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sound on disc

early 1930s sound on film

he sound is several frames away from the corresponding images. This is because the audio pickup, or reader, is set either above or below the lens assembly of the projector. Most analog pickups are in the basement (below the lens), while digital pickups are normally in the penthouse (fastened to the top of the projector). A test film is run to calibrate the sound to the picture. Once this calibration is done, projectionists can splice film together knowing that the sound will synchronize properly.

sound on film can be optical or magnetic

The most common method is an optical process whereby a transparent line is recorded along one side of the film. This strip varies in width according to the frequency of the sound. For this reason, it is known as a variable-area soundtrack. As the film passes the audio pickup, an exciter lamp provides a bright source of light, focused by a lens through the transparent line. The light that passes through the film shines on a photocell.

A variation of this method is known as variable-density soundtrack. It uses a strip that varies in transparency instead of width. The more transparent the strip is, the more light shines through. The biggest problem with this method is that the natural graininess of the film can create a lot of background noise.

In the 1950s, magnetic recording became popular. Magnetic sound-on-film had a couple of advantages over optical at the time:

    Magnetic was stereo, while optical was mono.
    Magnetic had better sound quality.

But there were disadvantages, too:

    Magnetic had to be added to the movie after it was filmed.
    Magnetic was more expensive.
    Magnetic didn't last as long as optical.
    Magnetic was more easily damaged.