Dolby Digital

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.

The Codec

Dolby Digital is the trademark for Dolby Laboratories' AC-3 lossy audio compression (or data reduction) system. It is 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.

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 screen channels and 2 surround channels) plus the "0.1" limited range (10 Hz-120 Hz) Low Frequency Effect channel LFE.

Dolby Digital EX provides a rear centre channel by matrixing it in the two 2 rear channels. It can then be extracted in a similar fashion to Dolby Pro-Logic's extraction of a front centre channel from a 2 channel source. Otherwise, Dolby Digital is a discrete format. (Although, cross channel information is used in the lossy compression process.) In some cases, 4 rear channels (7.1 total from a 5.1 source) are used; the rear centre channel is split across the rearmost 2 channels. 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.

AC-3 supports bitrates between 32 to 640 kbit/s; on DVD Video, up to 448 kbit/s.

AC-3 no longer represents the state-of-the-art in lossy audio compression, and 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.

External links

Dolby Laboratories

Referenced By