakers. For example, surround information is sent through the front speakers if surround speakers are unavailable or distributed from the centre channel to the left and right speakers if the centre speaker is not available.
Similarly, Dolby Digital Surround Ex encoding allows the centre channel information contained in the soundtrack to be reproduced by speakers that are placed directly behind the audience. This enhances the cinematic experience, creating a fuller, more realistic sound for increased dramatic effect in the theatre. If not equipped with the extra Surround EX speakers in the rear, the information intended for it will be played through the traditional left and right surround channels.
Dolby Digital is today the preferred multi-channel audio standard for broadcast in direct-to-home satellite and digital cable systems. In the motion picture industry, Dolby Digital soundtracks are optically encoded on the filmstrip allowing it to coexist with the analogue track. This is a cost effective and simple solution for film distributors and theatre owners.
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