Dolby Digital

See also: Surround Sound   Cinema Surround Sound   THX    Dolby Digital   Subwoofer

Dolby Digital was developed by the Dolby Laboratories and first introduced in 1992 with the movie 'Batman Returns'. After Dolby Surround and Dolby Pro Logic, Dolby Digital 5.1 and Dolby Digital Surround EX was developed.

Dolby Digital 5.1 - AC-3

provides six digital audio channels, the format '5.1'. The '5' channels are: Left, Center, Right, Left-Surround, and Right-Surround. The '.1' represents the so-called LFE channel, the limited bandwidth low-frequency subwoofer channel. It is a high-quality, low-complexity multichannel audio coding format. It can process 20-bit dynamic range digital audio signals over a frequency range from 20Hz to 20kHz (-3dB at 3Hz and 20.3 kHz). The lo-frequency effects channel covers 20 to 120 Hz (-3 dB at 3Hz and 121Hz).
Sampling rates are supported: 32kHz, 44.1kHz, and 48kHz. Data rates range from 32 kb/s for a single mono channel to up 640 kb/s for special applications.

It is the common sound format for digital television (DTV), DVD, high definition television (HDTV), and digital cable and satellite transmission. It operates at lower data rates for a given level of audio quality than an ensemble of single channels by coding several channels into a single transmission channel. By coding a multiplicity of channels as a single entity, AC-3 is able to achieve greater efficiency than with single channels. Dolby AC-3 was conceived as a multichannel audio coder operating at a bit-rate only little more than required for two channel coding. It takes advantage of the human auditory masking.
The AC-3 algorithm is independent of the number of coded channels. Parameters as bit-rate and number of channels can be tailored to particular applications. Current implementations have standardized on the SMPTE-recommended 5.1 channel arrangement. AC-3 conveys the 5.1 channel arrangement with data rate of 320 Kbps. It provides five full-bandwidth channels, front left, front right, center, surround left, and surround right plus a low-frequency effects channel (LFE). The LFE channel is one-tenth of the bandwidth of the other channels.
AC-3 also has a downmixing feature that ensures compatibility with devices that do not support the 5.1 format.
AC-3 is a block-structured coder with typically 512 samples per block. One or more blocks are collected in an input buffer from each channel before proceeding with the additional processing.
The decoder, like the encoder, is block structured and maintains sync with the incoming data stream.

Standard Dolby Digital can be encoded in different bit rates:
- 192 kb/s (for 1.0 or 2.0 soundtracks)
- 384 kb/s (good quality)
- 448 kb/s (5.1 soundtracks)

Transport of AC-3 (Dolby Digital 5.1):
- SPDIF (coax)
- Toslink (optical)

Dolby AC-l was first used in 1985 and has been adopted for DBS services and satellite communication networks.
Dolby AC-2 is a part of the Dolby FAX system, used to link remotely located studios and post production facilities.

Dolby Digital Plus (DD+)

Dolby Digital Plus offers higher bit rates and more efficient compression as standard Dolby Digital, resulting in an improved sound quality. It supports soundtracks up to 7.1 with discrete channels. DD+ is used on Blu-Ray Disks. The data stream is called "E-AC-3". The maximim bandwidth of DD+ / E-AC-3 is 6.144 Mbit/s and it can support up to 13 full range audio channels. Sample rates are 32kHz, 44.1kHz or 48 kHz, bit rates up to 24 bits per channel. Because of the high data rate, transport over SPDIF and Toslink is not possible, HDMI connections must be used.

Dolby TrueHD

Dolby TrueHD is the surround format for Blu-ray Disk by Dolby Laboratories. It can support up to 18 Mbps of audio. It is a lossless audio format with up to 4:1 compression that delivers audio content fully identical to the studio master source. The sampling rate can be in a range from 48 to 192 kHz and the word lengths can be from 16 to 24 bits. The formats are 5.1 and 7.1 Surround with 6 respectively 8 audio channels. Because of the high data rate, transport over SPDIF and Toslink is not possible, HDMI connections must be used.