Dolby Digital, originally synonymous withDolby AC-3 (see below), is the name for a family ofaudio compression technologies developed byDolby Laboratories. CalledDolby Stereo Digital until1995, it islossy compression (except forDolby TrueHD). The first use of Dolby Digital was to provide digital sound in cinemas from 35 mm film prints. It has since also been used for TV broadcast, radio broadcast via satellite, digital video streaming,DVDs,Blu-ray discs and game consoles.
Dolby AC-3 was the original version of the Dolby Digital codec. The basis of the Dolby AC-3 multi-channelaudio coding standard is themodified discrete cosine transform (MDCT), alossyaudio compression algorithm.[1] It is a modification of thediscrete cosine transform (DCT) algorithm, which was proposed byNasir Ahmed in 1972 forimage compression.[2] The DCT was adapted into the MDCT by J.P. Princen, A.W. Johnson and Alan B. Bradley at theUniversity of Surrey in 1987.[3]
Dolby Laboratories adapted the MDCT algorithm along withperceptual coding principles to develop the AC-3 audio format forcinema. The AC-3 format was released as the Dolby Digital standard in February 1991.[4][5] Dolby Digital was the earliest MDCT-based audio compression standard released, and was followed by others for home and portable usage, such asSony'sATRAC (1992), theMP3 standard (1993) andAAC (1997).[6]
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Batman Returns was the first movie to be announced as using Dolby SR-D (Spectral Recording-Digital) technology when it premiered in all selected movie theaters in the summer of 1992.[7] Dolby Digital cinema soundtracks are optically recorded on a35 mmrelease print using sequential data blocks placed between every perforation hole on the soundtrack side of the film. A constant bit rate of320 kbit/s is used. Acharge-coupled device (CCD) scanner in theimage 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-3bitstream. The data is then decoded into a 5.1 channel audio source. All film prints with Dolby Digital data also haveDolby Stereo analog soundtracks using Dolby SR noise reduction and such prints are known as Dolby SR-D prints. The analog soundtrack provides a fall-back option in case of damage to the data area or failure of the digital decoding; it also provides compatibility with projectors not equipped with digital soundheads. Almost all modern cinema prints are of this type and may also includeSDDS data and a timecode track to synchronize CD-ROMs carryingDTS soundtracks.
The simplest way of converting existing projectors is to add a so-calledpenthouse digital soundhead above the projector head. However, for new projectors it made sense to use dual analog/digital soundheads in the normal optical soundhead position under the projector head. To allow for the dual-soundhead arrangement the data is recorded 26 frames ahead of the picture. If a penthouse soundhead is used, the data must be delayed in the processor for the required amount of time, around 2 seconds. This delay can be adjusted in steps of the time between perforations, (approximately 10.4 ms).
Dolby Digital remains the predominant sound mixing format for movies, despite the introduction ofDolby Surround 7.1 andDolby Atmos in 2010 and 2012, respectively.
Dolby Digital[8] has similar technologies, included in Dolby Digital EX,[9] Dolby Digital Live,[10] Dolby Digital Plus,[11] Dolby Digital Surround EX,[12] Dolby Digital Recording,[13] Dolby Digital Cinema,[14] Dolby Digital Stereo Creator[15] and Dolby Digital 5.1 Creator.[16]
| Filename extension | .ac3 |
|---|---|
| Internet media type | audio/ac3 |
| Uniform Type Identifier (UTI) | public.ac3-audio[17] |
Dolby AC-3 (abackronym for Audio Codec 3, Advanced Codec 3, or Acoustic Coder 3), also known asATSC A/52 (name of the standard)[18] or simplyDolby Digital (DD), is the common version containing up to six discrete channels of sound. Before 1996 it was marketed asDolby Surround AC-3,Dolby Stereo Digital, andDolby SRD.[19]
The most elaborate mode of this codec in common use involves five channels for normal-range speakers (20 Hz – 20,000 Hz) (right, center, left, right surround, left surround) and one channel (20 Hz – 120 Hz allotted audio) for thesubwoofer drivenlow-frequency effects.[20]Mono andstereo modes are also supported. AC-3 supports audio sample rates up to 48 kHz.
In 1991, a limited experimental release ofStar Trek VI: The Undiscovered Country in Dolby Digital played in 3 US theatres.[19] In 1992,Batman Returns was the first movie to be released and presented in Dolby Digital.[21][22] In 1995, theLaserDisc version ofClear and Present Danger featured the firsthome theater Dolby Digital mix, quickly followed byTrue Lies,Stargate,Forrest Gump, andInterview with the Vampire among others.[23][24]
Dolby Digital Surround EX (sometimes shortened to Dolby Digital EX) is similar to Dolby's earlierPro Logic format, which utilizedmatrix technology to add acenter surround channel and single rear surround channel to stereo soundtracks.[25] EX adds an extension to the standard5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating6.1 or 7.1 channel output.
