Dolby Digital uses the same AC-3 compression system on both cinema and home formats, but the home formats typically run at higher data rates (320kb/s for cinema, 384kb/s and 448kb/s for DVD). Dolby Digital uses a copy protection system which prevents home Dolby Digital decoders from playing the DD bitstream off a 35mm print.

While DTS theatrical is actually a 5.0 format with the LFE in the surround channels (the advantage is even milder data compression levels on the 820 kilobit per second track), the home theater version uses a totally different compression format, a wider range of data rates, full range surrounds, separate subwoofer channel and the added option of DTS-ES Discrete 6.1.

The point with all this is that many DVDs don't sound very much like their theatrical counterparts. On action films the bass and surrounds usually seem more pumped up. But on some dramas and comedies the audio track may be more conservative than the previous theatrical release.

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Audio data reduction is not that simple. Just because you are missing x% of the bits does not mean you are missing x% of the audio signal. For instance, Delta PCM is a completely lossless coding scheme that has only 1/2 the number of bits recorded. It works by coding the difference between the current sample and the next sample. Upon decoding, 100% of the original information is there.

Dolby's AC-3 codec (and I believe DTS' Coherent Acoustics codec also) is a transform codec. The time domain PCM information is recorded as a set of frequency domain fourier transforms. This step alone gives a large amount of compression. For the most part with AC-3 data is not just thrown away. The number of bits used to code a forier coefficient is varied based on psychoacoustic masking techniques, ie. a loud 1 khz sound on the center channel will mask a soft 1 khz sound on the right channel so the right channel sound is given less bits (more noise) when it's coded. After this bit sharing is complete, some other techniques are used to get further reduction when needed and as a last resort some info is thrown away completely.

As to the original question, theatrical DTS and DD probably sound a little different. As to which sounds better, that's a tough one. Maybe somebody could design a blind test setup and invite some people to rate the two systems in the same theatre. I believe that Dolby did this once with an audience and the results came out with most people prefering the sound of Dolby Digital. Just the fact that there is a debate (when theatrical DTS is lossless compression) proves that 320 kbps AC-3 is not missing >90% of the PCM information when decoded.

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Dolby Digital uses a Cosine Transform encoder and SDDS uses what they call as Adaptive Transform encoder. Increased levels of compression are gained when you take an audio sample and turn it 90 degrees out of phase. Unfortunately, you also gain abberations in the audio and permanently lose even more of the original audio detail. Psychoacoustical masking has to be used to devote more bits and volume intensity levels to particular channels to mask out others in the background that listeners may not be able to hear anyway. Generally, the process works pretty well, but at low bitrate levels certain kinds of audio (music in particular) really fall apart.

Sony's MiniDisc format uses Consumer ATRAC encoding, which is similar to the Professional ATRAC encoding used in SDDS. MiniDisc recordings often sound pretty thin and trebly compared to their PCM Music CD counterparts.

Home DTS uses transform encoding, bitpooling and other "more advanced" techniques than its theatrical waveform counterpart. However, DTS operates at high enough bit rates that items like music material often sound better than a DD 5.1 counterpart. The best DTS material on DVD is usually encoded at the 1509kb/s bitrate instead of the lower 754kb/s data rate. Unfortunately, many DVD content creators think 754kb/s is good enough. On some movies yes, but for music oriented stuff I say you have to go with 1509kb/s. DTS music CDs use a 1234.8kb/s data rate. DTS Theatrical operates at only one fixed data rate, 820kb/s.

Digital Cinema often features uncompressed 20-bit Linear PCM 5.1 audio, and I think that is one of the only really good things about such setups at this point. The sound format is a no-compromise solution. It is just too bad the same cannot be said for the picture.

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