Dolby Digital, originally synonymous with Dolby AC-3, is the name for what has now become a family of audio compression technologies developed by Dolby Laboratories. Originally named Dolby Stereo Digital until 1995, except for Dolby TrueHD, the audio compression is lossy, based on the modified discrete cosine transform (MDCT) algorithm. The first use of Dolby Digital was to provide digital sound in cinemas from 35 mm film prints; today, it is now also used for applications such as TV broadcast, radio broadcast via satellite, digital video streaming, DVDs, Blu-ray discs and game consoles.
The main basis of the Dolby AC-3 multi-channel audio coding standard is the modified discrete cosine transform (MDCT), a lossy audio compression algorithm.It is a modification of the discrete cosine transform (DCT) algorithm, which was first proposed by Nasir Ahmed in 1972 and was originally intended for image compression. The DCT was adapted into the modified discrete cosine transform (MDCT) by J.P. Princen, A.W. Johnson and Alan B. Bradley at the University of Surrey in 1987.
Dolby Laboratories adapted the MDCT algorithm along with perceptual coding principles to develop the AC-3 audio format for cinema needs. The AC-3 format was released as the Dolby Digital standard in 1991.Dolby Digital was the earliest MDCT-based audio compression standard to be released, and was followed by other MDCT-based audio compression standards for home and portable usage, such as Sony's ATRAC (1992), the MP3 standard (1993) and AAC (1997).
This article's factual accuracy may be compromised due to out-of-date information. (September 2020)
Batman Returns was the first film to be announced as using Dolby SR-D technology when it premiered in theaters in the summer of 1992.Dolby Digital 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 constant bit rate of 320 kbit/s is used. A charge-coupled device (CCD) scanner in the image 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. The data is then decoded into a 5.1 channel audio source. All film prints with Dolby Digital data also have Dolby Stereo analogue soundtracks using Dolby SR noise reduction and such prints are known as Dolby SR-D prints. The analogue 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 current release cinema prints are of this type and may also include SDDS data and a timecode track to synchronize CD-ROMs carrying DTS soundtracks.
The simplest way of converting existing projectors is to add a so-called penthouse digital soundhead above the projector head. However, for new projectors it made sense to use dual analogue/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).
As of 2015 [update] , Dolby Digital in film sound mixing is being gradually replaced with Dolby Surround 7.1, with the more advanced Dolby Atmos technology also gaining in popularity. While the majority of movie theaters currently utilize Dolby Digital, virtually all films released today are mixed in Dolby Surround 7.1 and Dolby Atmos.
Dolby Digitalhas similar technologies, included in Dolby Digital EX, Dolby Digital Live, Dolby Digital Plus, Dolby Digital Surround EX, Dolby Digital Recording, Dolby Digital Cinema, Dolby Digital Stereo Creator and Dolby Digital 5.1 Creator.
Dolby Digital is the common version containing up to six discrete channels of sound. The most elaborate mode 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 the subwoofer driven low-frequency effects. Mono and stereo modes are also supported. AC-3 supports audio sample-rates up to 48 kHz.
This format has different names:
In 1991, a limited experimental release of Star Trek VI: The Undiscovered Country in Dolby Digital played in 3 US theatres.In 1992, Batman Returns is the first movie to be released in Dolby Digital. In 1995, the LaserDisc version of Clear and Present Danger featured the first home theater Dolby Digital mix, quickly followed by True Lies , Stargate , Forrest Gump , and Interview with the Vampire among others.
Dolby Digital EX is similar in practice to Dolby's earlier Pro Logic format, which utilized matrix technology to add a center surround 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.
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 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 cinema version of Dolby Digital EX was introduced in 1999, when Dolby and Skywalker Sound, a division of Lucasfilm Ltd., codeveloped Dolby Digital Surround EX™ for the release of Star Wars: Episode I – The Phantom Menace .Dolby Digital Surround EX has since been used on the DVD releases of the Star Wars prequel and original trilogies.
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 at 640 kbit/s and transports it via a single S/PDIF cable. A similar technology known as DTS Connect is available from competitor DTS. 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 digital PCM 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. The SoundStorm, used for the Xbox game console and certain nForce2 motherboards, used an early form of this technology. DDL is available on motherboards with codecs such as Realtek's ALC882D,ALC888DD and ALC888H. Other examples include some C-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 came Auzentech, 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 and Auzentech 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.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 based line 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.