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. Theextra surround channel is matrix encoded onto the discreteleft surround andright surround channels of the 5.1 mix, much like the frontcenter channel on Dolby Pro Logic 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 with Surround EX decoding and added speakers. A number of DVDs have a Dolby Digital Surround EX audio option.
The theater version ofDolby Digital Surround EX was introduced in 1999, when Dolby andTHX, a division ofLucasfilm Ltd., co-developedDolby Digital Surround EX™ for the release ofStar Wars: Episode I – The Phantom Menace.[21][26] Dolby Digital Surround EX has since been used on the DVD releases of theStar Wars prequel and original trilogies.[26]
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 a 5.1-channel 16-bit/48 kHz Dolby Digital format at640 kbit/s and transports it via a singleS/PDIF cable.[27] A similar technology known asDTS Connect is available from competitorDTS. 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 digitalPCM stereo or analog multichannel sound because S/PDIF over RCA, BNC, and TOSLINK can only support two-channel PCM, Dolby Digital multichannel audio, and DTS multichannel audio.HDMI was later introduced, and it can carry uncompressed multichannel PCM, lossless compressed multichannel audio, and lossy compressed digital audio. However, Dolby Digital Live is still useful with HDMI to allow transport of multichannel audio over HDMI to devices that are unable to handle uncompressed multichannel PCM.
Dolby Digital Live is available in sound cards using various manufacturers' audio chipsets. TheSoundStorm, used for theXbox game console and certainnForce2 motherboards, used an early form of this technology. DDL is available on motherboards with codecs such asRealtek's ALC882D,[28] ALC888DD and ALC888H. Other examples include someC-Media PCI sound cards and Creative Labs' X-Fi and Z series sound cards, whose drivers have enabled support for DDL.
NVIDIA later decided to drop DDL support in their motherboards due to the cost of involved royalties, leaving an empty space in this regard in the sound cards market.Then in June 2005 cameAuzentech, which with its X-Mystique PCI card, provided the first consumer sound card with Dolby Digital Live support.
Initially, no Creative X-Fi-based sound cards supported DDL (2005~2007) but a collaboration of Creative andAuzentech resulted in the development of the Auzentech Prelude, the first X-Fi card to support DDL. Originally planned to extend DDL support to all X-Fi-based sound cards (except the 'Xtreme Audio' line which is incapable of DDL hardware implementation), the plan was dropped because Dolby licensing would have required a royalty payment for all X-Fi cards and, problematically, those already sold.[29]In 2008, Creative released the X-Fi Titanium series of sound cards which fully supports Dolby Digital Live while leaving all PCI versions of Creative X-Fi still lacking support for DDL.
Since September 2008, all Creative X-Fi-based sound cards support DDL (except the 'Xtreme Audio' and its derivatives such as Prodigy 7.1e, which is incapable of DDL in hardware).X-Fi's case differs.
While they forgot about the plan, programmer Daniel Kawakami made a hot issue by applying Auzentech Prelude DDL module back to Creative X-Fi cards by disguising the hardware identity as Auzentech Prelude.[30]
Creative Labs alleged Kawakami violated their intellectual property and demanded he cease distributing his modified drivers.[31][32][33]
Eventually Creative struck an agreement with Dolby Laboratories regarding the Dolby license royalty by arranging that the licensing cost be folded into the purchase price of the Creative X-Fi PCI cards rather than as a royalty paid by Creative themselves.[29] Based on the agreement, in September 2008 Creative began selling theDolby Digital Live packs enabling Dolby Digital Live on Creative's X-Fi PCI series of sound cards. It can be purchased and downloaded from Creative. Subsequently, Creative added theirDTS Connect pack to the DDL pack at no added cost.[34]
| Internet media type | audio/eac3 |
|---|---|
| Uniform Type Identifier (UTI) | public.enhanced-ac3-audio[35] |
E-AC-3 (Dolby Digital Plus) is an enhanced coding system based on the AC-3codec. It offers increasedbitrates (up to6.144 Mbit/s), support for even moreaudio channels (up to 15.1 discrete channels[36] in the future), and improved coding techniques (only at low data rates) to reducecompression artifacts, enabling lower data rates than those supported by AC-3 (e.g. 5.1-channel audio at256 kbit/s). It is not backward compatible with existing AC-3 hardware, though E-AC-3codecs generally are capable oftranscoding to AC-3 for equipment connected viaS/PDIF. E-AC-3 decoders can also decode AC-3 bitstreams. The fourth generation Apple TV supports E-AC-3.[37] The discontinuedHD DVD system directly supported E-AC-3.Blu-ray Disc offers E-AC-3 as an option tograft added channels onto an otherwise5.1 AC-3 stream, as well as for delivery of secondary audio content (e.g. director's commentary) that is intended to be mixed with the primary audio soundtrack in the Blu-ray Disc player.
