What is Dolby Digital?

How Dolby Digital works in the cinema

olby Digital is a practical and cost-effective method of delivering digital sound to cinemas without the need for separate CDs. The six-channel digital soundtrack is printed directly onto 35 mm film in the form of data blocks in the areas between the perforations. A high-quality four-channel analog Dolby SR soundtrack is also provided on the same film, enabling playback in all cinemas.

The film is passed through the projector where the soundtracks are read by a sound reader. The sound reader consists of two light sources (LEDs) which shine light through the analog and digital soundtracks onto two light-sensitive devices which transform the light into electrical signals.

The digital signal is decoded by a Dolby Digital cinema processor into six channels of sound which are then routed through six power amplifiers and into the speakers situated around the cinema.

Unlike Dolby's analog four-channel systems, which use a single surround channel, Dolby Digital delivers sound to separate left and right surround channels through speakers mounted on the rear and side walls. There is also a separate subwoofer channel for low bass sounds.

What does "digital" mean?

Using the language of computers, digital audio consists of numbers (1s and 0s) representing the momentary strength of the sound sampled at very short time intervals. These numbers are reproduced on Dolby Digital soundtracks as small transparent and black pixels. In contrast, analog recordings use a medium with a continuously varying characteristic, such as the width of an analog film soundtrack.

Dolby Digital is here. With all the clarity and power you're used to from your CDs at home.
     A new kind of 35 mm film print delivers six channels of superb digital sound to put you right in the middle of the action. The improvements over analog which Dolby Digital can provide apply not just to loud dramatic effects such as crashes, explosions and planes passing overhead, but in much more subtle ways as well: clearer positioning of sounds at all times, cleaner dialogues, more lifelike atmospheres and quiet effects, and more accurate musical reproduction.
     Like all Dolby formats there are three screen channels (left, center, right), but instead of a single surround channel, there are separate left and right surround channels for an even greater sense of "being there," plus a subwoofer channel for low bass you can almost feel!
     Just like your CDs, the sound will stay free of snaps, crackles and pops for play after play after play, making a film sound just as impressive on the last day of its run as on the first.