How Movie Sound Works

Digital Theater Systems

The first commercial use of digital sound on a large scale debuted with the release of "Jurassic Park." It is called DTS, an acronym for Digital Theater Systems, the name of the company that patented the process. At its essence, DTS is an updated version of the classic sound-on-disc technology used in the early days of cinema. DTS employs a special optical time code that is part of the film. The time code is a series of dots and dashes along the side of each frame between the image and the analog optical sound tracks.

A special optical reader is mounted on the projector. The film is threaded through the reader before it enters the projector. Similar to the analog audio pickup, the DTS reader uses a light-emitting diode (LED) to focus light on a lens, through the film and onto a photocell. This creates pulses of current that the reader decodes as the time code. It sends this information via a serial cable to a computer. The computer controls an audio system with three CD players. The movie soundtrack consists of six tracks (right, left, center, left-surround, right-surround and subwoofer) compressed on one or two CDs, depending on the length of the movie. One CD holds about two hours of audio in the special compressed format used by DTS. The third CD player is used for a CD that contains current DTS movie previews.

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Both the film and the soundtrack CDs have an identifying code. The computer checks these codes to make sure that the correct soundtrack is played for the movie being shown. To make sure that the audio does not lag due to accessing the CD, the system buffers the audio in memory using the FIFO (first in, first out) method. Because the computer is constantly analyzing the timecode and matching the audio from the CD to it, the sound is seldom out of sync with the picture. And, since the sound is not actually encoded on the film, movie-goers don't hear that annoying "pop" that sometimes occurs when the audio pickup encounters a splice.

The downside to DTS is:

  • It requires additional steps in the production process to create the CDs.
  • DTS relies on additional equipment to operate.
  • The soundtrack CDs occasionally do not arrive at a theater with the film reels.

In the event of a failure of the DTS computer or CD players, the film still has the analog tracks. DTS Stereo, which is compatible with Dolby Stereo audio pickups, is the process used to create the analog tracks. As with all digital formats, the optical analog tracks are only used under certain conditions:

  • when there is no digital information (such as the local theater information or some previews)
  • when the digital format is incompatible with the local equipment (for example, a DTS film in a Dolby Digital setup)
  • when the digital equipment fails
  • when the digital information on the film is unreadable

DTS has lasted much longer than anyone expected. The original concept was viewed as a temporary solution while theaters made the transition to digital. But the ease of use, relatively low cost and the simple fact that many theaters have already made the investment in the format have combined to keep DTS a viable alternative to the sound-on-film digital formats.