It is a successor to Dolby Digital (AC-3), and has a number of improvements over that codec, including support for a wider range of data rates (32 kbit/s to 6144 kbit/s), an increased channel count, and multi-program support (via substreams), as well as additional tools (algorithms) for representing compressed data and counteracting artifacts.
Additional independent substreams may be used for secondary audio programs such as foreign language soundtracks, commentary, or descriptions/voiceovers for the visually impaired.
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.
A DD+ stream is a collection of fixed-length syncframe packets, each of which corresponds to either 256, 512, 768, or 1536 consecutive time-domain audio samples.
A syncframe consists of the following syntax elements (some of which may be elided when a Dolby Digital Plus service is encapsulated into another format or transport): At the heart of both Dolby Digital and DD+ is a modified discrete cosine transform (MDCT), which is used to transform the audio signal into the frequency domain; within each block up to 256 frequency coefficients may be transmitted.
Exponents for each channel are encoded in a highly packed differential format, with the deltas between consecutive frequency bins (other than the first) being given in the stream.
Dolby Digital Plus, like many lossy audio codecs, uses a heavily quantized frequency-domain representation of the signal to achieve coding gain; this section describes the operation of the base transform as well as various optional "tools" specified by the standard, which are used to achieve either greater compression or to reduce audible coding artifacts.
A floating-point format for frequency coefficients is used, and mantissas and exponents are stored and transmitted separately, with both being heavily compressed.
For highly stationary signals, such as long notes in musical performance, the Adaptive Hybrid Transform (AHT) is used.
Gain-adaptive quantization may be independently enabled for each frequency bin within a channel, and permits variable-length mantissa encoding.
Transient pre-noise processing (TPNP) is a Dolby Digital Plus-specific tool to reduce the resulting artifacts of signal quantization and other compression techniques.
In some devices, users may also have a choice (via menu) to select an alternate mode that suits their particular taste and/or application.
The combination of SMPTE 340-2008 and 337M allow the Dolby Digital Plus bitstream to be stored and transported within professional production, contribution and distribution workflows prior to emission to consumers.
Many such applications do not take advantage of its higher channel count or ability to support multiple independent programs; instead it is used as a higher-efficiency codec than AC-3.
The AC-3 core is encoded at 640 kbit/s, carries 5 primary channels (and 1 LFE), and is independently playable as a movie audio track by any Blu-ray Disc player.
[4][better source needed] Generally, a Dolby Digital Plus bitstream can only be transported over an HDMI 1.3 or greater link.
The DD+ specification explicitly defines downmixing modes and mechanics, so any source soundfield (up to 14.1) can be reproduced predictably for any listening environment (down to a single channel).