DTS, Dolby Digital and THX Sound - Difference Explained

Want to know the difference?? Here we go...


Dolby Digital

In 1965, an American physicist and engineer named Ray Dolby founded the Dolby labs in London. His idea was to develop noise reduction systems to improve sound quality for professionals and the general public alike. The Dolby name is now known throughout the world, and the surround sound standards he created are used both in movie theaters and at home. Here's a brief reminder of the two predecessors of the Dolby Digital flagship standard:

Dolby Surround: this has three channels, two for the front and one for the rear, with a bandwidth of 100 Hz to 7 kHz.
Dolby Pro Logic: this is an enhancement of Dolby Surround with four channels, including a center channel and two elements sharing a channel for the rear sound.
Dolby Digital 5.1, also called AC-3 (for Audio Code-3), has six channels - two front, two rear, one center and one for the subwoofer. Unlike Dolby Surround and Prologic, the bandwidth here ranges from 20 Hz to 20 kHz. The "5.1" refers to the five front and rear parts, plus the subwoofer, called LFE (for Low Frequency Effects), represented by the "1". The term AC-3 refers to a coding technology which eliminates sound data the user cannot hear and produces a Dolby Digital sound band coded on six channels. An absolute prerequisite for Dolby Digital sound is a decoder, such as the one on the Creative Labs Inspire 5700, or else a sound card like Audigy or Acoustic Edge, which decodes 5.1 sound and has the correct outputs.

There are also systems with just two speakers and a subwoofer which use Virtual Dolby Digital. These use a front channel mixing process to produce a virtual center channel; the rear channels are emulated by a processor which "virtualizes" surround sound using the HRTF filters via the front speakers.



Dolby Digital uses a fixed sound compression method of about 1:2. In other words, however much sound there is to encode, the compression will always be the same, because the compression algorithm has a constant output. Beyond this, there is a simultaneously positive (disk space) and negative aspect - negative because sound quality decreases as volume increases. But set against this is that the less space used on a DVD, the more space is available for different languages or bonuses, and this can compensate for a loss of quality. In general, AC-3 sound is coded in 18 bits, so the standard output of an AC-3 flow is 384 Kbps (6 channels x 18 x 48 kHz). On restitution, the Dolby Digital decoder transmits with a delay of one millisecond on the front channels because the listening position is usually closer to the rear speakers than the front or center ones. This optimizes simultaneous sound reception. With some decoders you can adjust this delay to get the best listening configuration.

The main advantage of this standard is the fact that it is the digital audio surround standard for DVD. In the rules defining the DVD standard, no other kind of digital sound band can be inserted on a DVD unless there is also a Dolby Digital-encoded sound band. So you'll never find a DTS sound band on its own. This leads to the second advantage of the standard - it is virtually universal. The first movie with a Dolby Digital 5.1 sound band was "Batman Returns" in 1992. Since then, practically all DVDs have perpetuated the standard.



DTS (Digital Theatre Sound)

DTS, for Digital Theater System, was introduced by Steven Spielberg with the release of "Jurassic Park" in 1993. So far, this standard applies more to the big screen than private homes. It, too, is 5.1, so the sound is coded over six channels as in Dolby Digital. There are now many DTS-compatible systems available. So surround systems, like sound cards, can decode the standard via software. However, while the excellent quality of DTS is undeniable, and even a bit higher than Dolby, remember that there are no movies available in DTS alone, and that Dolby is considered to be the digital sound standard and DTS is not.

The main feature of DTS is that its coding system favors sound quality over disk space. So a DTS sound band codes in 24 bits instead of the 18 bits with Dolby. Compression uses a dynamic process where the compression rate varies with the amount of sound to encode. This rate ranges from 1:1 to 1:40 and generally results in better sound quality than Dolby Digital with an average rate of 1.5 Mb/s. The main drawback is obviously that the sound band takes up much more space (about three times as much) than Dolby. So DVDs coded in DTS can only have one language and a limited number of bonuses. Because it is optional as a sound standard, there are not many DVDs with DTS on the market, though the number is growing.



THX LucasFilm Sound

THX is not a sound standard like DTS and Dolby Digital. It is a certification, a quality label created by Lucasfilm, designed to distinguish certain home cinema collections. The term THX is derived from "THX 1138", the first science fiction film by George Lucas, which has now become a cult movie. It was in 1986 that the daddy of Indiana Jones, Luke Skywalker and, later, Jar Jar Binks, began to develop the label, mainly for the big screen. This very demanding guy had noticed that cinema sound equipment produced sound of poor quality and that it varied depending on the movie theater. George reckoned that every single moviegoer had the right to expect the same sound with the same fidelity, regardless of the theater. And so, THX certification was born. It is now found in a great many movie theaters across the world.

Promedia by Klipsch and, more recently, Z-560 by Logitech opened the way for THX certification in multimedia sound systems. Though still marginal, it should not be long before these initials become a selling point. But while THX certification is obviously a sign of quality, it does not make one system better than another. There are systems of outstanding quality that match, or even exceed, the criteria set by Lucasfilm. It's just that their makers have not found it worthwhile, let alone necessary, to pay George Lucas to put their product on the THX test bench.

The certification involves three main criteria:

Sound quality. This is judged on the basis of bandwidth, frequency balance, consistency of front and rear speakers, sound position qualities and whether the system does actually comply with the output levels announced.
Interface ergonomics. This mainly means how easy it is to install and use.
Manufacturing quality. On a system with a THX certificate, the two front speakers and the center speaker are usually identical and dipolar (two diaphragms). The sound emitted should be directed towards the listener and not bounce off the floor or ceiling. The bandwidth should not be below 80 Hz, so all THX systems must have a subwoofer.

Info from Tom's Hardware
Hope this helps explain it!

Chris