Dolby Digital

**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 May 1995, 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 35mm 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).

Dolby Digital was the successor to both **Dolby Stereo** in its theatrical variant as well as the **Dolby Pro Logic** family in the home (especially **Dolby Surround**). It was extended with **Dolby Digital Surround EX** in 1999 and later with **Dolby Digital Plus**, and was eventually succeeded by **Dolby TrueHD** in 2004 and **Dolby Atmos** in 2012, both of which were lossless audio codecs.

## Trivia

### General/Theatrical

* The first film to feature a Dolby Digital soundtrack theatrically was *Batman Returns* on June 19th, 1992, however a test film in December 1991, Paramount's *Star Trek VI: The Undiscovered Country* was released with a Dolby Digital soundtrack in three theaters in the U.S.
* It is believed the final film to release with the format's old name of Dolby Stereo Digital was the original theatrical release of *Toy Story*, though this has yet to be confirmed.
* Due to AC-3's success, it was assigned as the standard audio format for the ATSC standard for HDTV in the United States, one of the mandatory formats on DVD (especially in PAL regions), and the baseline audio format on Blu-ray and HD-DVD.
* In October 1997, Dolby Digital releases topped 500 in number, with the 500th release being *Shine*. When the format celebrated 10 years of usage in June 2002, over 3,700 films had made use of a Dolby Digital soundtrack. By September 2004, this had reached around 6,500 films; and by the time of the introduction of Dolby Atmos in 2012, this number reached over 12,000 films.
* Due to its extreme length in the cinematic arena, both Dolby Digital and its EX extension marked the final evolution of 35mm audio capabilities for Dolby technologies; as both Dolby Surround 7.1 and Dolby Atmos are only intended for digital theaters (a digital theater version of Dolby Digital was also created in the mid-2000s).
* As of 2022, Dolby Digital soundtracks are still being produced for major films, although this is mainly relegated to independent and fallback tracks on the most recent films.

### Home Theater/Media

* The format made its home media debut on the LaserDisc release of *Clear and Present Danger* on January 31st, 1995 (which did not come with a trailer). The first release with a trailer was *True Lies* one week later on February 8th, 1995. (featuring the Train trailer)
* The maximum bitrate a Dolby Digital stream is capable of is 640 kbps. However, LaserDisc and DVD standards capped the bitrate of a Dolby Digital stream to 384 kbps and 448 kbps, respectively; while the D-VHS standard that also featured Dolby Digital capped out at 576 kbps. The full 640 kbps bitrate was not seen in the home until the HD disc formats, which allowed for the full bitrate. (mainly used on very early Warner Bros. and Paramount Blu-rays)
* On LaserDisc, this was seen until the format's end in 2001, with the final Japanese LaserDiscs from that year almost all having a Dolby Digital soundtrack.
* In the format's early days in the home, the format was sometimes known as **Dolby Surround Digital AC-3** (especially on LaserDiscs, where it had an extra suffix of **RF** due to the nature of needing a demodulator using an RF cable), due to the nature of its encoding on those discs.
* Due to confusion with the AC-3 name, the format's name was officially shortened in the home to just **Dolby Digital** in August 1996 for DVDs, and in 1997 this policy extended to LaserDiscs. Regardless, the AC-3 name kept around for some time on various releases until the early 2000s.
* Sometimes, the Dolby Digital logo is gratuitously seen on VHS releases from the late 90s and early 2000s, but these are just mislabels of a Dolby Surround soundtrack; as traditional VHS does not have the bandwidth necessary to encode a full AC-3 soundtrack. The later D-VHS and D-Theater tape formats *do* support Dolby Digital, however.
* Dolby Digital would be supported for the Xbox as a real-time 5.1 encoding solution (also available for the PC as Dolby Digital Live on some Creative sound cards), and would also be supported on the PlayStation 2 for FMV sequences only (which didn't require real-time mixing). All future Microsoft consoles starting from the Xbox 360 onward would integrate real-time Dolby Digital mixing as well.
* As of 2015, all legacy Dolby technologies prior to Dolby Atmos (including Dolby Digital) were folded into the generic name of **Dolby Audio**, though you can still find Dolby Digital mentions on a few modern releases.