Dolby Digital Plus

Dolby Digital Plus, also known as Enhanced AC-3 (and commonly abbreviated as DD+ or E-AC-3, or EC-3) is a digital audio compression scheme developed by Dolby Labs for transport and storage of multi-channel digital audio. It is a successor to Dolby Digital (AC-3), also developed by Dolby, and has a number of improvements including support for a wider range of data rates (32kbit/s to 6144kbit/s), increased channel count and multi-program support (via substreams), and additional tools (algorithms) for representing compressed data and counteracting artefacts. While Dolby Digital (AC-3) supports up to 5 full-bandwidth audio channels at a maximum bitrate of 640kbit/s, E-AC-3 supports up to 15 full-bandwidth audio channels at a maximum bitrate of 6.144 Mbit/s.

The full set of technical specifications for E-AC-3 (and AC-3) are standardized and published in Annex E of ATSC A/52:2012, as well as Annex E of ETSI TS 102 366 V1.2.1 (2008–08), published by the Advanced Television Systems Committee.

Dolby Digital Plus is capable of the following:

A Dolby Digital Plus service consists of one or more substreams. There are three types of substreams:

All DD+ streams must contain at least one independent substream or legacy substream, which contains the first (or only) 5.1 channels of the primary audio program. Additional independent substreams may be used for secondary audio programs such as foreign language soundtracks, commentary, or descriptions/voiceovers for the visually impaired. Dependent substreams may be provided for programs that have additional soundstage channels beyond 5.1.

Within each substream, provision is made for encoding five full-bandwidth channels, one low-frequency channel, and one coupling channel. The coupling channels is used for medium-to-high-frequency information which is common to multiple full-bandwidth channels. Its context is mixed in with the other channels in a fashion prescribed by the metadata, it is not reproduced as a discrete channel by the decoder.

Dolby Digital Plus includes comprehensive bitstream metadata for decoder control over output loudness (via dialnorm), downmixing, and reversible dynamic range control (via DRC).

Dolby Digital Plus is nominally a 16-bit aligned protocol, though very few fields in the syntax respect any byte or word boundaries. As many syntax elements are optional or variable-length, including some whose presence or length is dependent on complex preceding calculations, and there is little redundancy in the syntax, DD+ can be extremely difficult to parse correctly, with syntactically valid but incorrect parsings easily produced by defective encoders.

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