Dolby TrueHD is a
lossless,
multi-channel audio
codec
A codec is a computer hardware or software component that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder.
In electronic communications, an endec is a device that acts as both an encoder and a decoder o ...
developed by
Dolby Laboratories
Dolby Laboratories, Inc. (Dolby Labs or simply Dolby) is a British-American technology corporation specializing in audio noise reduction, audio data compression, audio encoding/compression, spatial audio, and high-dynamic-range television (H ...
for
home video
Home video is recorded media sold or Video rental shop, rented for home viewing. The term originates from the VHS and Betamax era, when the predominant medium was videotapes, but has carried over to optical disc formats such as DVD and Blu-ray. ...
, used principally in
Blu-ray Disc
Blu-ray (Blu-ray Disc or BD) is a Digital media, digital optical disc data storage format designed to supersede the DVD format. It was invented and developed in 2005 and released worldwide on June 20, 2006, capable of storing several hours of ...
and compatible hardware. Dolby TrueHD, along with
Dolby Digital Plus (E-AC-3) and
Dolby AC-4, is one of the intended successors to the
Dolby Digital (AC-3)
lossy
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
surround format. Dolby TrueHD competes with
DTS's
DTS-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 a
sampling rate
In signal processing, sampling is the reduction of a continuous-time signal to a discrete-time signal. A common example is the conversion of a sound wave to a sequence of "samples".
A sample is a value of the signal at a point in time and/or s ...
of up to 192 kHz 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
DVD-Audio (commonly abbreviated as DVD-A) is a digital format for delivering high-fidelity audio content on a DVD. DVD-Audio uses most of the storage on the disc for high-quality audio and is not intended to be a video delivery format.
The ...
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
A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.
A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...
carries program
metadata
Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive ...
, 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
Audio normalization is the application of a constant amount of gain to an audio recording to bring the amplitude to a target level (the norm). Because the same amount of gain is applied across the entire recording, the signal-to-noise ratio and ...
or
dynamic range compression
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is c ...
. 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 channel
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...
s (
7.1 surround) of 24-bit audio at 96 kHz, or up to 6 channels (
5.1 surround) at 192 kHz. The maximum
bitrate of 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). 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 (for example, a 7.1 track to a 5.1 output, or a 5.1 track to a
stereo
Stereophonic sound, commonly shortened to stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configurat ...
output) by merging discrete channels' signals (except the
low-frequency effects
The low-frequency effects (LFE) channel is a band-limited Audio signal, audio track that is used for reproducing deep and intense low-frequency sounds in the 3–120 Hertz, Hz frequency range.
This track is normally sent to a subwoofer—a ...
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 (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 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, 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. It has experienced a mild resurgence as the encoding used for
Dolby Atmos audio (especially in
Ultra HD Blu-ray
Ultra HD Blu-ray (4K Ultra HD, UHD-BD, or 4K Blu-ray) is a digital optical disc data storage format that is an enhanced variant of Blu-ray. Ultra HD Blu-ray supports 4K UHD (3840 × 2160 pixel resolution) video at frame rates up to 60 progre ...
titles),
but DTS-HD MA is still more common on titles with non-Atmos lossless audio. Regardless, publishers such as
Paramount Home Entertainment, and
Crunchyroll
Crunchyroll is an American Video on demand#Subscription models, subscription video on-demand Over-the-top media service, over-the-top Streaming media, streaming service owned by Sony, Sony Group Corporation. The service primarily distributes fi ...
still use Dolby TrueHD for their releases.
Universal Pictures Home Entertainment
Universal Pictures Home Entertainment LLC (UPHE) is the home video distribution division of Universal Pictures, an American film studio owned by NBCUniversal, the entertainment unit of Comcast.
UPHE is the home video distributor for all of the ...
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:
[ Thursday, 19 January 2017]
*Over 6 or 8
RCA connectors as analog audio, using the player's internal decoder and
digital-to-analog converter
In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.
DACs are commonly used in musi ...
(DAC).
*Over
HDMI
High-Definition Multimedia Interface (HDMI) is a proprietary digital interface used to transmit high-quality video and audio signals between devices. It is commonly used to connect devices such as televisions, computer monitors, projectors, gam ...
1.1 (or higher) connections as 6 or 8-channel
linear PCM
Pulse-code modulation (PCM) is a method used to digitally represent analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitud ...
, 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.
Because
S/PDIF
S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable using RCA connector, RCA or BN ...
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.
References
External links
Official website
{{Compression Software Implementations
Blu-ray Disc
Dolby Laboratories
HD DVD
Lossless audio codecs
Surround sound