"High Definition" Audio?

If you're an old hand at this home theater audio business, you know that both Dolby Digital and DTS first appeared in theaters, then on laserdiscs, and finally moved on to DVD. Because of the limited data space for audio on all of these delivery systems, the audio had to be heavily compressed—not in dynamic range (a common misconception) but to reduce the space it takes up on the film or disc. Both DTS and Dolby Digital use sophisticated encoding schemes to allow them to save space by discarding data that are not deemed audible. This "perceptual coding," together with other clever tricks, allow full-bodied, powerful sound to be squeezed into that itty-bitty living space.

It's remarkable how good the two formats can sound. DTS has even been used on CDs to carry high quality multichannel music. Because of that, it has gained a durable reputation as the best sounding of the two formats. I never completely agreed with that assessment, because it was based largely on projection based on DTS' higher data rate, and ignored fact that the coding schemes were completely different.

Furthermore, in order to be included on DVDs along with the essentially mandatory Dolby Digital, the DTS data rate on all DVDs containing a DTS track (still a small fraction of all DVDs released), apart from a few early titles, was slashed to half the original DTS data rate of 1.5Mbps. It was the original, higher data rate that gave DTS its sterling reputation, and I always felt that it lost something when it chopped that rate in half. But if it hadn't done so it would have failed in the DVD market.

Currently, the "which sounds better" argument is moot because all recent DVDs with multichannel DTS soundtracks also include a 5.1 Dolby Digital version. You can choose for yourself which you prefer.

But many audiophiles remain skeptical of any system in which data are discarded prior to playback. To answer that concern, DTS has now developed DTS-HD Master Audio. Dolby counters with Dolby TrueHD. Both formats claim to be lossless, but take up considerably less data space than PCM, the ancient digital format still used on CDs.

At the recent Las Vegas CES both Dolby and DTS were suggesting that these new formats might just show up in the upcoming HD DVD and Blu-ray high definition optical discs. Both companies put on elaborate demonstrations of their respective challengers. DTS had the upper hand with a larger room, a far superior sound system, and impressive video to add icing to the cake. Dolby, in a small prefab booth on the main floor, seemed more interested in distracting the audience with flashing lights and bionic chairs. But neither company made any real attempt to demonstrate the supposed superiority of their system to their standard formats—a challenge perhaps impossible amid the rattle and hum of a major trade show.

DTS-HD Master Audio (a name that's probably too long for its own good) can deliver sound at a data rate of up to 24Mbps on Blu-ray, carrying 7.1-channels of audio claimed to be bit-for-bit copies of the studio master. DTS-HD Master Audio recordings will play back at up to 1.5Mbps (that original DTS data rate) on conventional DTS-capable gear. But new decoders will be required in AV receivers and pre-pros to get the full quality available from the new format, which also supports sampling frequencies and bit depths up to 192kHz/24-bit.

Dolby TrueHD supports data rates of up to 18Mbps (I can already hear DTS acolytes shouting about the superiority of DTS yet again based on the more-bits-are-better paradigm). It is said to support up to 8 discrete channels of 24-bit/96kHz audio. TrueHD is a mandatory feature on HD DVD (which does not mean that every HD DVD must have a multichannel Dolby TrueHD track). It's optional on Blu-ray. Dolby TrueHD will require a v1.3 HDMI connection (a version of HDMI still under development). As with DTS-HD, bit-for-bit duplication of the master is claimed.

While Dolby states that a lossless recording always sounds like the source, recall that the PCM system used for CDs is also "lossless," and the last time anyone seriously claimed that CD produced perfect sound forever was 1984. Just a little reality check.

Naturally, the higher the data rate of both systems will require more space on the disc. How this relates to the length limitations of the recording will depend on a number of factors, including the type of compression used to encode the video and how much space that will demand. We can't yet predict if either format might be used for the soundtrack of a full length feature film, or merely for shorter music only recordings or music videos.

It promises to be an interesting year for both video and audio.

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