Dolby Digital Sound System
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Dolby AC-3 or Dolby Digital is an audio compression technology ie. It reduces the significant amount of data that is required for producing a high-quality sound. The Dolby digital has a maximum of 5.1 channel support which has a rate of 640 kilobits/s. With the limited amount of data, the Dolby digital promises us to provide. This video is to check your Sourround System (YouTube only supports 2 Channel Stereo Sound - but the sound is amazing)Try also the THX Surround Sound Video.

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 (32 kbit/s to 6144 kbit/s), increased channel count and multi-program support (via substreams), and additional tools (algorithms) for representing compressed data and counteracting artifacts. While Dolby Digital (AC-3) supports up to five full-bandwidth audio channels at a maximum bitrate of 640 kbit/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,^[1]^[2] as well as Annex E of ETSI TS 102 366^[3] V1.2.1 (2008–08), published by the Advanced Television Systems Committee.

Technical details[edit]

Specifications[edit]

Dolby Digital Plus is capable of the following:

Coded bitrate: 0.032 to 6.144 Mbit/s
Audio channels: 1.0 to 15.1 (i.e. from mono to 15 full range channels and a low frequency effects channel)
Number of audio programs per bitstream: 8
Sample rate: 32, 44.1 or 48 kHz

Structure[edit]

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

Independent substreams, which can contain a single program of up to 5.1 channels. Up to eight dependent substreams may be present in a Dolby Digital Plus stream. The channels present in an independent substream are limited to the traditional 5.1 channels: Left (L), Right (R), Center (C), Left Surround (Ls), and Right Surround (Rs) channels, as well as a Low Frequency Effects (Lfe) channel.
Legacy substreams, which contain a single 5.1 program, and which correspond directly to Dolby Digital content. At most a single legacy substream may be present in a DD+ stream.
Dependent substreams, which contain additional channels beyond the traditional 5.1 channels. As dependent substreams have the same structure as independent substreams, each dependent substream may contain up to five full-bandwidth channels and one low-frequency channel; however these channels may be assigned to different speaker placements. Metadata in the substream describes the purpose of each included channel.

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 channel is used for medium-to-high-frequency information which is common to multiple full-bandwidth channels. Its content 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).

Syntax[edit]

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. (The 1536-sample case is the most common case, and corresponds to Dolby Digital; the shorter subframe lengths are intended for use in interactive applications like video games where reducing encoder latency is an important concern). Each syncframe is independently decodable, and belongs to a specific substream within the service. 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):

A 16-bit sync word, which has the value 0x0b77.
A Bitstream Info (BSI) section, which includes key metadata such as the frame size, the bitstream identifier (which specifies the version of syntax used), channel mode, the substream identifier, the encoded dialog level (dialnorm), and metadata to guide decoder production of a downmix.
An Audio Frame section, which contains decoding information common to all audio blocks within the syncframe, including the necessary information to determine how exponents and mantissas are packed.
One, two, three, or six Audio Block sections. These sections contain additional decoding metadata, as well as the encoded and quantized frequency coefficients. Each Audio Block corresponds to 256 PCM samples in each channel.
A final section containing user-defined auxiliary data, any necessary padding to produce uniform syncframe lengths, and a 16-bit cyclic redundancy check code for error detection.

Storage of transform coefficients[edit]

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. Coefficients are transmitted in a binary floating-point format, with exponents transmitted separately from mantissas. This allows for highly efficient coding.

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. Three formats, or exponent strategies, are used; these are known as 'D15', 'D25', and 'D45'. In D15, each bin has a unique exponent, while in D25 and D45, delta values correspond to either pairs or quads of frequency bins. Audio blocks other than the first in a syncframe may additionally reuse the prior block's exponent set (this is required for channels that use the Adaptive Hybrid Transform).

The decoded exponents, along with a set of metadata parameters, is used to derive the bit allocation pointers (BAPs), which specify the number of bits allocated to each mantissa. Bins which correspond to frequencies in which human hearing is more precise are allocated more bits; bins which correspond to frequencies that humans are less sensitive to are allocated fewer. Anywhere between zero and 16 bits may be allocated for each mantissa; if zero bits are transmitted, a dither function may be optionally applied to generate the frequency coefficient.

Algorithm[edit]

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 redu

Dolby Digital Sound