Dolby Digital Plus

Dolby Digital Plus logo

Dolby Digital Plus (DD+ or E-AC-3 (Enhanced AC-3), and sometimes incorrectly as EC-3) is a digital audio compression scheme. It is an incompatible^[1] development of the technologies used in the earlier Dolby Digital system. E-AC-3 has a number of improvements aimed at increasing quality at a given bitrate 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 with legacy Dolby Digital decoders, and decoders that output audio over legacy S/PDIF connections must transcode 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 A/52B^[2], as well as in ETSI TS 102 366 V1.2.1 (2008-08).

Physical transport

As of 2007^[update], HDMI 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 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.

HD DVD and Blu-ray Disc

Main article: Comparison of high definition optical disc formats#Technical details

The maximum number of discrete coded channels is the same for both formats: 7.1. However, HD DVD and Blu-ray Disc impose different technical constraints on the supported audio-codecs. Hence, the usage of DD+ differs substantially between HD DVD and Blu-ray Disc.

Dolby Digital (AC-3) and Dolby Digital Plus (E-AC-3) bitrate comparison

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/PDIF 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 Disc 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 DVD 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 modes and mechanics, so any source soundfield (up to 14.1) can be reproduced predictably for any listening environment (down to a single channel).

See also

• DTS-HD High Resolution Audio

References

1. ^ Dolby Digital Plus technical specification Annex E, subsection E1 – "Scope"
2. ^ Dolby Digital Plus technical specification at atsc.org