杜比數碼

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Dolby Digital ，中文為杜比數碼，是美國公司杜比實驗室公司開發及持有的有損壓縮音樂格式的商標。

[编辑] 版本

杜比數碼包括數個類似的技術.

[编辑] Dolby Digital

Dolby Digital logo that is sometimes shown at the beginning of broadcasts, feature films, and games

Dolby Digital, or AC-3, 是分離六聲道最普遍的版本, 最知名的是5.1技術。5.1包括5個聲道，傳送至一般音域的喇叭 (20 Hz – 20,000 Hz) 。分別是右前(L)，中(C)，左前(R)，右環繞(RS)，左環繞(LS)以及1個低頻聲道 (20 Hz – 120 Hz) low frequency effects傳送至 重低音音箱。與此同時，杜比數碼格式支援單聲道及立體聲輸出。Batman Returns was the first film to use the Dolby Digital technology when it premiered in theaters in Summer 1992. The LaserDisc version of Clear and Present Danger featured the very first Home Theater Dolby Digital mix in 1995.

This codec has several aliases, which are different names for the same codec:

• Dolby Digital (promotion name, not accepted by the ATSC)
• DD (an abbreviation of above, often combined with channel count: DD 5.1)
• Dolby Surround AC-3 Digital (second promotional name, as seen on early film releases, and on home audio equipment until about 1995/1996)
• Dolby Stereo Digital (first promotional name, as seen on early releases, also seen on True Lies LaserDisc)
• Dolby SR-Digital (when the recording incorporates a Dolby SR-format recording for compatibility)
• SR-D (an abbreviation of above)
• Adaptive Transform Coder 3 (relates to the bitstream format of Dolby Digital)
• AC-3 (an abbreviation of above)
• Audio Codec 3, Advanced Codec 3, Acoustic Coder 3 (These are backronyms. However, Adaptive TRansform Acoustic Coding 3, or ATRAC3, is a separate format developed by Sony)
• ATSC A/52 (name of the standard, current version is A/52 Rev. B)

[编辑] Dolby Digital EX

Dolby Digital EX 類似杜比早期的 Pro-Logic 格式，整合了矩陣技術，在立體聲音軌上加入一中置和一後置聲道.。EX 在5.1 杜比數碼的基礎上加入後置聲道，創出6.1或7.1的聲道輸出。但是, 這技術並不能視為真正的 6.1或7.1編碼，與它的競爭對手不同 DTS-ES格式不同，它並不能提供完整獨立分離的6條或7條音軌.

[编辑] Dolby Digital Live

Dolby Digital Live (DDL) 是一種實時編碼技術應用在互動媒體如電子遊戲上。在個人電腦和遊戲機平台上，它把音訊編碼成5.1聲道並通過S/PDIF介面輸出。^[1] 。SoundStorm，應用在 Xbox和nForce2的電腦, 使用了這技術的雛型。不少使用c-media晶片的音效卡提供了這技術如Turtle Beach^[2] and Auzentech^[3] 。另外，主機版的晶片也有使用這技術，如 Realtek's ALC882D,^[4] ALC888DD and ALC888H。DTS公司也有類似功能的技術DTS Connect。

這個技術的好處是它能令只支援類比輸出的多聲道音效裝置能以數碼多聲道輸出。

[编辑] Dolby Digital Surround EX

Dolby Digital Surround EX 是Dolby和Lucasfilm THX在1999年5月面世的星際大戰首部曲：威脅潛伏電影中合作的產品。在考慮經濟效益和向下兼容性，在左環繞和右環繞中間，加插了後環繞聲道，形成6.1輸出。它使用了矩陣編碼，這是一條獨立分離聲道，情形就像前置左右聲道和中置聲道的關係一樣。所以它能在標准的5.1裝置上輸出5.1，又能同時左支援Dolby Digital Surround EX的器材上輸出6.1。 在星際大戰系列中使用了這技術。不少DVD支援Dolby Digital Surround EX輸出。

[编辑] Dolby Digital Plus

主条目：Dolby Digital Plus

又稱為E-AC-3，在AC3的基礎進行加強。它把碼率提升至( 6.144 Mbit/s)，支援更多聲道。(最高至13.1),減少壓縮對音質的損害。而且向下兼容AC-3器材。

[编辑] Dolby TrueHD

主条目：Dolby TrueHD

Dolby TrueHD, from Dolby Laboratories, is an advanced lossless audio codec based on Meridian Lossless Packing. Support for the codec is mandatory for HD DVD and optional for Blu-ray Disc hardware. TrueHD supports 24 bit, 96 kHz audio channels at up to 18 Mbit/s over 8 channels (although HD DVD and Blu-ray Disc standards currently limit their maximum number of audio channels to eight, Dolby TrueHD can output up to 14 sound channels simultaneously). It also supports extensive metadata, including dialog normalization and Dynamic Range Control.

