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9.7 Surround Encoding Technology
Some feature films have carried the multi-channel sound
tracks necessary for “surround sound” since the 1950s. But
only since the mid 80’s has surround sound been available to
domestic consumers. The technology that first enabled four
channels of audio to be decoded from the stereo soundtrack of
consumer media such as video cassette is Dolby Surround Pro
Logic. Since Dolby Surround Pro Logic, and especially following
the introduction of digital products such as DVD, enhanced
encode and decode technologies have been introduced that
enable a greater number of higher quality channels of audio to
be encoded. With digital encoding techniques such as Dolby
Digital and DTS Surround, appropriately encoded feature films,
music and even computer games can be reproduced in full-
bandwidth surround sound with up to eight audio channels (left,
right, centre, surround left, surround right, surround extra, low-
frequency effects).
The following few paragraphs provide a short description
and explanation of encoding technologies. Further
technical information can be found at www.dolby.com and
www.dtsonline.com.
9.7.1 Dolby Surround Pro Logic
Dolby Surround Pro Logic is a matrix decoding process that
generates four output signals (left, right, centre, surround)
from a Dolby Surround encoded stereo input signal. It is built
into virtually every home theatre audio system. The nature of
Pro Logic decoding constrains the single surround channel to
relatively narrow bandwidth.
9.7.2 Dolby Surround Pro Logic II
Dolby Surround Pro Logic II is an improved analogue matrix
technology that provides improved surround performance
on Dolby Surround encoded program material. While earlier
surround programme material is fully compatible with Pro Logic
II, appropriately encoded soundtracks can take full advantage
of its enhancements - which include full bandwidth left and
right surround channels. Pro Logic II also features two distinct
decoding options for “music” and “movie” programme material.
9.7.3 Dolby Digital
Dolby Digital is a fully digital decoding technology that provides
three full bandwidth front channels, two full bandwidth surround
channels, and one low-frequency effects channel - a channel
scheme known generically as “3/2.1” (or “5.1”). The encoding
technique for Dolby Digital, known as Dolby AC-3, has since
1995 been used on many Video Laser Discs and more recently
on DVD. Dolby AC-3 encoding can also be found on digital
television services. In addition to encoding audio for six channel
replay, Dolby AC-3 incorporates compression techniques that
ease audio data storage and transmission demands and can
enable, for example, a single DVD to carry a complete movie.
9.7.4 Dolby Digital 2/0
A variation of Dolby Digital 3/2.1 is Dolby Digital 2/0. Dolby
Digital 2/0 takes advantage of Dolby AC-3 data compression
in order to reduce the data storage demands of stereo
programme material. The “2/0” denotes the use of just two
main audio channels with no low frequency effects channel.
Programme material encoded for a “2.1” (two main channels
with one subwoofer channel) scheme is also available and can
be handled within Dolby 2/0 decoding.
9.7.5 DTS Surround
DTS Surround is an alternative digital audio encoding format
that has become popular with feature film producers and can
therefore be found on many DVDs. The first feature film to
be DTS encoded was Jurassic Park in 1993. DTS Surround
provides a similar 3/2.1 channel scheme to Dolby Digital with
the encoding technology also providing data compression. The
DTS data compression ratio is lower than that in AC-3 however
and it is argued that DTS can provide better audio quality. The
downside of any quality improvement over AC-3 is however
higher data storage requirements.
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