Dolby TrueHD
Dolby TrueHD is a lossless, multi-channel audio codec developed by Dolby Laboratories for home video, used principally in Blu-ray Disc and compatible hardware. Dolby TrueHD, along with Dolby Digital Plus and Dolby AC-4, is one of the intended successors to the Dolby Digital lossy surround format. Dolby TrueHD competes with DTS's DTS-HD Master Audio, another lossless surround sound codec.
The Dolby TrueHD specification provides for up to 16 discrete audio channels, each with a sampling rate of up to and sample depth of up to 24 bits. Dolby's compression mechanism for TrueHD is Meridian Lossless Packing ; prior to Dolby TrueHD, MLP was used for the DVD-Audio format, although the two formats' respective implementations of MLP are not mutually compatible. A Dolby TrueHD audio stream varies in bitrate, as does any other losslessly compressed audio format.
Like its predecessor, Dolby TrueHD's bitstream carries program metadata, or non-audio information that a decoder uses to modify its interpretation of the audio data. Dolby TrueHD metadata may include, for example, audio normalization or dynamic range compression. In addition, Dolby Atmos, a multi-dimensional surround format encoded using Dolby TrueHD, can embed more advanced metadata to spatially place sound objects in an Atmos-compatible speaker system.
Blu-ray Disc
In the Blu-ray Disc specification, Dolby TrueHD tracks may carry up to 8 discrete audio channels of 24-bit audio at 96 kHz, or up to 6 channels at. The maximum bitrate of an audio stream including metadata is , and a TrueHD frame is either 1/1200 seconds long or 1/1102.5 seconds long. Any Blu-ray player or AV receiver that can decode TrueHD can also downmix a multi-channel TrueHD track into any smaller amount of channels for final playback by merging discrete channels' signals.Dolby TrueHD is an optional codec, which means that Blu-ray hardware may decode it, but also may not. Consequently, all Blu-rays that include Dolby TrueHD audio also include a fail-safe track of Dolby Digital, a mandatory codec. Unlike the competing DTS-HD Master Audio, which encodes its primary track in terms of differences from the companion mandatory track, a Dolby TrueHD-equipped Blu-ray's primary and companion tracks are redundant; the Dolby TrueHD bitstream has no data in common with the AC-3 bitstream, but AC-3 is used to construct E-AC3 stream. Similar to DTS-HD MA, however, Dolby TrueHD's dual tracks are opaque to the user; a Blu-ray player loaded with a Dolby TrueHD disc will automatically fall back to AC-3 if it cannot decode or pass through the lossless bitstream, with no explicit selection required.
Dolby TrueHD's prominence relative to DTS-HD MA began to decline around 2010. It has experienced a mild resurgence as the encoding used for Dolby Atmos audio, but DTS-HD MA is still more common on titles with non-Atmos lossless audio. Regardless, publishers such as Paramount Home Entertainment and Crunchyroll still use Dolby TrueHD for their non-Atmos releases. Universal Pictures Home Entertainment has recently used Dolby TrueHD on occasion.
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:- Over 6 or 8 RCA connectors as analog audio, using the player's internal decoder and digital-to-analog converter.
- Over HDMI 1.1 connections as 6 or 8-channel linear PCM, using the player's decoder and the AV receiver's DAC.
- Over HDMI 1.3 connections as the original Dolby TrueHD bitstream encapsulated in MAT frames, with decoding and DAC both done by the AV receiver. This is the transport mode mandated by Dolby Atmos.