Dolby Digital

Dolby Digital, initially known as Dolby AC-3, represents a collection of audio compression technologies created by Dolby Laboratories. Formerly referred to as Dolby Stereo Digital until 1995, it employs lossy compression (excluding Dolby TrueHD). Initially, Dolby Digital was employed to deliver digital sound in cinemas through 35 mm film prints. Subsequently, it found applications in TV broadcast, satellite radio broadcast, digital video streaming, DVDs, Blu-ray discs, and game consoles.
The Dolby AC-3 multi-channel audio coding standard is founded upon the modified discrete cosine transform (MDCT), an algorithm for compressing audio in a lossy manner. This MDCT is a variation of the discrete cosine transform (DCT) algorithm, initially introduced by Nasir Ahmed in 1972 for compressing images. The MDCT, as we know it today, was developed by J.P. Princen, A.W. Johnson, and Alan B. Bradley at the University of Surrey in 1987.
The AC-3 audio format for cinema was developed by Dolby Laboratories by adapting the MDCT algorithm and incorporating perceptual coding principles. In February 1991, the AC-3 format was introduced as the Dolby Digital standard. This release marked the earliest MDCT-based audio compression standard, paving the way for subsequent standards like Sony's ATRAC (1992), the MP3 standard (1993), and AAC (1997) for home and portable usage.
== Cinema ==
Batman Returns was the initial movie to be announced as utilizing Dolby SR-D (Spectral Recording-Digital) technology during its release in selected theaters in the summer of 1992. Dolby Digital soundtracks for cinema are recorded optically on a 35 mm print with sequential data blocks placed between each perforation hole on the soundtrack side of the film. The data is recorded at a constant bit rate of 320 kbit/s. A CCD scanner in the image projector scans the video image of this area, and a processor extracts the digital data as an AC-3 bitstream by correlating the image area. This data is then decoded into a 5.1 channel audio source. Film prints with Dolby Digital data also contain Dolby Stereo analogue soundtracks with Dolby SR noise reduction, known as Dolby SR-D prints. The analogue soundtrack serves as a backup in case of data area damage or digital decoding failure, and ensures compatibility with projectors lacking digital soundheads. Most modern cinema prints are of this kind and may also feature SDDS data and a timecode track for synchronizing CD-ROMs carrying DTS soundtracks.
One method of converting existing projectors is by incorporating a penthouse digital soundhead above the projector head. However, for new projectors, it is more practical to utilize dual analogue/digital soundheads positioned below the projector head in the usual optical soundhead location. In order to accommodate the dual-soundhead setup, the data is recorded 26 frames ahead of the picture. If a penthouse soundhead is employed, the data needs to be delayed in the processor for approximately 2 seconds. This delay can be fine-tuned in increments of the time between perforations, which is roughly 10.4 ms.
Dolby Digital continues to be the primary sound mixing format for films, even with the launch of Dolby Surround 7.1 and Dolby Atmos in 2010 and 2012, respectively.
== Versions ==
Dolby Digital offers a range of technologies, such as 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 ===
Dolby Digital is a widely used format that includes a maximum of six separate sound channels. The most advanced configuration commonly used consists of five channels for regular speakers (ranging from 20 Hz to 20,000 Hz) - right, center, left, right surround, and left surround. Additionally, there is one channel dedicated to the subwoofer, which handles low-frequency effects ranging from 20 Hz to 120 Hz. Mono and stereo modes are also compatible with Dolby Digital. Furthermore, AC-3 supports audio sample rates of up to 48 kHz.
This particular format is known by various other names such as Dolby Digital, DD (an abbreviation for Dolby Digital, often accompanied by the channel count like DD 2.0, DD 5.1), AC-3 (Audio Codec 3, Advanced Codec 3, Acoustic Coder 3), and ATSC A/52 (the name of the standard). Prior to 1996, it was marketed as Dolby Surround AC-3, Dolby Stereo Digital, and Dolby SRD. It is worth noting that AC-3 is a backronym for Adaptive Transform Acoustic Coding 3, which is a separate format developed by Sony.