[编辑] Channel configurations

Although most commonly associated with the 5.1 channel configuration, Dolby Digital allows a number of different channel selections. The full list of available options is:

• Mono (Center only)
• 2-channel stereo (Left + Right), optionally carrying matrixed Dolby Surround
• 3-channel stereo (Left, Center, Right)
• 2-channel stereo with mono surround (Left, Right, Surround)
• 3-channel stereo with mono surround (Left, Center, Right, Surround)
• 4-channel quadrophonic (Left, Right, Left Surround, Right Surround)
• 5-channel surround (Left, Center, Right, Left Surround, Right Surround)

All of these configurations can optionally include the extra Low Frequency Effect (LFE) channel. The last two with stereo surrounds can optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.

Many Dolby Digital decoders are equipped with downmixing functionality to distribute encoded channels to available speakers. This includes such functions as playing surround information through the front speakers if surround speakers are unavailable, and distributing the center channel to left and right if no center speaker is available. When outputting to separate equipment over a 2-channel connection, a Dolby Digital decoder can optionally encode the output using Dolby Surround to preserve surround information.

The '.1' in 5.1, 7.1 etc. refers to the LFE channel, which is also a discrete channel.

[编辑] Applications of Dolby Digital

Dolby Digital recording on 35 mm film (between the sprocket holes). Look very closely and you will see the Dolby "Double-D" logo in the middle of the code pattern. The blue edge track is SDDS, a competing system.

Dolby Digital SR-D cinema soundtracks are optically recorded on a 35 mm release print using sequential data blocks placed between every perforation hole on the sound track side of the film. A CCD scanner in the projector picks up a scanned video image of this area, and a processor correlates the image area and extracts the digital data as an AC-3 bitstream. These data are finally decoded into a 5.1 channel audio source.

Dolby Digital audio is also used on DVD-Video and other purely digital media, like home cinema. In this format, the AC-3 bitstream is interleaved with the video and control bitstreams.

The system is used in many bandwidth-limited applications other than DVD-Video, such as digital TV. The AC-3 standard allows a maximum coded bit rate of 640 kbit/s. 35 mm film prints use a fixed rate of 320 kbit/s. HD-DVD and DVD-Video discs are limited to 448 kbit/s, although many players can successfully play higher-rate bitstreams (which are non-compliant with the DVD specification). ATSC and Digital cable standards limit AC-3 to 448 kbit/s. Blu-ray Disc, the Sony PlayStation 3 and the Microsoft Xbox game console can output an AC-3 signal at a full 640 kbit/s. Some Sony PlayStation 2 console games are also capable to output AC-3 standard audio as well.

Dolby is part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and also considered the successor to MP3. AAC outperforms AC-3 at any bitrate, but is more complex. The advantages of AAC become clearly audible at less than 400 kbit/s for 5.1 channels, and at less than 180 kbit/s for 2.0 channels.

Dolby Digital Plus (DD-Plus) is supported in HD DVD, as a mandatory codec, and in Blu-ray Disc, as an optional codec.

[编辑] Dolby Technologies in packaged media formats

[编辑] See also

• Dialnorm – Dolby Digital metadata parameter controlling decoder gain.
• Dolby Laboratories – company history and technology development.
• Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
• Dolby Stereo – first cinema analogue surround sound system
• Dolby Pro Logic – consumer version of the Dolby Stereo analogue surround sound system.
• Dolby TrueHD – lossless codec for HD DVD and Blu-ray Disc
• Home cinema
• Loudspeaker
• List of Dolby Digital films
• SoundStorm – a real-time AC3 encoder for nForce2-based computers.
• C-Media – Producer of DDL audio chipsets used in many sound cards and motherboards.

[编辑] Technical details

The data layout of AC-3 is described by simplified "C-like" language in official specifications. An AC-3 stream is made up by a series of synchronization frames, which are composed of six audio blocks. Each audio block contains 256 audio samples per channel. Note 6×256 = 1536 = Audio frame size. Below is a simplified AC-3 header intended to give an introduction into the data syntax. A detailed description of the header can be found in the ATSC "Digital Audio Compression (AC-3) Standard", section 5.4.

[编辑] References

1. ↑ Dolby Digital Live – Dolby.com
2. ↑ Montego DDL – Turtlebeach.com
3. ↑ HDA X-Plosion 7.1 DTS Connect – Auzentech.com
4. ↑ NVIDIA nForce 500 motherboard reviews – AnandTech.com, June 8 2006

[编辑] External links

• Dolby Laboratories
• Dolby Chart
• AC3Filter - open source DirectShow AC3 decoder
• Open source liba52 dolby digital decoder
• ATSC standards, from which the A/52B "Digital Audio Compression (AC-3) Standard" may be downloaded
• A52Codec QuickTime codec, enables QuickTime to play back AC3 streams on PPC and Intel Macs
• Dolby Digital 5.1 television programs

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