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The purpose of this page is to explain THX, DTS, and Dolby Digital in the context of movie theatres. These terms are relevant to home theatre systems as well, but there are important differences. A future page or set of pages on home theatre systems may be created that will explain these terms in the home theatre environment. For more information about these companies, visit their WWW sites:
First, people tend to put THX, DTS, and Dolby Digital into one "basket", and attempt to compare them. I am going to try to eliminate some of the confusion that results when people do this.
I have heard these types of (incorrect) comments on many occasions:
First of all, THX is not a sound format. DTS and Dolby Digital are sound formats. By that, I mean that DTS and Dolby Digital are methods of encoding the movie audio digitally and reading it and decoding it in the movie theatre. THX is not a method of encoding sound, but has to do with quality certification. Nearly any THX theatre will use Dolby Digital or DTS as the sound format. SDDS is a sound format from Sony that is also used, but since none of the theatres in Huntsville or Decatur currently use it, I will not go into it.
THX is a set of quality certification standards developed by Lucasfilm that attempt to ensure that the movie's picture and sound will be reproduced in the theatre in a similar manner to the way it was in the mixing studio. The standards involve speaker positions, auditorium acoustics, and image brightness, among other things. A theatre advertising THX must meet the standards and pay a yearly licensing fee to Lucasfilm.
It is not necessarily true that any given THX theatre is better than any given non-THX theatre. A theatre may be built with exceptional equipment and may have an environment that meets or exceeds THX specifications, but may choose not to spend the money to get tested and licensed. When the THX advertisement is used at a theatre, it simply means that the theatre met the THX specifications when inspected, and Lucasfilm was paid the licensing fee by the theatre. At the time of this writing, the actual THX specifications and the details of recertification are proprietary and are not available to the general public, making it difficult to analyze or comment on THX quality certifications.
Before talking about the digital sound formats, it is necessary to understand the standard optical audio records that have been used on film for a very long time. The optical audio records work based on how much light passes through the soundtrack as the film moves along. It is read by shining light through the soundtrack and using a solar cell device to convert the light into an electrical audio signal. Originally, the sound was monophonic, but stereophonic sound was made possible by dividing the original soundtrack into two soundtracks and using separate solar cells for each. After stereo was made possible, Dolby developed the matrixed Dolby Stereo format, which was introduced with Star Wars in 1977. The matrixed format encoded 4 channels (left, center, right, and rear) of information into 2, and this format is very similar to the Dolby Surround and Dolby Pro-Logic systems that have been with us for home theatres for quite a while. The theatrical version uses Dolby A or SR instead of Dolby B noise reduction on the rear channel signal.
DTS is a digital sound format, and it stands for Digital Theatre Systems, which is the company that developed the format. DTS was the first digital sound format to be widely used in theatres (starting with Jurassic Park, 1993). DTS is a "5.1" format, meaning that it has 5 full-bandwidth channels (left, center, right, left surround, and right surround) and a low-bandwidth subwoofer channel. DTS, unlike the PCM 2-channel stereo format found on common compact discs, uses a lossy compression method to encode the 5.1 channels of audio into a reasonable size that can be stored on compact disc media and played back from that media in the theatre. Films with DTS sound have a time-code track on the film between the image area and the optical audio records. The information on the time-code track is sent to a computer which then reads the compact disc media to get the compressed DTS sound information. There are three CD-ROM drives, one for a trailer disc, and two for the movie sound. The trailer disc contains the DTS soundtracks for all of the previews being shown in theatres. During a movie, if for any reason the DTS information cannot be read, the standard optical audio records are used as a backup.
Dolby Digital is, of course, a digital sound format from Dolby Laboratories. It is a 5.1-channel format, like DTS. The differences in Dolby Digital and DTS are in the encoding method, and the way that the digital information is stored. Dolby Digital compresses the sound information using a different algorithm than DTS. Like DTS, it is a lossy compression method, but the compression ratio is much higher for Dolby Digital, meaning that the Dolby Digital soundtrack requires less data to store than the DTS soundtrack. Dolby Digital soundtracks are placed between the perforations on the film, and are read by a reader that captures the digital information and passes it to the Dolby Digital decoder. Like DTS, the standard optical audio records are used as a backup in case the Dolby Digital audio information cannot be read.
DTS and Dolby Digital are both better in quality than the standard optical audio records with a Dolby Stereo soundtrack. Whether DTS is better than Dolby Digital or Dolby Digital is better than DTS is one of those "religious debates" that I prefer to not get into.