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 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 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.
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 2 channels 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.
All Dolby TrueHD players are capable of downmixing the decoded Dolby TrueHD audiotrack to an arbitrary number of channels more suitable for player output. For example, all Dolby TrueHD-capable players can create a 2-channel (stereo-compatible) downmix from a 6-channel source audiotrack.
Dolby TrueHD sees its most popular usage as a high definition audio codec for Blu-ray Disc. While early Blu-ray Disc players didn't support Dolby TrueHD, many current models support Dolby TrueHD, and an increasing percentage of movie releases feature a Dolby TrueHD audiotrack.
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.
Dolby TrueHD may be transported to AV receivers in 5.1 or 7.1 channels, at full uncompressed quality, in one of three ways depending on player and/or receiver support:
- Over 6 or 8 RCA connectors as analog audio, using the player's internal decoder and digital-to-analog converter (DAC).
- Over HDMI 1.1 (or higher) connections as 6- or 8-channel Linear PCM, using the player's decoder and the AV receiver's DAC.
- Over HDMI 1.3 (or higher) connections as the original TrueHD bitstream, with decoding and DAC both done by the AV receiver.
- ↑ "HDMI Part 5 - Audio in HDMI Versions". 2006-08-09. http://www.hdtvmagazine.com/articles/2006/08/hdmi_part_5_-_audio_in_hdmi_versions.php.