Dolby TrueHD - An Advanced Lossless Multi-channel Audio Codec

What is TrueHD?

Dolby TrueHD is an advanced lossless multi-channel audio codec developed by Dolby Laboratories which is intended primarily for high-definition home-entertainment equipment such as Blu-ray Disc and HD DVD. It is the successor to the AC-3 Dolby Digital surround sound codec which was used as the audio standard for DVD discs. In this application, Dolby TrueHD competes with DTS-HD Master Audio, another lossless codec from DTS.

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 Dolby 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, Dolby 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 is a variable bit-rate codec.

TrueHD on Blu-ray Disc

In the Blu-ray specification, Dolby TrueHD is an optional codec. Dolby TrueHD audiotracks may carry up to 24 bit audio in up to 8 discrete audio channels at 96 kHz or up to 6 channels (5.1 surround) at 192 kHz. The maximum encoded bitrate is 18 Mbit/s (the same as the uncompressed rate). Since TrueHD is an optional codec, a separate (primary) AC-3 audiotrack must be present on Blu-ray discs that contain a TrueHD track.

TrueHD on HD DVD

In the HD DVD format, Dolby TrueHD was a mandatory codec. The HD DVD specification permitted the sole (primary) audiotrack of a movie to be encoded in Dolby TrueHD, and the specification requires support for up to 2 channel (stereo) decoding. In practice all HD DVD players decoded multichannel (5.1) programming, and of the few HD DVD movies to be encoded with Dolby TrueHD audiotracks, all were multichannel.

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Related External Source

TrueHD - Full TrueHD explanation on Wikipedia.

Blu-ray Disc - Blu-ray Disc (official abbreviation BD) is an optical disc storage medium designed to supersede the DVD format.

About the Author

Donna Peng@DonnaPeng

Donna Peng's fascination with multimedia began at an early age - shortly after she licked the physical disc and then she's been obsessed ever since. Her decade-long career at Digiarty after the graduation has seen her unmatched expertise in the field of DVD, digital video, software and anything related to home theatre. She is currently fascinated with photography.