Dolby AC-4 is an audio compression standard supporting multiple audio channels and/or audio objects. Support for 5.1 channel audio is mandatory and additional channels up to 7.1.4 are optional.[38] AC-4 provides a 50% reduction in bit rate over AC-3/Dolby Digital Plus.[38]
Dolby TrueHD, developed by Dolby Laboratories, is an advanced lossless audio codec based onMeridian Lossless Packing. Support for the codec was mandatory for HD DVD and is optional forBlu-ray Disc hardware. Dolby TrueHD supports 24-bit bit depths and sample rates up to 192 kHz. Maximum bitrate is18 Mbit/s while it supports up to 16 audio channels (HD DVD and Blu-ray Disc standards currently limit the maximum number of audio channels to eight). It supports metadata, including dialog normalization and Dynamic Range Control.
Although commonly associated with the 5.1 channel configuration, Dolby Digital allows a number of different channel selections. The options are:
These configurations optionally include the extralow-frequency effects (LFE) channel, but only if at least three channels are present.[39] The last two with stereo surrounds can optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel, indicated via a 2-bit flag.
Many Dolby Digital decoders are equipped withdownmixing to distribute encoded channels to speakers. This includes such functions as playing surround information through the front speakers if surround speakers are unavailable and distributing thecenter 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.
Dolby Digital audio is 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 bandwidth-limited applications other than DVD-Video, such as digital TV. The AC-3 standard allows a maximum coded bit rate of640 kbit/s. 35 mm film prints use a fixed rate of320 kbit/s, which is the same as the maximum bit rate for 2-channelMP3. DVD-Video discs are limited to448 kbit/s, although many players can successfully play higher-rate bitstreams (which are non-compliant with the DVD specification). HD DVD limits AC-3 to448 kbit/s, as doATSC and digital cable standards. Blu-ray Disc, the PlayStation 3 and the Xbox game console can output an AC-3 signal at a full640 kbit/s. Some Sony PlayStation 2 console games are able to output AC-3 standard audio as well, primarily during pre-rendered cutscenes.
Dolby is part of a group of organizations involved in the development ofAAC (Advanced Audio Coding), part of MPEG specifications, and considered the successor to MP3.
Dolby Digital Plus (DD-Plus) and TrueHD are supported in HD-DVD, as mandatory codecs, and in Blu-ray Disc, as optional codecs.
| HD DVD | Blu-ray Disc | DVD-Video | DVD-Audio | LaserDisc | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 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 | Player support | Channels (max) | Max bit rate |
| Dolby Digital | Mandatory | 5.1 | 504 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 | Optional | 5.1 | 384 kbit/s |
| Dolby Digital Plus | 7.1 | 3 Mbit/s | Optional | 7.1 | 1.7 Mbit/s | N/A | |||||||||
| Dolby TrueHD | 7.1 | 18 Mbit/s | 7.1 | 18 Mbit/s | |||||||||||
In the LaserDisc worldAC3RF is the term widely placed on connectors of players that support Dolby Digital.[40] Specific demodulators and receivers from theLaserDisc era (1990s thru early 2000s) also include placement of this term on connectors.[40]
LaserDisc titles with Dolby Digital tracks often have theTHX logo on their covers.
The data layout of AC-3 is described by simplified "C-like" language in official specifications. An AC-3 stream is a series of frames; the frame size code is used along with the sample rate code to determine the number of (2-byte)words before the next syncword. Channel blocks can be either long, in which case the entire block is processed as singlemodified discrete cosine transform, or short, in which case two half-length transforms are performed on the block. Below is a simplified AC-3 header. A detailed description is in theATSC "Digital Audio Compression (AC-3) (E-AC-3) Standard", section 5.4.
| Field Name | # of bits | Description |
|---|---|---|
| Syncword | 16 | 0x0B77, data transmission is left bit first:big endian |
| Cyclic redundancy check | 16 | |
| Sampling frequency | 2 | '11'=reserved '10'=32 kHz '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 |
AC3 was covered by patents that expired in March 2017. Patents were used to ask to pay a commercial license to publish an application that decodes AC3. This led some audio app developers to ban AC3 from their apps, although the open sourceVLC media player supported AC-3 audio without having paid a patent license fee.[41]
In Dolby's 2005 original and amended S-1 filings with the SEC, Dolby acknowledged that "Patents relating to our Dolby Digital technologies expire between 2008 and 2017."[42][43][44]
The last patent covering AC-3 expired March 20, 2017, rendering it free to use.[45][46]
A free ATSC A/52 (AC3) stream decoder,liba52, is available under theGNU General Public License.FFmpeg and the VLC media player each include code for handling AC-3.
, Dolby Laboratories