Friday, December 14, 2007

Dolby TrueHD

Dolby TrueHD logo
Dolby TrueHD logo

Dolby TrueHD, from Dolby Laboratories, is an advanced lossless multi-channel audio codec, intended primarily for high-end home-entertainment equipment, such as Blu-ray Disc and HD DVD. In this application, Dolby TrueHD competes with DTS-HD Master Audio, another lossless codec from Digital Theater System.

Dolby TrueHD uses Meridian Lossless Packing (MLP) as its mathematical basis for compressing audio samples. MLP was used on the earlier DVD-Audio format, but details of TrueHD and DVD-Audio differ substantially. A Dolby TrueHD bitstream can carry up to 14 discrete sound channels. Sample-depths up to 24 bits/sample and audio sample-rates up to 192 kHz are supported. Like the more common legacy codec Dolby Digital, TrueHD bitstreams carry program metadata. Metadata is separate from the coding format and compressed audio samples, but stores relevant information about the audio waveform. For example, dialog normalization and Dynamic range compression are controlled by metadata embedded in the TrueHD bitstream.

TrueHD and high-definition optical disc

In the HD DVD format, TrueHD is a mandatory codec – all HD DVD players must support the extraction and decoding of a 2-channel (stereo) waveform from a TrueHD bitstream, although all HD DVD players currently support 5.1 decoding. HD DVD allows the sole (primary) audiotrack of a movie to be encoded in TrueHD, with no (other) secondary audiotracks present. In Blu-ray Disc, TrueHD is an optional codec, meaning that TrueHD may only be present on a disc which already contains a primary Dolby Digital soundtrack. The primary Dolby Digital audiotrack ensures all Blu-ray players, including those incapable of processing TrueHD, can access a playable audiotrack.

For both Blu-ray and HD DVD, TrueHD's capabilities are the same: the program may carry up to 8 discrete audio channels, at a sample depth & rate of 24-bit/96 kHz. The maximum (disc) encoded bitrate is 18 Mbit/s, although movie-titles (thus far) have remained below 5 Mbit/s. All TrueHD players are capable of downmixing the decoded TrueHD audiotrack to an arbitrary number of channels more suitable for player output. For example, all TrueHD-capable players can create a 2-channel (stereo-compatible) downmix from a 6-channel source audiotrack.

Connecting a TrueHD source to a TrueHD receiver requires a digital-link capable of transporting either the encoded bitstream (up to 18 Mbit/s), or the unpacked linear-PCM audio (>35 Mbit/s). HDMI 1.1 (and higher) can transport multichannel PCM-audio, and therefore can transport an unpacked TrueHD audiotrack. An HDMI 1.3 (or higher) link is required to transport TrueHD in raw bitstream form.[1] TOSLINK (and SPDIF) cannot carry TrueHD without transcoding, due to limitations of the specification.

HDMI-equipped players can internally decode TrueHD to LPCM, and output the LPCM over an HDMI 1.1 (or higher) interface; all HD DVD players currently support this.[2] If a player cannot internally decode TrueHD, it can transport the bitstream over HDMI 1.3 to a receiver capable of decoding it; this feature is now supported on the Samsung 1400 BD player with a firmware upgrade. HD DVD players can also transcode the TrueHD bitstream into a different legacy format (such as Dolby Digital or DTS), providing a high-quality approximation of TrueHD audio over a legacy TOSLINK cable for those that do not have HDMI.

The Phantom of the Opera, released April 18, 2006 on HD DVD, was the first movie to offer a TrueHD soundtrack.

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