| Dolby TrueHd | |
|---|---|
| Filename extension | .thd |
Dolby TrueHD is alossless,multi-channel audiocodec developed byDolby Laboratories forhome video, used principally inBlu-ray Disc and 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)lossy surround 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 rate of up to192 kHz andsample depth of up to 24 bits. Dolby'scompression mechanism for TrueHD isMeridian Lossless Packing (MLP); prior to Dolby TrueHD, MLP was used for theDVD-Audio format, 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'sbitstream carries 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 normalization ordynamic range compression. In addition,Dolby Atmos, a multi-dimensional surround format encoded using Dolby TrueHD, can embed more advanced metadata to spatially placesound objects in an Atmos-compatible speaker system.
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) at192 kHz.[1] The maximumbitrate of an audio stream including metadata is18 Mbit/s (instantaneous, since it is variable bitrate),[2] and a TrueHD frame is either 1/1200 seconds long (for 48000, 96000 or 192000 Hz) or 1/1102.5 seconds long (for 44100, 88200 or 176400 Hz). Any Blu-ray player orAV receiver that can decode TrueHD can also downmix 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 astereo output) by merging discrete channels' signals (except thelow-frequency effects channel, 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. 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 (or offered).
Dolby TrueHD's prominence relative to DTS-HD MA began to decline around 2010.[3] It has experienced a mild resurgence as the encoding used forDolby Atmos audio (especially inUltra HD Blu-ray titles),[4] but DTS-HD MA is still more common on titles with non-Atmos lossless audio. Regardless, publishers such asParamount Home Entertainment[5][6][7] andCrunchyroll[8][9][10][11] still use Dolby TrueHD for their non-Atmos releases.Universal Pictures Home Entertainment has recently used Dolby TrueHD on occasion.[12][13][14]
Audio encoded using Dolby TrueHD may be transported to A/V receivers in one of three ways depending on player and/or receiver support:[15][16]
BecauseS/PDIF does 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 mixing the TrueHD track down to stereo.