Dolby Digital Plus

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Dolby Digital Plus (DD+ or E-AC-3), is a digital audio compression scheme developed specifically for the introduction of HDTV and HD DVD/Blu-ray Disc. It is a development of the earlier Dolby Digital system. Compared to Dolby Digital it supports a greater range of bitrates ^[1] at higher quality and up to thirteen full range audio channels compared with Dolby Digital's five. The compression algorithm also has a number of improvements aimed at increasing quality at a given bitrate compared with Dolby Digital.

Dolby Digital Plus software is not directly backward compatible with existing Dolby Digital decoders. However, Dolby Digital Plus capable players are required to be able to transform the output to a backwards compatible Dolby Digital signal.^[2][3]

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.^[4]

Specifications

• Coded bitrate: 0.032 to 6.144 Mbit/s
• Audio Channels: up to 13.1 (13 full range channels and a subwoofer channel)
• Sample rate: 32, 44.1 or 48 kHz
• Bit depth: up to 24 bits per channel.

Physical transport

As of 2007, HDMI 1.3 is the only means to transport a raw DD+ bitstream between two pieces of consumer equipment. The older and more widespread TOSLINK and S/PDIF-interfaces can only be used if the bitstream is transcoded. See the section below on downmixing.

HD DVD and Blu-ray

The maximum number of discrete coded channels is the same for both formats: 7.1.^[5] However, HD DVD and Blu-ray impose different technical constraints on the supported audio-codecs. Hence, the usage of DD+ differs substantially between HD DVD and Blu-ray.

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 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.^[6]

Media players and downmixing

As of 2006, a DD+ bitstream can only be transported over an HDMI v1.3 link. Connecting players to audio receivers is problematic, as HDMI-audio is not yet a standard feature on mainstream consumer A/V receivers. Delivery of DD+ encoded audio, between the audio-source and audio-receiver, is accomplished in one of the following ways:

• Raw bitstream – Transport of the encoded (compressed) DD+ bitstream requires both ends of the audio-link to support DD+ audio-packets over HDMI 1.3. Some players support HDMI 1.3, but not the optional advanced audio-codecs over the link.
• Decoded linear PCM – The source internally decodes the bitstream. The resulting audio-waveforms are output over an HDMI 1.1 link, or a set of discrete analog audio outputs. 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).
• Transcoded legacy AC-3 bitstream – Legacy AC-3 is supported by virtually all consumer surround sound equipment, and provides a consistent way to deliver an approximation of the DD+ source. All DD+ decoder implementations are required to support the legacy AC-3 output mode, which supports up to 5.1 channels. The transcoding operation may introduce distortion and artifacts not present in the original program, but the resulting DTS/AC-3 bitstream can be transmitted via a standard S/PDIF/TOSLINK cable (or HDMI 1.1). 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 practice, most HD DVD titles have been authored with 'mixer' enabled, which requires players to combine multiple audio-sources. Since audio-mixing can only be done in the LPCM domain, such HD DVD titles force all players to a full end-to-end decompression/recompression cycle.

Of the above methods, only the decoding method is mandatory for all decoder implementations, though most decoder implementations with SPDIF/TOSLINK output also support the 3rd method. Thus far, delivery methods (of DD+) have favored the 2nd and 3rd methods. Beyond basic compliance with the DD+ specification, some models of HD DVD players can also transcode DD+ to DTS-audio. DTS is less compressed than AC-3 (1536 kbit/s vs. 640 kbit/s), making it preferred by some consumers.

Blu-ray players do not downmix DD+ content to AC-3, for the simple reason that the Blu-ray format mandates the DD+ audiotrack must be accompanied by a separate, core AC-3 bitstream. Furthermore, the Blu-ray spec allows the disc's individual audiotracks to be output directly (without modification/decompression). Hence, a DD+ Blu-ray title always gives the user the option to play the core AC-3 audiotrack, ensuring compatibility with all Blu-ray players.

References