Creative Labs alleged Kawakami violated their intellectual property and demanded he cease distributing his modified drivers.
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.Based on the agreement, in September 2008 Creative began selling the Dolby 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 their DTS Connect pack to the DDL pack at no added cost.
E-AC-3 (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 even more audio channels (up to 15.1 discrete channelsin the future), and improved coding techniques (only at low data rates) to reduce compression artifacts, enabling lower data rates than those supported by AC-3 (e.g. 5.1-channel audio at 256 kbit/s). It is not backward compatible with existing AC-3 hardware, though E-AC-3 codecs generally are capable of transcoding to AC-3 for equipment connected via S/PDIF. E-AC-3 decoders can also decode AC-3 bitstreams. The fourth generation Apple TV supports E-AC-3. The discontinued HD DVD system directly supported E-AC-3. Blu-ray Disc offers E-AC-3 as an option to graft added channels onto an otherwise 5.1 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.AC-4 provides a 50% reduction in bit rate over AC-3/Dolby Digital Plus.
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. Dolby TrueHD supports 24-bit bit depths and sample rates up to 192 kHz. Maximum bitrate is 18 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 extra low-frequency effects (LFE) channel. The last two with stereo surrounds optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.
Many Dolby Digital decoders are equipped with downmixing 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 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.
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 of 640 kbit/s. 35 mm film prints use a fixed rate of 320 kbit/s, which is the same as the maximum bit rate for 2-channel MP3. 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). HD DVD limits AC-3 to 448 kbit/s. ATSC and digital cable standards limit AC-3 to 448 kbit/s. Blu-ray Disc, the PlayStation 3 and the Xbox game console can output an AC-3 signal at a full 640 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 of AAC (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|
|Dolby TrueHD||7.1||18 Mbit/s||7.1||18 Mbit/s|
In the LaserDisc world AC3RF is the term widely placed on connectors of players that support Dolby Digital.Specific demodulators and receivers from the LaserDisc era (1990s thru early 2000s) also include placement of this term on connectors.
LaserDisc titles with Dolby Digital tracks often have THX 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 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. A detailed description is in the ATSC "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 (expired since 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 source VLC media player supported AC-3 audio without having paid for any kind of patent license fee.
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."
The last patent covering AC-3 expired March 20, 2017, so it is now generally free to use.
A free ATSC A/52 (AC3) stream decoder, liba52, is available under the GNU General Public License. FFmpeg and the VLC media player each include code for handling AC-3.
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to about 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting (DAB) practical.
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods, which permit storage and transmission of movies using currently available storage media and transmission bandwidth. While MPEG-2 is not as efficient as newer standards such as H.264/AVC and H.265/HEVC, backwards compatibility with existing hardware and software means it is still widely used, for example in over-the-air digital television broadcasting and in the DVD-Video standard.
Windows Media Audio (WMA) is a series of audio codecs and their corresponding audio coding formats developed by Microsoft. It is a proprietary technology that forms part of the Windows Media framework. WMA consists of four distinct codecs. The original WMA codec, known simply as WMA, was conceived as a competitor to the popular MP3 and RealAudio codecs. WMA Pro, a newer and more advanced codec, supports multichannel and high resolution audio. A lossless codec, WMA Lossless, compresses audio data without loss of audio fidelity. WMA Voice, targeted at voice content, applies compression using a range of low bit rates. Microsoft has also developed a digital container format called Advanced Systems Format to store audio encoded by WMA.
Adaptive Transform Acoustic Coding (ATRAC) is a family of proprietary audio compression algorithms developed by Sony. MiniDisc was the first commercial product to incorporate ATRAC in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time as CD while storing audio information with minimal loss in perceptible quality. Improvements to the codec in the form of ATRAC3, ATRAC3plus, and ATRAC Advanced Lossless followed in 1999, 2002, and 2006 respectively.
Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, samples are taken 44,100 times per second, each with 16-bit sample depth. Digital audio is also the name for the entire technology of sound recording and reproduction using audio signals that have been encoded in digital form. Following significant advances in digital audio technology during the 1970s and 1980s, it gradually replaced analog audio technology in many areas of audio engineering and telecommunications in the 1990s and 2000s.
Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo was developed by Dolby in 1976 for analog cinema sound systems. The format was adapted for home use in 1982 as Dolby Surround when HiFi capable consumer VCRs were introduced. It was further improved with the Pro Logic decoding system in 1987.
