Today, many theaters boast digital surround-sound systems. Digital sound works on a very different principle from analog sound systems.
In analog recordings, sound is encoded as a long, fluctuating stream of information. In digital recordings, sound is encoded as a series of 1s and 0s, just like a computer program. With this approach, you can encode a lot more information in a limited space, making for crisper, more precise audio tracks. (See How Analog and Digital Recording Works for details.)
Digital theater sound was introduced to the public with the release of "Jurassic Park" in 1993. "Jurassic Park" used a technology called DTS Digital Sound®, named for Digital Theater Systems, the company that patented the process.
In this sound system, six separate audio channels are encoded onto one or two CDs. The theater is equipped with a CD player and a decoder that splits these channels up and plays them on different speakers arranged throughout the theater. As in Dolby Stereo, DTS has three front sound channels and a subwoofer. But instead of a single surround channel, it has separate channels for speakers on the left side of the theater and speakers on right side of the theater.
The CD is synchronized with the picture by a special time code on the film. The code, a series of dots and dashes along the side of each frame, is read by a special optical reader mounted on the projector. The reader shines light on the film with a light-emitting diode (LED). The light that passes through the film hits a small photocell. The photocell sends pulses of current representing these flashes of light to the DTS processor. The dash pattern corresponds to a pattern encoded onto the CD. The processor makes sure the two codes are synchronized so that the sound and picture fit together. (See How Movie Sound Works for more information.)