Dolby Digital

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Dolby Digital is the marketing name for a series of lossy audio compression technologies by Dolby Laboratories.

Contents

[edit] Versions

Dolby Digital includes several similar technologies.

[edit] Dolby Digital

Dolby Digital, or AC-3, is the common version containing up to 6 discrete channels of sound, with 5 channels for normal-range speakers (20 Hz - 20,000 Hz) (Right front, Center, Left Front, Right Rear and Left Rear) and one channel (20 Hz - 120 Hz) for the LFE, or subwoofer. The Dolby Digital format supports Mono and Stereo usages as well.

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 till about 1995/6 or so)
  • 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)

[edit] Dolby Digital EX

Dolby Digital EX is similar in practice to Dolby's earlier Pro-Logic format, which utilized Matrix technology to add a center and single rear surround channel to stereo soundtracks. EX adds an extension to the standard 5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating 6.1 or 7.1 channel output. However, the format is not considered a true 6.1 or 7.1 channel codec because it lacks the capability to support a discrete 6th channel unlike the competing DTS-ES codec.

[edit] Dolby Digital Live

Dolby Digital Live (DDL) is a real-time encoding technology for interactive media such as video games. It converts any audio signals on a PC or game console into the 5.1-channel Dolby Digital format and transports it via a single S/PDIF cable.<ref name=DDL>Dolby Digital Live. Dolby.com.</ref> The SoundStorm, used for the Xbox game console and certain nForce2-based PCs, used an early form of this technology. Dolby Digital Live is currently available in sound cards from manufacturers such as Turtle Beach<ref name=turtlebeach>Montego DDL. Turtlebeach.com.</ref> and Auzentech<ref name=auzentech>HDA X-Plosion 7.1 DTS Connect. Auzentech.com.</ref>, as well as on motherboards with codecs such as Realtek's ALC-882D<ref name=ddl882>anandtech.com. Jun 8, 2006.</ref> and ALC-888<ref name=ddl888>Realtek ALC888.</ref> A similar technology known as DTS Connect is available from competitor DTS.

[edit] Dolby Digital Surround EX

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This section has been tagged since October 2006.</div>

The name of Dolby Digital EX for cinema. Since the encoding of the additional back surround channel uses the same technique, it is fully downwards compatible to Dolby Digital EX.

[edit] Dolby Digital Plus

Dolby Digital Plus is an enhanced coding system based on the AC-3 codec. It offers increased bitrates (up to 6.144 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce compression artifacts, and backward compatibility with existing AC-3 hardware.

[edit] 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.

[edit] Applications of Dolby Digital

Image:35mm film audio macro.jpg
Dolby Digital recording on 35mm film (between the sprocket holes)

Dolby Digital SR-D cinema soundtracks are optically recorded on a 35mm 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. 35mm 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 TV standards limit AC-3 to 448 kbit/s. Both Blu-Ray Disc and the Microsoft Xbox game console can output an AC-3 signal at a full 640Kbps.

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.

[edit] Dolby Technologies in packaged media formats

HD DVD Blu-ray DVD DVD-Audio
Codec Player support Channels (max) Max Bit Rate Player support Channels (max) Max Bit Rate Player support Channels (max) Max Bit Rate Player support Channels (max) Max Bit Rate
Dolby Digital Mandatory 5.1 448 kbit/s Mandatory 5.1 640 kbit/s Mandatory 5.1 448 kbit/s Optional in video zone for playback compatibility on DVD-Video players 5.1 448 kbit/s
Dolby Digital Plus 7.1 3 Mbit/s Optional 7.1 1.7 Mbit/s N/A
Dolby TrueHD 8 18 Mbit/s 8 18 Mbit/s

[edit] See also

[edit] 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 6 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.

Field Name # of bits Description
syncword 16 0x0B77   Transmission of data is left bit first also known as Big Endian.
CRC 16
Sampling frequency 2 '11'=reserved '10'=32KHz '01'=44.1 '00'=48
Frame Size Code 6
Bit Stream Identification 5
Bit Stream Mode 3 '000'=main audio service
Audio Coding Mode 3 '010'=left, right channel ordering
Center Mix level 2
Surround Mix Level 2
Dolby Surround Mode 2 '00'=not indicated '01'=Not surround encoded '10'=Yes, surround encoded.

[edit] References

<references/>

[edit] External links

Multimedia compression formats
Video compression formats ISO/IEC MPEG-1 | MPEG-2 | MPEG-4 | MPEG-4/AVC ITU-T H.261 | H.262 | H.263 | H.264 Others AVS | Dirac | Indeo | MJPEG | RealVideo | VC-1 | Theora | VP6 | VP7 | WMV
Audio compression formats ISO/IEC MPEG MPEG-1 Layer III (MP3) | MPEG-1 Layer II | AAC | HE-AAC ITU-T G.711 | G.722 | G.722.1 | G.722.2 | G.723 | G.723.1 | G.726 | G.728 | G.729 | G.729.1 | G.729a Others AC3 | ATRAC | FLAC | iLBC | Monkey's Audio | Musepack | RealAudio | SHN | Speex | Vorbis | WavPack | WMA
Image compression formats ISO/IEC/ITU-T JPEG | JPEG 2000 | JPEG-LS | JBIG | JBIG2 -- -- Others BMP | GIF | ILBM | PCX | PNG | TGA | TIFF | WMP
Media container formats General 3GP | ASF | AVI | FLV | Matroska | MP4 | MXF | NUT | Ogg | Ogg Media | QuickTime | RealMedia Audio only AIFF | AU | WAV -- --
cs:Dolby Digital

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