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Optical soundtrack

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  • optical sound formats
  • white light vs. red light
  • color of optical tracks (cyan, high magenta, silver etc.)
  • types of optical readers
  • issues related to optical sound
  • variable area and variable density tracks

How an Optical Sound Reader Works (the very basics) - As the film runs through the sound head of the projector, light rays from the exciter lamp focus on the optical track through a slit lens. The optical track creates a variance in light, which is absorbed by a solar cell in the sound head and converted to variance in electrical impulses. These impulses are then amplified, sent to the sound processor, then to the speakers where they are converted to sound waves. Optical sound is the most widely used type of sound reproduction on film prints to this day even with the advent of digital sound. Many 35mm film prints beginning in the 90s will have both an optical track and a digital track (Dolby Digital, SDDS, or DTS).

Optical Sound Formats


16mm (see 16mm page

Sound Issues

  • clipping or distortion resulting from incorrect lateral positioning of the sound head
  • Motorboating: a term use to describe the sound heard when sprocket holes are passing in front of the exciter lamp. This can be caused because of a threading error, or because the film is shrunken.
  • Dirt build up: Always make sure your sound rollers and optics in the sound reader are clean. Never get liquid on the sound reader. Optics can be gently cleaned with a Q-Tip dipped in electronics grade alcohol. Sound drum should be clean and turn freely.


See Also

Magnetic sound Dolby Digital DTS