Dolby Digital

Type of format	Lossy audio format
First published	February 1991
Developed by	Dolby Laboratories
Compression Type	CBR
Bitrate	640 kbit/s (80 KB memory)
Channels	5.1
Bits	16, 24
Sample	48 kHz
Filename extension	.ac3
Open Format?	Yes
Free Format?[1]	Yes
Magic Number	0B 77

   Dolby Digital (or AC3) is a lossy audio codec created by Dolby Laboratories in 1991. First used for Laserdisc and then dominating the DVD market, it has become the international standard for home theater audio. The codec is based on the modified discrete cosine transform (MDCT) compression algorithm. It has a high bitrate of 640 kbit/s (80 KB memory) and can go up to 5.1 channels. On Blu-ray, it's used commonly for special features and audio commentaries due to its advantage of MP3-like lossy compression, low bit rate (192 kbit/s) without losing quality, and conserving disc space. The last patent expired on March 20, 2017, so it is now generally free to use.

Like DTS, Dolby has optional extensions that play on top of the main Dolby Digital codec, Dolby Digital Plus, Dolby TrueHD, and Dolby Atmos. Dolby Digital Plus is an extension of DD with more bitrate, while Dolby TrueHD is a different codec that plays on top of DD (based on MLP). Dolby Atmos is an extension of Dolby True HD that expands on existing Dolby surround sound systems by adding height channels, allowing sounds to be interpreted as three-dimensional objects with neither horizontal nor vertical limitations; it is similar to DTS:X, and it is very popular with movies and music albums. It is under the .ac3 extension name and its syncword is 0x0B77. 

Open source implementation

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.

Footnotes

• [1] https://news.ycombinator.com/item?id=13910925

Links

• Encoding / decoding Guidelines: http://www.dolby.com/assets/pdf/tech_library/46_DDEncodingGuidelines.pdf
•  Short description: http://focus.ti.com/lit/an/spra724/spra724.pdf
•  Dolby AC-3 Spec from ATSC - 1995: http://web.archive.org/web/20061231025413/http://www.sparta.lu.se/~bjorn/whitney/references/ac3spec.pdf
•  Dolby Digital Encoding Guidelines - 2007: http://web.archive.org/web/20070126232025/https://www.dolby.com/assets/pdf/tech_library/46_DDEncodingGuidelines.pdf

Author(s) : Æ Firestone