Dolby TrueHD

Dolby TrueHD logo as introduced in 2012.

Dolby TrueHDis alossless,multi-channelaudiocodecdeveloped byDolby Laboratoriesforhome video,used principally inBlu-ray Discand compatible hardware. Dolby TrueHD, along withDolby Digital Plus(E-AC-3) andDolby AC-4,is one of the intended successors to theDolby Digital(AC-3)lossysurround format. Dolby TrueHD competes withDTS'sDTS-HD Master Audio(DTS-HD MA), another lossless surround sound codec.

The Dolby TrueHD specification provides for up to 16 discrete audio channels, each with asampling rateof up to 192 kHz andsample depthof up to 24 bits. Dolby'scompressionmechanism for TrueHD isMeridian Lossless Packing(MLP); prior to Dolby TrueHD, MLP was used for theDVD-Audioformat, although the two formats' respective implementations of MLP are not mutually compatible. A Dolby TrueHD audio streamvaries in bitrate,as does any other losslessly compressed audio format.

Like its predecessor, Dolby TrueHD'sbitstreamcarries programmetadata,or non-audio information that a decoder uses to modify its interpretation of the audio data. Dolby TrueHD metadata may include, for example,audio normalizationordynamic range compression.In addition,Dolby Atmos,a multi-dimensional surround format encoded using Dolby TrueHD, can embed more advanced metadata to spatially placesound objectsin an Atmos-compatible speaker system.

Blu-ray Disc

In the Blu-ray Disc specification, Dolby TrueHD tracks may carry up to 8 discreteaudio channels(7.1 surround) of 24-bit audio at 96 kHz, or up to 6 channels (5.1 surround) at 192 kHz.The maximumbitrateof an audio stream including metadata is 18 Mbit/s (instantaneous, since it is variable bitrate),and a TrueHD frame is either 1/1200 seconds long (for 48000 Hz, 96000 Hz or 192000 Hz) or 1/1102.5 seconds long (for 44100 Hz, 88200 Hz or 176400 Hz). Uncompressed (LPCM) it can be >35 Mbit/s. Any Blu-ray player orAV receiverthat can decode TrueHD can also mix a multi-channel TrueHD track into any smaller amount of channels for final playback (for example, a 7.1 track to a 5.1 output, or a 5.1 track to astereooutput) by merging discrete channels' signals (except thelow-frequency effectschannel, the ".1," in a stereo mixdown, which is discarded due to its sound not playing back well without a dedicatedsubwoofer).

Dolby TrueHD is an optional codec, which means that Blu-ray hardware may decode it, but also may not (for example, inexpensive or early players, Blu-ray computer software, or pre–Blu-ray AV receivers). Consequently, all Blu-rays that include Dolby TrueHD audio also include a fail-safe track ofDolby Digital(AC-3), a mandatory codec. Unlike the competingDTS-HD Master Audio,which encodes its primary (optional) 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. Similarly 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 (or offered).

Dolby TrueHD's prominence relative to DTS-HD MA began to decline around 2010.It has experienced a mild resurgence as the encoding used forDolby Atmosaudio (especially inUltra HD Blu-raytitles),but DTS-HD MA is still more common on titles with non-Atmos lossless audio. Regardless, publishers such asParamount Home Entertainment,Funimation,andCrunchyrollstill use Dolby TrueHD for their releases.Universal Pictures Home Entertainmenthas 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 8RCA connectorsas analog audio, using the player's internal decoder anddigital-to-analog converter(DAC).
• OverHDMI1.1 (or higher) connections as 6 or 8-channellinear 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 encapsulated in MAT(Metadata-Enhanced Audio Transport) frames, with decoding and DAC both done by the AV receiver. This is the transport mode mandated by Dolby Atmos.

BecauseS/PDIFdoes not have sufficient bandwidth to carry a TrueHD bitstream, or more than two channels of PCM audio, using S/PDIF requires either falling back to a disc's Dolby Digital track or mi xing the TrueHD track down to stereo.

External links

• Official website