Perhaps the most controversial question asked in the context of DVDs is: does DTS sound better than Dolby Digital? As in so many debates, there is the strong position and the weak one. The strong position is that, yes, of course DTS sounds better. The weak one comes from those who have listened to both and, often shamefacedly, admit that they can't tell any difference.
Naturally, DTS advocates are in the winning position because they can say that they have heard a difference. The poor old Dolby Digital defenders look like they have cloth ears. But before we delve too deeply into the debate, let's look at what the two formats are, and where they came from.
Neither was developed for DVD. Both were intended to provide 5.1 discrete channels of digital sound in cinemas on 35mm film while remaining fully compatible with standard equipment. Dolby placed its digital sound track as optical marking on the edge of the film, between the sprocket holes. This left Dolby with a bandwidth of around 400 kilobits per second. Six channels of uncompressed CD-quality digital audio requires ten times this capacity, so Dolby developed AC-3, an MPEG-type 'lossy' audio compression system.
Dolby Digital's first appearance was in 1992's Batman Returns, slightly pipping Jurassic Park's 1993 premiering of DTS.
The DTS sound track isn't on the film at all, but on a separate CD, synchronised with SMPTE time code markings on the film. Since CDs output around 1,400 kilobits per second of digital audio, DTS was designed to use this bandwidth (actually, due to different error correction techniques, full DTS produces 1,536 kilobits per second). This had the additional advantage of allowing easier distribution of other-language sound tracks, without the expense of having to prepare new film prints with dubbed dialogue. In their DVD incarnations, Dolby Digital and DTS are similar systems. Both use a lossy compression system and in their 5.1 varieties, each offers the DVD producer two bit rates. Most Dolby Digital DVDs use 384kb/s but some use 448kb/s, which Dolby says is the maximum possible on a DVD. DTS can come at the original rate of 1536kb/s or the half-rate of 768kb/s (eg. Gladiator, Santana: Supernatural Live and all of Columbia TriStar Superbit DVDs).
So which sounds better?
That's where the controversy arises. Dolby and DTS are only of slight help in this. They have been sniping at each other for a while. DTS says that of course it sounds better because its compression isn't as lossy as Dolby's. Dolby says that you can't draw any conclusions from the raw compression figures because it all depends on how well the codec (compression/decompression system) is designed. This makes sense. Sony's ATRAC3 sounds better than MP3 at similar compression rates. Still, we are talking about a big difference in compression levels here.
Dolby also says that the DTS 0.1 channel rolls off the bass by a few dB at the top end of the range. DTS counters that Dolby Digital's 0.1 channel imparts a huge phase shift due to its brick-wall low pass filter. Dolby says that the half-rate DTS 'maxes out at 15kHz' (later amended to a 3dB attenuation at 15kHz) while 384kb/s Dolby goes to 18kHz and 448kHz reaches 20kHz. DTS responds that the higher level Dolby system mixes the channels above 15kHz, and the lower level one as low as 10kHz.
Dolby says that it organised listening tests with professionals who preferred its system. DTS says that the tests ought to be independently conducted.
Dolby says DTS tracks often play louder than Dolby Digital ones due to the latter's dialogue normalisation feature (which is often set to reduce the playback level by 4dB) and an alleged 0.6dB boost in DTS's encoding of broadband material. DTS says that there is no 0.6dB boost.
And so on.
Reviewer opinions are varied, however the range is from DTS-is-better to they're-pretty-much-the-same. The more carefully conducted the trials, the greater the attention paid to removing non-codec related variables, the more equivocal the findings.
As for me, I find that there is very little, if any, difference. If you hear a DTS sound track sounding obviously better than a Dolby Digital one, you can be fairly confident that a different mix was used. For identically sourced audio, changing from Brand A of speakers to Brand B will change the sound a lot more than changing from the Dolby Digital to the DTS audio track.
© 2001 by Stephen Dawson