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DTS-HD Master Audio

DTS-HD Master Audio (DTS-HD MA; known as DTS++ before 2004^[1]) is a multi-channel, lossless audio codec developed by DTS as an extension of the lossy DTS Coherent Acoustics codec (DTS CA; usually itself referred to as just DTS). Rather than being an entirely new coding mechanism, DTS-HD MA encodes an audio master in lossy DTS first, then stores a concurrent stream of supplementary data representing whatever the DTS encoder discarded. This gives DTS-HD MA a lossy "core" able to be played back by devices that cannot decode the more complex lossless audio. DTS-HD MA's primary application is audio storage and playback for Blu-ray media; it competes in this respect with Dolby TrueHD, another lossless surround format.

DTS-HD MA can store up to 8 discrete channels of audio (7.1 surround) at up to a 24 bit sample depth and 192 kHz sampling frequency (96 kHz for 6.1 or 7.1 surround).^[2] Although DTS-HD MA, and the related DTS-HD, allow virtually any number of channels in the abstract,^[3] these limits are imposed for practical reasons of limited storage and bitrate availability. A DTS-HD MA bitstream may have a bitrate no greater than 24.5 Mbit/s (instantaneous), of which no greater than 1.5 Mbit/s may be lossy DTS (as per the DTS CA specification).^[4]

The Blu-ray specification stipulates DTS-HD MA as an optional codec, which means that some Blu-ray hardware may not decode it. This is the reason for the bifurcated nature of a DTS-HD MA audio stream; DTS CA, unlike its MA extension, is mandatory, so a player that is not MA-capable can automatically default to an MA-encoded disc's base DTS stream and simply ignore the supplementary data.^[4] Alternatively, even if a player is MA-capable, the base stream may be needed for backward compatibility with an older AV receiver (for example, one manufactured during the DVD era).^[5]

DTS-HD MA is the encoding format for DTS:X, an object-based surround-sound format that competes with Dolby Atmos. A DTS-HD MA bitstream carrying DTS:X can contain up to 9 simultaneous sound objects, which are dynamically mapped to a user's speaker system during playback, unlike the rigid number and placement of speakers required by channel-based surround (a DTS marketing executive referred to DTS:X in an interview as "whatever.1").^[6]

DTS-HD Master Audio may be transported to AV receivers in 5.1, 6.1, or 7.1 channels, in full quality, in one of three ways depending on player and/or receiver support:^[7]

• Over 6, 7 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-, 7-, 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 DTS-HD Master Audio bitstream, with decoding and DAC both done by the AV receiver. (This is the transport mode required for DTS:X playback.)

S/PDIF (optical or coaxial) cannot carry linear PCM in more than 2 channels, and players connected this way typically output DTS-HD MA as either the lossy DTS core or downmixed stereo PCM.^[7]

The bitstream itself is not tied to HDMI, however: IEC 61937-5, the standard for carrying DTS bitstreams over the IEC 60958 (S/PDIF) interface, defines a packet type for DTS-HD (DTS type IV) with a selectable transmission rate.^[9] At 192 kHz, the highest sampling rate of S/PDIF, two 16-bit channels provide about 6.1 Mbit/s, compared with about 24.5 Mbit/s over HDMI's High Bitrate Audio mode (768 kHz); FFmpeg's IEC 61937 muxer implements DTS type IV output at a selectable rate.^[10] A DTS-HD MA stream can therefore be carried over S/PDIF as long as every frame fits within this limit. For 5.1 channels at 48 kHz and 24 bits (6.9 Mbit/s uncompressed), the lossless compression has to reduce each frame by at least about 11%; streams authored for Blu-ray, which may use up to 24.5 Mbit/s, are not guaranteed to fit. Whether a given player sends, or a given AV receiver accepts, DTS-HD over S/PDIF depends on the device.