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 (AC-3) lossy surround format. Dolby TrueHD competes with DTS's DTS-HD Master Audio (DTS-HD MA), another lossless surround sound codec.

Dolby TrueHD logo

The Dolby TrueHD specification provides for up to 16 discrete audio channels, each with a sampling rate of up to 192kHz and sample depth of up to 24 bits. Dolby's compression mechanism for TrueHD is Meridian Lossless Packing (MLP); 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 (7.1 surround) of 24-bit audio at 96 kHz, or up to 6 channels (5.1 surround) at 192 kHz.[1] The maximum bitrate of an audio stream including metadata is 18 Mbit/s (instantaneous). Any Blu-ray player or AV receiver that 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 a stereo output) by merging discrete channels' signals (except the low-frequency effects channel, the ".1," in a stereo mixdown, which is discarded due to its sound not playing back well without a dedicated subwoofer).

Dolby TrueHD is an optional codec, which means that Blu-ray hardware may decode it, but also may not (especially in the case of very inexpensive players, Blu-ray computer software, or players manufactured during Blu-ray's infancy). Consequently, all Blu-rays that include Dolby TrueHD audio also include a fail-safe track of Dolby Digital (AC-3), a mandatory codec. Unlike the competing DTS-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. 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.[2] It has experienced a mild resurgence as the encoding used for Dolby Atmos audio (especially in Ultra HD Blu-ray titles),[3] but DTS-HD MA is still more common on titles with non-Atmos lossless audio.

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:[4][5]

  • 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 encapsulated in MAT[3] (Metadata-Enhanced Audio Transport) frames, with decoding and DAC both done by the AV receiver. This is the transport mode mandated by Dolby Atmos.[3]

References
  1. "Dolby TrueHD". Dolby Laboratories. Retrieved August 22, 2014.
  2. "DTS-HD Master Audio Becoming the Blu-ray Standard". Blu-raystats.com. January 15, 2010. Retrieved November 15, 2010.
  3. "Dolby Atmos for the Home Theater" (PDF). Dolby Laboratories. Archived from the original (PDF) on May 21, 2020. Retrieved 2018-06-29.
  4. Maestra, Rodolfo La (August 8, 2006). "HDMI Part 5 - Audio in HDMI Versions". HDTV Magazine. Archived from the original on May 12, 2008. Retrieved November 15, 2010.
  5. "HDMI versions". Thursday, 19 January 2017
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