Star Trek VI: The Undiscovered Country had a limited experimental release in Dolby Digital in 1991, being shown in three theaters in the United States. The following year, in 1992, Batman Returns became the first film to be released in Dolby Digital. Clear and Present Danger, released on LaserDisc in 1995, featured the first Dolby Digital mix for home theaters, which was soon followed by True Lies, Stargate, Forrest Gump, Interview with the Vampire, and several others.
=== Dolby Digital EX ===
Dolby Digital EX is akin to Dolby's previous Pro Logic system, incorporating matrix technology to introduce a center surround channel and a single rear surround channel to stereo soundtracks. EX expands upon the standard 5.1 channel Dolby Digital codec by incorporating matrixed rear channels, resulting in 6.1 or 7.1 channel output.
=== Dolby Digital Surround EX ===
It offers a cost-effective and backward-compatible solution for 5.1 soundtracks to include a sixth, center back surround channel to enhance the localization of effects. The additional surround channel is encoded using matrix onto the discrete left surround and right surround channels of the 5.1 mix, similar to the front center channel in Dolby Pro Logic encoded stereo soundtracks. This allows the content to be played without any loss of information on standard 5.1 systems, or on systems with Surround EX decoding and additional speakers in 6.1 or 7.1 configurations. Many DVDs feature a Dolby Digital Surround EX audio option.
In 1999, Dolby and THX, a division of Lucasfilm Ltd., collaborated to introduce the theater edition of Dolby Digital Surround EX. This groundbreaking technology was specifically developed for the release of Star Wars: Episode I – The Phantom Menace. Since then, Dolby Digital Surround EX has been utilized in the DVD versions of both the Star Wars prequel and original trilogies.
=== Dolby Digital Live ===
Dolby Digital Live (DDL) is an encoding technology that operates in real-time and is specifically designed for interactive media like video games. It has the capability to convert audio signals from a PC or game console into a 5.1-channel 16-bit/48 kHz Dolby Digital format at a rate of 640 kbit/s. This converted audio is then transmitted through a single S/PDIF cable. A competing technology called DTS Connect is also available from DTS. One of the key advantages of DDL is that it allows consumer sound cards to utilize digital multichannel sound, which would otherwise be limited to digital PCM stereo or analog multichannel sound due to the restrictions of S/PDIF over RCA, BNC, and TOSLINK connections. These connections can only support two-channel PCM, Dolby Digital multichannel audio, and DTS multichannel audio. HDMI, which was introduced later, can carry uncompressed multichannel PCM, lossless compressed multichannel audio, and lossy compressed digital audio. However, Dolby Digital Live remains valuable when used with HDMI as it enables the transmission of multichannel audio to devices that are incapable of handling uncompressed multichannel PCM.
Dolby Digital Live can be found in sound cards that utilize a variety of audio chipsets from different manufacturers. The SoundStorm, which was utilized in the Xbox game console and specific nForce2 motherboards, was an early adopter of this technology. Motherboards equipped with codecs like Realtek's ALC882D, ALC888DD, and ALC888H also offer DDL support. Additionally, some C-Media PCI sound cards and Creative Labs' X-Fi and Z series sound cards have drivers that enable Dolby Digital Live functionality.
NVIDIA made the decision to discontinue DDL support in their motherboards due to the associated royalties, creating a gap in the sound card market. However, in June 2005, Auzentech introduced the X-Mystique PCI card, becoming the first consumer sound card to offer Dolby Digital Live support.
Between 2005 and 2007, no Creative X-Fi based sound cards were compatible with DDL. However, a partnership between Creative and Auzentech led to the creation of the Auzentech Prelude, the first X-Fi card to offer DDL support. The original intention was to expand DDL support to all X-Fi based sound cards, with the exception of the 'Xtreme Audio' line which could not support DDL hardware implementation. This plan was ultimately abandoned due to the fact that Dolby licensing would have necessitated royalty payments for all X-Fi cards, including those that had already been sold. In 2008, Creative introduced the X-Fi Titanium series of sound cards, which fully supported Dolby Digital Live, while the PCI versions of Creative X-Fi continued to lack DDL support.
