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. Similarly, a Dolby Atmos encoded Dolby TrueHD stream contains metadata to extract and place the objects in relevant positions. Like all compressed lossless codecs, Dolby TrueHD is a variable bit-rate codec.
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) and 20 objects of 24-bit audio at 96 kHz or up to 6 channels (5.1 surround) at 192 kHz. The maximum encoded bitrate is 18 Mbit/s. Since Dolby TrueHD is an optional codec, a companion Dolby Digital bitstream (2.0 192 Kb/s or 5.1 448 Kb/s/640 Kb/s) 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 bitstreaming capabilities.
All Dolby TrueHD-enabled Blu-ray Disc players are capable of decoding the 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) mix from a 6-channel source audio track.
The most popular Dolby TrueHD application is as a high definition audio codec for Blu-ray Disc, although early Blu-ray players did not support it. All current Blu-ray players support Dolby TrueHD decoding or bitstreaming. Since 2010 however, Dolby TrueHD steadily lost its lossless audio market share to rival DTS-HD Master Audio.