Dolby TrueHD

Dolby TrueHD, commonly known in Chinese as 杜比TrueHD, is a lossless multichannel audio coding format developed by Dolby Laboratories. Based on Meridian Lossless Packing (MLP) technology, it entered the consumer market in the mid-2000s alongside high-definition optical discs such as HD DVD and Blu-ray. The PCM sample data decoded from it matches the encoder’s input bit-for-bit.

TrueHD reduces the volume of PCM data through prediction, inter-channel decorrelation, and entropy coding, without relying on the removal of signal components deemed inaudible. The compression ratio varies depending on the audio content, so the bitstream uses a variable bit rate. "Lossless" merely indicates that the decoded data is identical to the encoded input; it does not imply that the recording, mixing, mastering, or sampling specifications are superior to those of another release copy. This format uses hierarchical substreams to organize different audio channels and playback configurations. The bitstream can carry metadata such as various channel combinations, downmix information, dialogue level control, and dynamic range control, allowing the decoder to generate multi-channel or reduced-channel output based on the device’s capabilities. The sampling frequency range listed in Dolby’s published specifications is 44.1 to 192 kHz, with a maximum quantization bit depth of 24 bits.

In Blu-ray applications, the maximum bitrate for Dolby TrueHD is 18 Mbit/s. The format can carry up to eight 96 kHz, 24-bit full-range audio channels, or up to six 192 kHz, 24-bit full-range audio channels. Commercial film and television soundtracks are typically produced at 48 kHz, while music discs sometimes use 96 kHz or 192 kHz; the presence of the TrueHD logo alone does not indicate the actual sampling rate, bit depth, or number of channels.

Unlike the common “core plus lossless extension” structure of DTS-HD Master Audio, the TrueHD bitstream itself does not contain an AC-3 core for direct playback by standard Dolby Digital decoders. Blu-ray producers may include a separate Dolby Digital audio track or provide compatible audio within specific program structures to accommodate older devices and basic playback requirements. When analyzing media, the separate AC-3 stream and the TrueHD main stream should be recorded separately.

TrueHD can be multiplexed with video, subtitles, and other data in M2TS programs on Blu-ray and Ultra HD Blu-ray. HD DVD once specified it as one of its supported high-definition audio formats; following the discontinuation of that format, physical releases of TrueHD have remained primarily within the Blu-ray ecosystem. File containers can also store TrueHD audio streams, but the file extension alone does not indicate the internal audio format.

Supported players can decode TrueHD into multichannel LPCM for output via HDMI; alternatively, they can pass the compressed bitstream through HDMI to an amplifier or soundbar for decoding. Traditional optical and coaxial S/PDIF connections are not capable of transmitting full TrueHD; devices connected via these interfaces typically fall back to separate compatible audio tracks, transcoded lossy audio, or stereo PCM. The fact that a player displays “PCM” does not necessarily mean the source audio track has undergone lossy conversion, as lossless TrueHD is inherently PCM after decoding.

Dolby Atmos can be used alongside TrueHD as supplementary information on Blu-ray and Ultra HD Blu-ray discs. Atmos-compatible decoders read soundstage, object, or spatial metadata and render it according to the speaker layout; TrueHD decoders that do not support Atmos can still play compatible traditional multichannel representations. Therefore, a Dolby TrueHD audio track may include Atmos, or it may simply be a standard 2.0, 5.1, or 7.1-channel lossless program.

Dolby TrueHD, Dolby Digital, and Dolby Digital Plus are three distinct encoding formats. Dolby Digital and Dolby Digital Plus use lossy compression, while TrueHD uses lossless compression; Dolby Atmos is an immersive audio system that can be delivered via various Dolby codecs. When a player interface displays Dolby Atmos as the primary indicator, the underlying format must still be confirmed via media information or disc specifications.