Dolby TrueHD
Dolby TrueHD is a lossless multi-channel audio codec developed by Dolby Laboratories which is used in home-entertainment equipment such as Blu-ray Disc players and A/V receivers. It is one of the successors to the Dolby Digital (AC-3) surround sound codec, which is used as the audio standard for the DVD-Video format. In this application, Dolby TrueHD competes with DTS-HD Master Audio, a lossless codec from DTS.
Dolby TrueHD uses Meridian Lossless Packing (MLP) as its mathematical basis for compressing audio samples. MLP is also used in the DVD-Audio format, but details of Dolby TrueHD and the MLP Lossless format as used on DVD-Audio differ substantially. A Dolby TrueHD bitstream can carry up to 16 discrete audio 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 and provides control over the decoding process. For example, dialog normalization and dynamic range compression are controlled by metadata embedded in the Dolby TrueHD bitstream. Dolby TrueHD is a variable bit-rate codec.
Blu-ray Disc
In the Blu-ray Disc specification, Dolby TrueHD is an optional codec. Dolby TrueHD audiotracks may carry up to 8 discrete audio channels (7.1 surround) of 24-bit audio at 96 kHz or up to 6 channels (5.1 surround) at 192 kHz.[1] The maximum encoded bitrate is 18 Mbit/s. Since Dolby TrueHD is an optional codec, a companion Dolby Digital bitstream (2.0 192kbs or 5.1 448kbs) must accompany the Dolby TrueHD bitstream on Blu-ray discs. Blu-ray Disc players consider this combination of two audio bitstreams as a single logical audio track, and a Blu-ray Disc player will automatically select the Dolby Digital or Dolby TrueHD bitstream depending on its decoding and/or audio bitstream output capabilities.
All Dolby TrueHD-enabled Blu-ray Disc 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. Some early Blu-ray players did not support Dolby TrueHD coding, and in some cases, the player would resample it as standard Dolby Digital. All current Blu-ray players support Dolby TrueHD, along with the ability to upmix the source to a higher resolution format (Bitstream or LPCM), or downmix to a lower channel format depending on the player or equipment's requirements. However, by late 2009 and early 2010, Dolby TrueHD had been steadily losing the lossless audio market share to rival DTS-HD Master Audio.[2]
Transport
Audio encoded using Dolby TrueHD may be transported to A/V receivers in one of three ways depending on player and/or receiver support:[3]
- 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 Dolby TrueHD bitstream, with decoding and DAC both done by the AV receiver.
References
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