From September 2008 onwards, all sound cards based on Creative X-Fi have been equipped with DDL support, with the exception of the 'Xtreme Audio' and its related models like Prodigy 7.1e, which lack DDL capability in their hardware. The case of X-Fi is unique.
However, programmer Daniel Kawakami gained attention when he successfully applied the Auzentech Prelude DDL module to Creative X-Fi cards by disguising the hardware identity as Auzentech Prelude.
Creative Labs accused Kawakami of infringing their intellectual property rights and demanded that he stop distributing his modified drivers.
Creative and Dolby Laboratories reached an agreement on the Dolby license royalty, deciding to include the licensing cost in the purchase price of the Creative X-Fi PCI cards instead of Creative paying a separate royalty. As per the agreement, Creative started selling the Dolby Digital Live packs in September 2008, allowing Dolby Digital Live on Creative's X-Fi PCI series of sound cards. The packs can be bought and downloaded from Creative. Additionally, Creative later included their DTS Connect pack to the DDL pack without any extra charge.
=== Dolby Digital Plus ===
Creative and Dolby Laboratories reached an agreement on the Dolby license royalty, deciding to include the licensing cost in the purchase price of the Creative X-Fi PCI cards instead of Creative paying a separate royalty. As per the agreement, Creative started selling the Dolby Digital Live packs in September 2008, allowing Dolby Digital Live on Creative's X-Fi PCI series of sound cards. The packs can be bought and downloaded from Creative. Additionally, Creative later included their DTS Connect pack to the DDL pack without any extra charge.
=== Dolby AC-3 ===
Dolby AC-4 is a standardized audio compression format that enables the transmission of multiple audio channels and/or audio objects. It is required to support 5.1 channel audio, while additional channels up to 7.1.4 are optional. AC-4 offers a significant 50% decrease in bit rate compared to AC-3/Dolby Digital Plus.
=== Dolby TrueHD ===
Dolby TrueHD, created by Dolby Laboratories, is a sophisticated lossless audio codec utilizing Meridian Lossless Packing technology. While HD DVD required support for this codec, it is not mandatory for Blu-ray Disc hardware. Dolby TrueHD is capable of handling 24-bit bit depths and sample rates reaching 192 kHz. With a maximum bitrate of 18 Mbit/s, it can accommodate up to 16 audio channels (although both HD DVD and Blu-ray Disc standards currently cap the maximum number of audio channels at eight). Additionally, Dolby TrueHD includes metadata features such as dialog normalization and Dynamic Range Control.
== Channel Configurations ==
Despite being commonly linked with the 5.1 channel setup, Dolby Digital offers a variety of channel configurations to choose from. The available options include: Dolby Digital 1/0 – Mono (center only) Dolby Digital 2/0 – 2-channel stereo (left + right), with the option of carrying matrixed Dolby Surround Dolby Digital 3/0 – 3-channel stereo (left, center, right) Dolby Digital 2/1 – 2-channel stereo with mono surround (left, right, surround) Dolby Digital 3/1 – 3-channel stereo with mono surround (left, center, right, surround) Dolby Digital 2/2 – 4-channel quadraphonic (left, right, left surround, right surround) Dolby Digital 3/2 – 5-channel surround (left, center, right, left surround, right surround).
The LFE channel is an optional configuration that can be included in these setups. In the case of the last two configurations, they have the additional feature of stereo surrounds and can use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.
Many Dolby Digital decoders have the capability to downmix encoded channels and distribute them to speakers. This includes functions like playing surround information through the front speakers if surround speakers are not available, and distributing the center channel to the left and right speakers if there is no center speaker. When connected to separate equipment through a 2-channel connection, a Dolby Digital decoder can choose to encode the output using Dolby Surround to preserve the surround information.
