Dolby Digital Plus
Encyclopedia
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
. It is an incompatible development of the technologies used in the earlier Dolby Digital
Dolby Digital
Dolby Digital is the name for audio compression technologies developed by Dolby Laboratories. It was originally called Dolby Stereo Digital until 1994. Except for Dolby TrueHD, the audio compression is lossy. The first use of Dolby Digital was to provide digital sound in cinemas from 35mm film prints...
system. E-AC-3 has a number of improvements aimed at increasing quality at a given bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
compared with legacy Dolby Digital (AC-3). While legacy AC-3 supports up to 5 full-range audio channels at a coded bitrate of 640 kbit/s, E-AC-3 supports up to 13 full range audio channels at a coded bitrate of 6.144 Mbit/s peak.
Dolby Digital Plus bitstreams are not backward compatible
Backward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...
with legacy Dolby Digital decoders, and decoders that output audio over legacy S/PDIF connections must transcode
Transcode
Transcoding is the direct digital-to-digital data conversion of one encoding to another, such as for movie data files or audio files. This is usually done in cases where a target device does not support the format or has limited storage capacity that mandates a reduced file size, or to convert...
the bitstreams to an older format such as PCM, AC-3, or DTS.
Codec changes
The codec used by Dolby Digital Plus is based on the original Dolby Digital codec, but with several enhancements to improve coding efficiency:- Transient pre-noise processing – to reduce "pre-noise" artifacts before sharp transients.
- Enhanced channel coupling – which maintains phase relationships between channels, and improves performance of matrix decoders.
- Adaptive hybrid transform processing – an improved bit allocation and quantization algorithm.
Dolby claims that these changes can result in bitrate improvements of up to 50% while still allowing for the signal to be efficiently converted to Dolby Digital for backwards compatibility.
Specifications
Dolby Digital Plus is capable of the following:- Coded bitrate: 0.032 to 6.144 Mbit/s
- Audio Channels: 1.0 to 13.1 (i.e. from mono to 13 full range channels and a low frequency effects channel)
- Sample rate: 32, 44.1 or 48 kHz
- Bit depth: up to 24 bits per channel.
The full set of technical specifications for E-AC-3 is published in Annex E of ATSC
ATSC
ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....
A/52B, as well as in ETSI TS 102 366 V1.2.1 (2008-08).
Physical transport
, HDMIHDMI
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...
1.3 is the only means to transport a raw DD+ bitstream between two pieces of consumer equipment. The older and more widespread S/PDIF
S/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...
interface cannot directly transport DD+ bitstreams. A number of methods of transcoding exist to convert an E-AC-3 bitstream into a S/PDIF compatible bitstream. See the section below on downmixing
Downmixing
Downmixing is a general term used for manipulating audio where a number of distinct audio channels are mixed together to produce a lower number of channels...
.
HD DVD and Blu-ray Disc
The maximum number of discrete coded channels is the same for both formats: 7.1. However, HD DVDHD DVD
HD DVD is a discontinued high-density optical disc format for storing data and high-definition video.Supported principally by Toshiba, HD DVD was envisioned to be the successor to the standard DVD format...
and Blu-ray Disc
Blu-ray Disc
Blu-ray Disc is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs being the norm for feature-length video discs...
impose different technical constraints on the supported audio-codecs. Hence, the usage of DD+ differs substantially between HD DVD and Blu-ray Disc.
| Codec | HD DVD | Blu-ray Disc | ||||
|---|---|---|---|---|---|---|
| Decoding | Channels | Bitrate | Decoding | Channels | Bitrate | |
| AC-3 | mandatory | 1 to 5.1 | 504 kbit/s | mandatory | 1 to 5.1 | 640 kbit/s |
| E-AC-3 | mandatory | 1 to 7.1 | 3.0 Mbit/s | optional, available for rear channels only | 6.1 to 7.1 | 1.7 Mbit/s |
| TrueHD | mandatory optional |
1 or 2 3 to 8 |
18.0 Mbit/s 18.0 Mbit/s |
optional | 1 to 8 | 18.0 Mbit/s |
On HD DVD, DD+ is designated a mandatory audio codec. An HD DVD movie may use DD+ as the primary (or only) audio track. An HD DVD player is required to support DD+ audio by decoding and outputting it to the player's output jacks. As stored on disc, the DD+ bitstream can carry for any number of audio channels up to the maximum allowed, at any bitrate up to 3.0 Mbit/s.
On Blu-ray Disc, DD+ is an optional codec, and is deployed as an extension to a "core" AC-3 5.1 audiotrack. 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. The DD+ extension bitstream is used on players that support it by replacing the rear channels in the 5.1 setup with higher fidelity versions, along with providing a possible channel extension to 6.1 or 7.1. The complete audio track is allowed a combined bitrate of 1.7 Mbit/s: 640 kbit/s for the AC-3 5.1 core, and 1 Mbit/s for the DD+ extension. During playback, both the core and extension bitstreams contribute to the final audio-output, according to rules embedded in the bitstream metadata.
Media players and downmixing
Generally, a Dolby Digital Plus bitstream can only be transported over an HDMI 1.3 or greater link. Older receivers support earlier versions of HDMI, or only have support for the S/PDIFS/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...
system for digital audio, or analog inputs.
For non-HDMI 1.3 links, the player can decode the audio and then transmit it via a variety of different methods.
- Earlier versions of HDMI, such as HDMI 1.1, support PCM audio, where the player decodes the audio and transmits it losslessly as PCM over HDMI to the receiver.
- Some receivers and players support analog surround sound, and the player can decode the audio, and transmit it to the receiver as analog audio.
Most receivers and players support S/PDIF. This lower bandwidth digital connection is not capable of transmitting lossless audio with more than two channels, but a player can transmit a S/PDIF compatible audio stream to the receiver in one of the following ways:
- Blu-ray DiscBlu-ray DiscBlu-ray Disc is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs being the norm for feature-length video discs...
players can take advantage of the legacy 5.1 AC-3 bitstream embedded in the E-AC-3 bitstream, transmitting just the AC-3 bitstream with no modifications. - Players supporting the HD DVDHD DVDHD DVD is a discontinued high-density optical disc format for storing data and high-definition video.Supported principally by Toshiba, HD DVD was envisioned to be the successor to the standard DVD format...
standard can transcode the decoded audio into another format. Depending upon the method and options available to the player, this can be done with relatively little quality loss. Dolby's reference decoder, available to all licensees, exploits the common heritage between AC-3 and DD+ by performing the operation in the frequency domain. Hybrid re-compression avoids unnecessary end-to-end decompression and subsequent recompression (DD+ → LPCM → AC-3.) In addition to AC-3, some HD DVD players transcode audio compatible with S/PDIF into 1.5 Mbit/s DTS audio.
Should the player need to decode the audio for a non-HDMI 1.3 receiver, the results should be predictable. The DD+ specification explicitly defines downmixing
Downmixing
Downmixing is a general term used for manipulating audio where a number of distinct audio channels are mixed together to produce a lower number of channels...
modes and mechanics, so any source soundfield (up to 14.1) can be reproduced predictably for any listening environment (down to a single channel).