Dolby Digital

Dolby Digital, originally synonymous with Dolby AC-3 (see below), is the name for a family of audio compression technologies developed by Dolby Laboratories. Called Dolby Stereo Digital until 1995, it is lossy compression (except for Dolby 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-channel audio coding standard is the modified discrete cosine transform (MDCT), a lossy audio compression algorithm.[1] It is a modification of the discrete cosine transform (DCT) algorithm, which was proposed by Nasir Ahmed in 1972 for image compression.[2] The DCT was adapted into the MDCT by J.P. Princen, A.W. Johnson and Alan B. Bradley at the University of Surrey in 1987.[3]
Dolby Laboratories adapted the MDCT algorithm along with perceptual coding principles to develop the AC-3 audio format for cinema. 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 as Sony's ATRAC (1992), the MP3 standard (1993) and AAC (1997).[6]
== Cinema[edit] ==
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Batman Returns was the first film 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 a 35 mm release print using sequential data blocks placed between every perforation hole on the soundtrack 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 modern cinema prints are of this type and may also include SDDS data and a timecode track to synchronize CD-ROMs carrying DTS soundtracks.
link=https://en.wikipedia.org/wiki/File:Dolby_Digital_track_reader.jpg|left|thumb|A Dolby Digital Penthouse Soundhead mounted on a mid-1950s vintage Kalee model 20 projector
link=https://en.wikipedia.org/wiki/File:35mm_film_audio_macro.jpg|right|thumb|A photo of a 35 mm film print featuring all four audio formats (or quad track) - from left to right: Sony Dynamic Digital Sound (SDDS) (blue area to the left of the sprocket holes), Dolby Digital (grey area between the sprocket holes labelled with the Dolby Double-D logo in the middle), analog optical sound (the two white lines to the right of the sprocket holes), and the DTS time code (the dashed line to the far right)
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).
Dolby Digital remains the predominant sound mixing format for movies, despite the introduction of Dolby Surround 7.1 and Dolby Atmos in 2010 and 2012, respectively.
== Versions[edit] ==
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]
=== Dolby AC-3[edit] ===
Dolby AC-3
Filename extension 	.ac3
Internet media type 	audio/ac3
Uniform Type Identifier (UTI)	public.ac3-audio[17]
link=https://en.wikipedia.org/wiki/File:Dolbytrailerdvd07.png|right|thumb|The former Dolby Digital logo, entitled "Aurora", that was sometimes shown at the start of broadcasts, feature films, and video games
link=https://en.wikipedia.org/wiki/File:Dolby_Digital_old_logo.svg|thumb|The old Dolby Digital logo
Dolby AC-3 (a backronym for Audio Codec 3, Advanced Codec 3, or Acoustic Coder 3), also known as ATSC A/52 (name of the standard)[18] or simply Dolby Digital (DD), is the common version containing up to six discrete channels of sound. Before 1996 it was marketed as Dolby Surround AC-3, Dolby Stereo Digital, and Dolby 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 the subwoofer driven low-frequency effects.[20] Mono and stereo modes are also supported. AC-3 supports audio sample rates up to 48 kHz.
In 1991, a limited experimental release of Star 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 in Dolby Digital.[21][22] 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.[23][24]
=== Dolby Digital EX[edit] ===
Dolby Digital EX is similar 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.
=== Dolby Digital Surround EX[edit] ===
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 theater version of Dolby Digital Surround EX was introduced in 1999, when Dolby and THX, a division of Lucasfilm Ltd., co-developed Dolby Digital Surround EX™ for the release of Star Wars: Episode I – The Phantom Menace.[21][25] Dolby Digital Surround EX has since been used on the DVD releases of the Star Wars prequel and original trilogies.[25]
link=https://en.wikipedia.org/wiki/File:Dolby_Near.JPG|thumb|A Dolby home theater badge on a laptop computer
=== Dolby Digital Live[edit] ===
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.[26] 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,[27] 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.[28] 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.[29]
Creative Labs alleged Kawakami violated their intellectual property and demanded he cease distributing his modified drivers.[30][31][32]
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.[28] 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.[33]
Category:American inventions
Category:Audio codecs
Category:Consumer electronics
Category:Dolby Laboratories
Category:Film sound production
Category:High-definition television
Category:Surround sound
Category:Audiovisual introductions in 1986