Sound

Regardless of the sound format, any sound represented on the film image itself will not be the sound for the particular frame it occupies. In the gate of the projector head, there is no space for a reader. Consequently, all optical sound formats must be offset from the image because the sound reader is usually located above (for magnetic readers) or below (for optical readers) the projector head.

Optical

Optical sound constitutes the recording and reading of amplitude based on the amount of light that is projected through a soundtrack area on a film using an illuminating light or laser and a photocell or photodiode. As the photocell picks up the light in varying intensities, the electricity produced is intensified by an amplifier, which in turn powers a loudspeaker, where the electrical impulses are turned into air vibrations and thus, sound waves. In 16 mm, this optical soundtrack is a single mono track placed on the right side of the projected image, and the sound head is 26 frames after the gate. In 35 mm, this can be mono or stereo, on the left side of the projected image, with the sound head 21 frames after the gate.

Magnetic sound

Magnetic sound is no longer used, but between 1952 and the late '90s, was the standard for motion picture high fidelity sound because of its wider frequency range than optical sound. There are two forms of magnetic sound in conjunction with projection: double-head and striped.

The first form of magnetic sound was the double-head system, in which the movie projector was interlocked with a dubber playing a 35 mm reel of a full-coat, or film completely coated with magnetic iron-oxide. This was introduced in 1952 with Cinerama, holding six tracks of stereophonic sound. Stereophonic releases throughout 1953 also used an interlocked full-coat for three-channel stereophonic sound.

Although magnetic audio was excellent quality, it was prone to damage and erasure over time. As analog optical stereo gained popularity (it was also more durable and far less expensive to include on a film print), 35 mm four-track magnetic sound was increasingly only used for special road show screenings, and the development of digital sound systems made it completely obsolete.

Digital

Modern theatrical systems use optical representations of digitally encoded multi-channel sound. An advantage of digital systems is that the offset between the sound and picture heads can be varied and then set with the digital processors. Digital sound heads are usually above the gate. All digital sound systems currently in use have the ability to instantly and gracefully fall back to the optical sound system should the digital data be corrupt or the whole system fail.

Cinema Digital Sound (CDS)

Created by Kodak and ORC (Optical Radiation Corporation), Cinema Digital Sound was the first attempt to bring multi-channel digital sound to first-run theaters. CDS was available on both 35 mm and 70 mm films. Film prints equipped with CDS did not have the conventional analog optical or magnetic soundtracks to serve as a "back-up" in case the digital sound was unreadable. Another disadvantage of not having an analog back-up track is that CDS required extra film prints be made for the theaters equipped to play CDS. The three formats that followed, Dolby Digital, DTS and SDDS, can co-exist with each other and the analog optical soundtrack on a single version of the film print. This means that a film print carrying all three of these formats (and the analog optical format, usually Dolby SR) can be played in whichever format the theater is equipped to handle. CDS did not achieve widespread use and ultimately failed. It premiered with the film Dick Tracy and was used with several other films, such as Days of Thunder and Terminator 2: Judgement Day.

Sony Dynamic Digital Sound (SDDS)

SDDS runs on the outside of 35 mm film, between the perforations and the edges, on both edges of the film. It was the first digital system that could handle up to eight channels of sound. The additional two tracks are for an extra pair of screen channels (Left Center and Right Center) located between the 3 regular screen channels (Left, Center and Right). A pair of CCDs located in a unit above the projector read the two SDDS tracks. The information is decoded and decompressed before being passed along to the cinema sound processor. By default, SDDS units use an onboard Sony Cinema Sound Processor, and when the system is set up in this manner, the theatre's entire sound system can be equalized in the digital domain. In contrast, both DTS and Dolby Digital soundtracks must be passed through to standard analog cinema sound processors - which are also used for analog optical sound, so equalization of the sound remains in the analog domain. The audio data in an SDDS track is compressed in the 20-bit ATRAC2 compression scheme at a ratio of about 4.5:1. SDDS premiered with the film Last Action Hero. Sony ceased the sale of SDDS processors in 2001-2002.

Dolby Digital

Also known as Spectral Recording Digital or "SR•D." Sound is printed between the perforations and is 26 frames before the picture (the offset can be varied based on processing presets). Dolby Digital produces 6 discrete channels. In a variant called SR•D EX, the left and right surround channels can be dematrixed into left, right, and back surround, using Dolby Pro Logic. The audio data in a Dolby Digital track is compressed in the 16-bit AC-3 compression scheme at a ratio of about 12:1. The images between each perforation are read by a CCD located either above the projector or in the regular analog sound head below the film gate. The information is then decoded, decompressed, and converted to analog; this can happen either in a separate SR-D processor that feeds signals to the cinema sound processor, or SR-D decoding can be built-in to the cinema processor.

Digital Theater Systems (DTS)

DTS actually stores the sound information on separate CD-ROMs supplied with the film. The CDs are fed into a special modified computer which syncs up with the film through the use of DTS time code, decompresses the sound, and passes it through to a standard cinema processor. The time code is placed between the optical sound tracks and the actual picture, and is read by an optical LED ahead of the gate. The time code is actually the only sound system which is not offset within the film from the picture, but still needs to be physically set offset ahead of the gate in order to maintain continuous motion. Each disc can hold slightly over 90 minutes of sound, so longer films will require a second disc. Three types of DTS sound exist: DTS-ES (Extended Surround), an 8 channel digital system; DTS-6, a 6 track digital system, and a now obsolete 4 channel system. DTS-ES derives a back surround channel from the left surround and right surround channels using Dolby Pro Logic. The audio data in a DTS track is compressed in the 20-bit APTX-100 compression scheme at a ratio of about 4.5:1. Of the three digital formats currently in use, DTS is the only one that has been used with 70 mm presentations. DTS was premiered on Jurassic Park. A consumer version of DTS is available on some DVDs.