Dolby Digital Plus
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Dolby Digital Plus (DD+), also known as E-AC-3, is an audio compression system that was developed specifically for the introduction of HDTV and HD DVD/Blu-ray Disc. It is a development of the Dolby Digital AC-3 audio compression system. It supports data rates up to 6.144 Mbit/s, compared to 0.640 Mbit/s for the original Dolby Digital AC-3 allowing for higher resolution audio to be carried at higher quality. It can carry 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.
The new compression algorithm is not directly backward compatible with AC-3 — existing AC-3 decoders will not be able to decode E-AC-3 streams — but DD+ capable equipment is required to be able to convert E-AC-3 streams to 0.640 Mbit/s AC-3 for compatibility with existing Dolby Digital A/V processors. [1]
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[edit] Codec changes
The codec used by Dolby digital plus is based on the original AC-3 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 converted to AC-3 for backwards compatibility. [2]
[edit] Technical details
- 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, 48, or 96 kHz
- Bit depth: up to 24 bits per channel.
[edit] Physical transport
As of 2006, HDMI 1.3 is the only means to transport a DD+ stream between two pieces of consumer equipment. The older and more widespread S/PDIF-interface cannot provide the sufficient bandwidth for a DD+ stream (up to 6144 kbit/s). See the section below on downmixing.
[edit] 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 sole) audio track, while an HD DVD player is required to decode DD+ audio (among other forms of audio.) Since DD+ is allowed as a primary audio track, the bitstream is stored as an independent, self-contained unit, with no companion "core" (legacy) AC-3 audio track. HD DVD allows DD+ bitrate up to 3 Mbit/s, which is short of Dolby's maximum defined bitrate of 6 Mbit/s.
On Blu-ray Disc, DD+ is an optional codec. If DD+ is present on a Blu-ray movie, a "core" AC-3 bitstream must accompany it. The AC-3 5.1 core is encoded at 640 kbit/s, with a separate DD+ "extension" bitstream. Both the core AC-3 audio-program and extended DD+ program are user-playable. For 5.1 channel sound, the complete DD+ program uses both the AC-3 core, plus an additional 1.024 Mbit/s 5.1 channel extension packet, yielding a total bitrate of 1.7 Mbit/s.[3]
The maximum number of discrete coded channels is the same for both formats: 7.1 [4]
[edit] Media players and Downmixing
HD DVD players are required to mix interactive-audio with program (pre-recorded) audio from the disc. Furthermore, as of 2006, DD+ can only be transported over an HDMI v1.3 link. Consequently, current players internally decode DD+ to linear PCM. All current players output the decoded and re-mixed audio as LPCM to HDMI, and some will output to discrete analog line-out.
HD DVD players can also transcode the original DD+ audio-program to DTS/AC-3, which are both well supported in mainstream equipment. The transcoding process is performed in realtime, and thus may introduce some distortion and artifacts not present in the original program. However, the resulting DTS/AC-3 bitstream can be sent via a standard S/PDIF/toslink cable (or HDMI.) Thus, the consumer gains access to (re-mixed) HD DVD audio tracks that are acoustically superior to their DVD/AC-3 rendition, on a legacy AC-3/DTS audio receiver.
Common downmix formats:
- Multi-channel analog line-out
- Bitstream (AC-3/5.1 or DTS/5.1)
- Linear PCM
Due to rapid changes in HDMI in the last few years, not all receivers and processors can handle audio over HDMI properly or at all.
[edit] References
- ^ Dolby Digital plus FAQ at dolby.com
- ^ Dolby Digital Plus technical overview at dolby.com
- ^ Dolby Digital plus True HD whitepaper at www.dolby.com
- ^ Dolby Digital plus HD promo at www.dolby.com