The '1' in 5.1, 7.1, etc. refers to the LFE channel, which is a discrete channel.
== Applications ==
Dolby Digital audio is utilized in DVD-Video and other digital media formats, such as home cinema. In this particular format, the AC-3 bitstream is combined with the video and control bitstreams. This system finds application in scenarios with limited bandwidth, apart from DVD-Video, such as digital TV. The AC-3 standard permits a maximum coded bit rate of 640 kbit/s. 35 mm film prints maintain a fixed rate of 320 kbit/s, which aligns with the maximum bit rate for 2-channel MP3. DVD-Video discs are restricted to 448 kbit/s, although numerous players can successfully handle higher-rate bitstreams (which do not comply with the DVD specification). HD DVD imposes a limit of 448 kbit/s on AC-3. ATSC and digital cable standards also restrict AC-3 to 448 kbit/s. Blu-ray Disc, the PlayStation 3, and the Xbox game console have the capability to output an AC-3 signal at the full 640 kbit/s. Additionally, certain Sony PlayStation 2 console games are capable of outputting AC-3 standard audio, primarily during pre-rendered cutscenes.
Dolby is a member of a consortium of organizations engaged in the advancement of AAC (Advanced Audio Coding), which is a component of MPEG specifications and is regarded as the next generation of MP3.
In HD-DVD, Dolby Digital Plus (DD-Plus) and TrueHD are essential codecs, while in Blu-ray Disc, they are offered as optional codecs.
== AC3RF ==
LaserDisc players that are compatible with Dolby Digital are commonly labeled with AC3RF connectors. This term is also found on connectors of specific demodulators and receivers from the LaserDisc era (1990s thru early 2000s). Additionally, LaserDisc titles featuring Dolby Digital tracks frequently display the THX logo on their covers.
== Technical Details ==
The AC-3 data layout is outlined using a simplified "C-like" language in the official specifications. Each AC-3 stream consists of frames, with the frame size code and sample rate code used to determine the number of (2-byte) words before the next syncword. Channel blocks can be either long, where the entire block undergoes a single modified discrete cosine transform, or short, where two half-length transforms are applied to the block. The simplified AC-3 header is provided below, with a more detailed explanation available in the ATSC "Digital Audio Compression (AC-3) (E-AC-3) Standard", section 5.4.
== License ==
AC3, a technology developed by Dolby, was protected by patents that expired in March 2017. These patents required developers to obtain a commercial license in order to include AC3 decoding in their applications. As a result, some audio app developers chose to exclude AC3 from their apps. However, the open source VLC media player continued to support AC-3 audio without requiring a patent license fee. 
In Dolby's 2005 original and amended S-1 filings with the SEC, the company acknowledged that the patents related to their Dolby Digital technologies would expire between 2008 and 2017. 
The final patent covering AC-3 expired on March 20, 2017, making it freely available for use.
=== Open Source Implementation ===
The liba52 decoder, which is a free ATSC A/52 (AC3) stream decoder, can be accessed under the GNU General Public License. Both FFmpeg and the VLC media player contain code designed to manage AC-3 streams.
== Collaborations ==
Although the list of collaborations provided may not encompass all instances, below are a few noteworthy examples of Dolby Digital being utilized or forming partnerships with other companies:
=== Disney ===
Disney and Dolby Digital have had significant collaborations, enhancing the audio experience for audiences in various ways:
# Disney+ and Dolby Atmos:
#* Disney+, the streaming service from Disney, offers content with Dolby Atmos support.
#* Dolby Atmos provides an immersive audio experience by placing sound effects, music, and dialogue anywhere within a 3D space.
#* Whether you’re watching classic Disney films or brand new content, Dolby Atmos adds sparkle and depth to the audio.
#* Titles on Disney+ include both live-action and animated releases from Disney, Lucasfilm, and Marvel Studios 
=== Universal ===
Universal Music Group (UMG) and Dolby Digital have embarked on a groundbreaking collaboration that revolutionizes music production, playback quality, and the in-car audio experience. While there isn’t a specific mention of Universal Animation Studios collaborating directly with Dolby, it’s essential to recognize that Dolby Digital technologies benefit the entire film industry.
