Dolby Digital

Dolby Digital logotype

Table of contents

1 Alias names
2 Description
3 Applications of Dolby Digital
4 Related articles
5 External links

Alias names

Dolby Digital (promotion name, not accepted by the ATSC), often combined with channel count (DD 5.1)
DD
Dolby SR-Digital (for more than 2 channels)
SR-D
Audio Codec 3
AC-3
ATSC A/52 (name of the standard, current version is A/52 Rev. A)

These are all different names for the same codec.

Dolby Digital is the trademark for Dolby Laboratories' AC-3 lossy audio compression (or data reduction) system. It is a system for coding and decoding (codec) digital audio sound so that it occupies less space on the recording medium. It is termed lossy because a perceptual coding scheme is utilised which attempts to remove information that is inaudible: for example, when a louder sound masks a quieter sound. (e.g., it is not possible to hear the noise on a poor quality tape recording of heavy metal.) In this sense it is an example of lossy data compression as these frequencies are not restored on playback.

Its main application is for multichannel audio, but it supports anywhere from 1.0 channels (mono) to 5.1 channels (full surround) and also dual channel (1+1).

"5.1" surround sound consists of:

The 5 full-range (10 Hz-22 kHz) channels:
- 3 front channels (left, center and right)
- And 2 surround side channels (left, right).
Plus the "0.1" limited range (10 Hz-120 Hz) Low Frequency Effect channel LFE, that carries deep bass sound efects to the subwoofer.

Dolby Digital 5.1 EX (also known as 6.1) provides a rear centre channel by matrixing it in the two 2 back surround channels/speakers. The additional center rear information is split between the left and right back surround channels, in a similar fashion to Dolby Pro Logic's extraction of a front centre channel from a 2 channel source.

Support for EX is more readily found in movie theatres, where typically 8-20 rear loudspeakers are already installed in a rear array, to provide good coverage of the auditorium and a "diffuse" soundfield. It should be noted, then, that Dolby Digital EX is really a separate system: it is a "bolt on" to the core 5.1 audio compression standard in order to provide a sixth channel that could otherwise not be accommodated.

Applications of Dolby Digital

Dolby Digital SR-D cinema soundtracks are optically recorded on a 35mm release print using sequential data blocks placed between every perforation hole on the sound track side of the film. A CCD scanner in the 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. This data is finally decoded into a 5.1 channel audio source.

Dolby Digital audio is also 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 many bandwidth-limited applications other than DVD Video, such as digital TV.

On DVD Video, the AC-3 compression algorithm is used on top of a 16 bit 48 kHz PCM bitstream. It supports bitrates between 64 to 448 kpbs with 384 kbps being the normal rate for 5.1 channels and 192 kbps the normal rate for stereo.

According to the AC-3 standard, the maximum bit rate is 32 to 640 kbps.

Dolby is part of a group of organisations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and also considered the successor to MP3. AAC outperforms AC-3 at any bitrate, but is more complex. The advantages of AAC become clearly audible at less than 400 kbit/s for 5.1 channels, and at less than 180 kbit/s for 2.0 channels.

Dolby Laboratories - company history and technology development.
Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
Dolby Pro Logic - analogue surround sound system.