Dolby Digital (DD/AC-3) is a digital audio compression system that uses auditory masking for compression. It works with from 1 to 5.1 channels of audio and can carry Dolby Surround coded two-channel material. It applies audio masking over all channels and dynamically allocates bandwidth from a ‘common pool’. Dolby Digital is a constant bit rate system supporting from 64 kb/s to 640 kb/s rates; typically 64 kb/s mono, 192 kb/s two-channel, 320 kb/s 35 mm Cinema 5.1, 384 kb/s Laserdisc/DVD 5.1 and DVD 448 kb/s 5.1.
DVD players and ATSC receivers with Dolby Digital capability can provide a backward-compatible mix-down by extracting the five main channels and coding them into analog Dolby Surround for Pro Logic playback.
Dolby Digital Plus offers more, better quality, channels and supports data rates up to 6 Mb/s. is backwards compatible with Dolby Digital players and is offered as 7.1 channels on HD DVD and Blu-ray with data rates up to 3 and 1.7 Mb/s respectively.
Dolby E is an audio compression scheme which can encode/decode up to eight channels plus metadata – typically 5.1 mix (six channels) and Rt/Lt (Right Total/Left Total surround) or stereo two-channel mix, etc – onto two AES/EBU bitstreams at 1.92 Mb/s (20-bit audio at 48 kHz). Thus video recorders, typically with four channels, can support the greater channel requirements of DVD and some DTV systems (e.g. ATSC). With audio frames matching video frames, Dolby E is a professional distribution coding system for broadcast and post production which maintains quality up to 10 code/recode cycles.
Dolby E is widely used in HD production to carry 5.1 sound. As it is locked to video frames it has to be decoded and re-coded to work with a frame-rate conversion process.
Dolby Surround (a.k.a. Dolby Stereo, Dolby 4:2:4 Matrix) offers analog coding of four audio channels – Left, Center, Right, Surround (LCRS), into two channels referred to as Right Total and Left Total (Rt, Lt). On playback, a Dolby Surround Pro Logic decoder converts the two channels to LCRS and, optionally, a sub-woofer channel. The Pro Logic circuits steer the audio and increase channel separation. The Dolby Surround system, originally developed for the cinema, is a method of getting more audio channels but suffers from poor channel separation, a mono limited bandwidth surround channel and other limitations. A Dolby Surround track can be carried by analog audio or linear PCM, Dolby Digital and MPEG compression systems.
Dolby TrueHD is a lossless compression system designed for high-definition disk-based media and claims to be bit-for-bit identical to the studio master. Running up to 18 Mb/s up to eight 24-bit/96 kHz channels are supported on HD DVD and Blu-ray Disc standards, and is expected to feature in future A/V receivers and downloadable media. It can connect over HDMI.