=== THX ===
* Dolby Digital and THX serve different purposes:
** Dolby Digital is a codec used for audio encoding and decoding.
** THX focuses on quality standards and post-processing for audio and video.
* While they are not direct collaborators, they often coexist in the same systems.
* For example, THX Surround EX applies post-processing to audio decoded by Dolby Digital EX in AV receivers.
* THX modes enhance audio after it has been decoded by codecs like Dolby Digital or DTS.
=== DreamWorks ===
Dolby Digital collaborated with DreamWorks to promote Bee Movie in 2007
=== Act III ===
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=== Other ===
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== Logos ==
See: Dolby Digital/Logos
The Dolby Digital trademark is recognized globally, frequently seen at the start of movies, games, and TV shows. The company behind it registered both the trademark and logo to highlight their corporate identity. The logo showcases the audio technology's name, honoring its founder, Ray Milton Dolby, who established Dolby Labs in 1965. The original logo debuted in 1967, but it has since evolved into the modern "Double D" version, crafted anew by designers.
=== 1995-2007 ===
The brand's logo was initially designed to be complex and open to interpretation. Each component of the logo evokes associations with the brand's name, technology, or historical significance. Positioned in the top left corner, there is a mirrored arrangement of two letter "D"s, forming a double icon. These letters are set against a dark background and resemble incisions within vertical rectangles. Adjacent to the double icon is a horizontal rectangle in white, featuring a bold black border and the precise inscription "Dolby" in the center. Below these elements, the word "Digital" spans the length of the upper components, serving to balance the logo. Notably, it lacks any surrounding frame.
=== 2003-2007 ===
The designers kept the original structure and style of the logo intact, with only minor adjustments made. Specifically, they opted to increase the size of the Dolby lettering, resulting in it now filling the entire rectangular space.
=== 2007-Today ===
The latest iteration features a consolidation of the vertical rectangles housing the dual "D" letters into a single large shape with white incisions. The border surrounding the term "Dolby" was eliminated, while on the contrary, it was introduced to the "Digital" label. However, this border is not conventional; rather, it consists of two slender lines meeting at a corner.
=== 2012-Today ===
In 2012, a new iteration was unveiled. The letters "Dolby" within it are closely intertwined, while the term "Digital" has been completely shifted to the opposite side - towards the left, in proximity to the black rectangle. It has been magnified to occupy the space of two lines. The borders and lines have vanished.
== Trivia ==
Origin and Name Change:
* Dolby Digital, originally synonymous with Dolby AC-3, is a family of audio compression technologies developed by Dolby Laboratories.
* It was initially named Dolby Stereo Digital until May 1995 when it became known as Dolby Digital.
* The audio compression is lossy and relies on the modified discrete cosine transform (MDCT) algorithm.
Film Debut:
* The first film to feature a Dolby Digital soundtrack theatrically was Batman Returns on June 19th, 1992.
* However, a test film, Paramount’s Star Trek VI: The Undiscovered Country, was released with a Dolby Digital soundtrack in three U.S. theaters in December 1991.
* Toy Story is believed to be the final film to release with the format’s old name of Dolby Stereo Digital.
Widespread Adoption:
* Due to its success, Dolby Digital became the standard audio format for ATSC (HDTV) in the United States.
* It is also one of the mandatory formats on DVDs (especially in PAL regions) and the baseline audio format on Blu-ray and HD-DVD.
* Over 12,000 films have made use of a Dolby Digital soundtrack by the time of the introduction of Dolby Atmos in 2012.
Success and Evolution:
* Dolby Digital was succeeded by Dolby TrueHD in 2004 and Dolby Atmos in 2012, both of which are lossless audio codecs.
* Its impact extends beyond theaters, enhancing home entertainment systems and streaming services.
== Gallery ==
See: Dolby Digital/Gallery
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