Dolby Digital

Dolby Digital logo

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

Versions

Dolby Digital^[1] includes several similar technologies, which include Dolby Digital EX,^[2] Dolby Digital Live,^[3] Dolby Digital Plus,^[4] Dolby Digital Surround EX,^[5] Dolby Digital Recording,^[6] Dolby Digital Cinema,^[7] Dolby Digital Stereo Creator^[8] and Dolby Digital 5.1 Creator.^[9]

Dolby Digital

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

Dolby Digital is the common version containing up to six discrete channels of sound. The most elaborate mode in common usage involves five 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 allotted audio) for the subwoofer driven low-frequency effects. Mono and stereo modes are also supported. AC-3 supports audio sample-rates up to 48 kHz. Batman Returns was the first film to use Dolby Digital technology when it premiered in theaters in Summer 1992. The Laserdisc version of Clear and Present Danger featured the first Home theater Dolby Digital mix in 1995.

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

• Dolby Digital
• DD (an abbreviation for Dolby Digital, often combined with channel count; for instance, DD 2.0, DD 5.1)
• AC-3 (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)^[10]
• ATSC A/52 (name of the standard)^[11]

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 channel 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.

Dolby Digital Surround EX

The Cinema Version of "Dolby Digital EX" is called Dolby Digital Surround Ex and works the same way. Dolby Digital Surround EX was co-developed by Dolby and Lucasfilm THX in time for the release in May 1999 of Star Wars Episode I: The Phantom Menace. It provides an economical and backwards-compatible means for 5.1 soundtracks to carry a sixth, center back surround channel for improved localization of effects. The extra surround channel is matrix encoded onto the discrete Left Surround and Right Surround channels of the 5.1 mix, much like the front center channel on Dolby Surround encoded stereo soundtracks. The result can be played without loss of information on standard 5.1 systems, or played in 6.1 or 7.1 on systems equipped with Surround EX decoding and additional speakers. Dolby Digital Surround EX has since been used for the Star Wars prequels on the DVD versions and also the remastered original Star Wars trilogy. A number of DVDs have Dolby Digital Surround EX audio option.

Dolby Digital Live

Dolby Digital Live (DDL) is a real-time hardware encoding technology for interactive media such as video games. It converts any audio signals on a PC or game console into a 5.1-channel Dolby Digital format and transports it via a single S/PDIF cable.^[12] A similar technology known as DTS Connect is available from competitor DTS.

Dolby Digital Live is currently available in sound cards from manufacturers such as Creative Labs, Diamond Multimedia, TerraTec, Turtle Beach,^[13] HT OMEGA SYSTEM,^[14] Auzentech^[15] and Asus^[16] using C-Media chipsets. The SoundStorm, used for the Xbox game console and certain nForce2-based PCs, used an early form of this technology.

DDL is also available on motherboards with codecs such as Realtek's ALC882D,^[17] ALC888DD and ALC888H.

DDL is also supported by all Creative X-Fi based sound cards (except the Xtreme Audio which is not capable of DDL in hardware), but was intentionally disabled in the original drivers by Creative on all but the Auzentech Prelude. A programmer named Daniel Kawakami had re-enabled this feature and fixed other bugs in the Windows drivers through a series of modified drivers that he made publicly available. Creative Labs had alleged that Daniel had violated their intellectual property and demanded he cease distributing his modified drivers.^[18][19]

In 2008 Creative released the X-Fi Titanium series of sound cards which fully support Dolby Digital Live. In addition, in September 2008 Creative began selling the "Dolby Digital Live" and "DTS Connect" packs which enables Dolby Digital Live on Creative's X-Fi series of sound cards. It can be purchased and downloaded from Creative.^[20]

An important benefit of this technology is that it enables the use of digital multichannel sound with consumer sound cards, which are otherwise limited to PCM stereo or multichannel analog.

Dolby Digital Plus

E-AC-3, more commonly known as 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), and improved coding techniques to reduce compression artifacts. It is not backward compatible with existing AC-3 hardware, though E-AC-3 decoders generally are capable of transcoding to AC-3 or DTS for equipment connected via S/PDIF. Only the discontinued HD DVD system directly supported E-AC-3, though Blu-ray disc offers E-AC-3 as a option to graft additional channels onto an otherwise 5.1 AC-3 stream.

Dolby TrueHD

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

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 quadraphonic (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.

Applications of Dolby Digital

Various audio track formats on 35 mm film. L to R: Sony Dynamic Digital Sound (SDDS, a competing system); Dolby Digital (between the sprocket holes); analog Optical; DTS time code. The Dolby Digital code pattern contains a small version of the Dolby "Double-D" logo in the middle.

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. This data is 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 able to output AC-3 standard audio as well.

Dolby is also part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and considered the successor to MP3. AAC outperforms MP3 at any bitrate, but is more complex.^[21]

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

Dolby Digital output from the Wii game console is rare, although some games such as Tiger Woods PGA Tour 09, and Tiger Woods PGA Tour 10 use Dolby Digital. The Wii mostly uses Dolby Pro Logic II, and is compatible with Dolby Pro Logic IIx.

Dolby Technologies in packaged media formats

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 frames; each frame has a fixed number of 6 audio blocks; each audio block contains 256 audio samples per channel. For example, a 5.1 AC-3 bitstream contains 1536 samples per audio block (6 channels × 256 samples/channel). Channel blocks can be either long, in which case the entire block is processed as single modified discrete cosine transform or short, in which case two half length transforms are performed on the block. 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.

Free Open Source Project liba52

A free ATSC A/52 stream decoder, liba52, is available under the GPL license.

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
• Digital Theater Systems
• Home cinema
• Loudspeaker
• SoundStorm – a real-time AC3 encoder for nForce2-based computers.
• C-Media – Producer of DDL audio chipsets used in many sound cards and motherboards.

References

1. ^ http://www.dolby.com/consumer/technology/dolby-digital-details.html
2. ^ url=http://www.dolby.com/consumer/technology/dolby_ex.html
3. ^ url=http://www.dolby.com/consumer/technology/dolby_live.html
4. ^ url=http://www.dolby.com/consumer/technology/dolby-digital-plus-details.html
5. ^ url=http://www.dolby.com/consumer/technology/dolby_surround.html
6. ^ url=http://www.dolby.com/consumer/technology/dolby_record.html/
7. ^ url=http://www.dolby.com/consumer/technology/dolby-digital-cinema-details.html
8. ^ url=http://www.dolby.com/consumer/technology/dolby-digital-stereo-creator-details.html
9. ^ url=http://www.dolby.com/consumer/technology/dolby_creator.html
10. ^ url=http://www.sony.com
11. ^ url=http://www.atsc.org/standards/a52.php
12. ^ Dolby Digital Live – Dolby.com
13. ^ Montego DDL – Turtlebeach.com
14. ^ [1] – Htomega.com
15. ^ HDA X-Plosion 7.1 DTS Connect – Auzentech.com
16. ^ [2] – techreport.com
17. ^ NVIDIA nForce 500 motherboard reviews – AnandTech.com, June 8, 2006
18. ^ Silence From Sound Card Maker After Customer Revolt Wired.com
19. ^ Daniel_K, Who Fixed Creative's Broken Vista Drivers, Speaks Out Wired.com
20. ^ Dolby Digital Live pack
21. ^ http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf

External links

• Dolby Laboratories
• ATSC standards, from which the A/52B "Digital Audio Compression (AC-3) Standard" may be downloaded
• Dolby Digital 5.1 television programs
• Minnetonka Audio's SurCode for Dolby Digital is capable of encoding LPCM files into Dolby Digital stereo and surround for home and theatrical playback.