Dolby Laboratories, Inc. is an American company specializing in audio noise reduction and audio encoding/compression. Dolby licenses its technologies to consumer electronics manufacturers.
Near Instantaneous Companded Audio Multiplex (NICAM) is an early form of lossy compression for digital audio. It was originally developed in the early 1970s for point-to-point links within broadcasting networks. In the 1980s, broadcasters began to use NICAM compression for transmissions of stereo TV sound to the public.
SoundStorm is a brand by Nvidia regarding to a SIP block integrating 5.1 surround sound technology found on the die of their nForce and nForce2 chipsets for x86 CPUs. It is also the name of a certification to be obtained by Nvidia when complying with their specifications.
High-Efficiency Advanced Audio Coding (HE-AAC) is an audio coding format for lossy data compression of digital audio defined as an MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC (AAC-LC) optimized for low-bitrate applications such as streaming audio. The usage profile HE-AAC v1 uses spectral band replication (SBR) to enhance the modified discrete cosine transform (MDCT) compression efficiency in the frequency domain. The usage profile HE-AAC v2 couples SBR with Parametric Stereo (PS) to further enhance the compression efficiency of stereo signals.
DTS, Inc. is an American company that makes multichannel audio technologies for film and video. Based in Calabasas, California, the company introduced its DTS technology in 1993 as a competitor to Dolby Laboratories, incorporating DTS in the film Jurassic Park (1993). The DTS product is used in surround sound formats for both commercial/theatrical and consumer-grade applications. It was known as The Digital Experience until 1995. DTS licenses its technologies to consumer electronics manufacturers.
Dolby Digital Plus, also known as Enhanced AC-3 is a digital audio compression scheme developed by Dolby Labs for transport and storage of multi-channel digital audio. It is a successor to Dolby Digital (AC-3), also developed by Dolby, and has a number of improvements including support for a wider range of data rates, increased channel count and multi-program support, and additional tools (algorithms) for representing compressed data and counteracting artifacts. While Dolby Digital (AC-3) supports up to five full-bandwidth audio channels at a maximum bitrate of 640 kbit/s, E-AC-3 supports up to 15 full-bandwidth audio channels at a maximum bitrate of 6.144 Mbit/s.
Dolby TrueHD is a lossless, multi-channel audio codec developed by Dolby Laboratories for home video, used principally in Blu-ray Disc and compatible hardware. Dolby TrueHD, along with Dolby Digital Plus and Dolby AC-4, is one of the intended successors to the Dolby Digital (AC-3) lossy surround format. Dolby TrueHD competes with DTS's DTS-HD Master Audio, another lossless surround sound codec.
MPEG Multichannel is an extension to the MPEG-1 Layer II audio compression specification, as defined in the MPEG-2 Audio standard which allows it provide up to 5.1-channels of audio. To maintain backwards compatibility with the older 2-channel (stereo) audio specification, it uses a channel matrixing scheme, where the additional channels are mixed into the two backwards compatible channels. Extra information in the data stream contains signals to process extra channels from the matrix.
MPEG Surround, also known as Spatial Audio Coding (SAC) is a lossy compression format for surround sound that provides a method for extending mono or stereo audio services to multi-channel audio in a backwards compatible fashion. The total bit rates used for the core and the MPEG Surround data are typically only slightly higher than the bit rates used for coding of the core. MPEG Surround adds a side-information stream to the core bit stream, containing spatial image data. Legacy stereo playback systems will ignore this side-information while players supporting MPEG Surround decoding will output the reconstructed multi-channel audio.
Dolby E is a lossy audio compression and decoding technology developed by Dolby Laboratories that allows 6 to 8 channels of audio to be compressed into an AES3 digital audio stream that can be stored as a standard stereo pair of digital audio tracks.
An audio coding format is a content representation format for storage or transmission of digital audio. Examples of audio coding formats include MP3, AAC, Vorbis, FLAC, and Opus. A specific software or hardware implementation capable of audio compression and decompression to/from a specific audio coding format is called an audio codec; an example of an audio codec is LAME, which is one of several different codecs which implements encoding and decoding audio in the MP3 audio coding format in software.
Dolby AC-4 is an audio compression technology developed by Dolby Laboratories. Dolby AC-4 bitstreams can contain audio channels and/or audio objects. Dolby AC-4 has been adopted by the DVB project and standardized by the ETSI.
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