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How Movie Sound Works

Analog Sound
Optical audio pickup
Optical audio pickup

The mechanism for delivering sound in the early days of cinema was incredibly simple. Vitaphone, used in "The Jazz Singer," consisted of a record player playing a wax record. This was known as sound-on-disc. The sound recording was usually done after the movie was filmed. The record was played on a turntable that synchronized sound to the film by controlling the speed of the projector. It was a simple but very effective way to add audio to a movie.

In the early 1930s, sound-on-film began to supplant sound-on-disc as the technology of choice for adding a soundtrack to a movie. An interesting thing about sound-on-film is that the 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 uses one of two technologies:

  • Optical
  • 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.

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The light is changed to electrical current by the photocell. The amount of current is determined by the amount of light received by the photocell. The wider parts of the strip allow more light, which causes the photocell to produce more current. Since the width of the transparent strip changes the amount of light, this results in a variable electric current that can be sent to a pre-amplifier. The pre-amplifier boosts the signal and sends it to the amplifier, which distributes the signal to the speakers.

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.

Even though magnetic recording provided as many as six discrete tracks of sound on a film, the expense was simply too much. There had been experiments with stereo optical tracks, but there was too much noise to make that sound system worthwhile. But when Dolby Laboratories introduced Dolby A in 1965, a noise reduction method originally developed for professional recording studios, the movie industry saw an opportunity to reinvent the optical track.

Dolby A breaks the incoming audio signal into four discrete bands. A technique called pre-emphasis boosts the signal of each band above 10 decibels, the level of ambient noise. Each signal then travels through a compander, where the signal is compressed to further eliminate low-level noise and is then expanded again. The signals are combined, and the result is much cleaner sound.

The main compromise in Dolby A is a narrower frequency response, resulting in a smaller dynamic range. Dolby noise reduction has evolved from Dolby A to Dolby Spectral Recording, an enhanced process that reduces noise twice as much as Dolby A.

In 1971, "A Clockwork Orange" used Dolby A on magnetic sound-on-film with great success. Eastman Kodak worked with RCA and Dolby in the early 1970s to develop stereo variable area (SVA), an optical method that offered stereo sound by using two variable width lines in the space that was originally allocated for one.

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