How It Works: Science and Technology - Vol. 15

By Wendy Horobin | Go to book overview

Soundtrack

This technician is
sitting at a mixing desk in
a recording studio where a
man is reading a narration
script for a television
program. The recording
settings are computer
controlled from the desk.
Mixing desks improve the
quality of the reproduced
sound by recording it
digitally in tiny packets
of separate information so
that when it is combined
the sound mimics the
subtleties of the original
sound. Other sounds can
be added to the soundtrack
at this point before it is
laid down on the finished
film or video.

Until recently, a soundtrack was simply the sound area on film used to show moving pictures. Film soundtracks are an established technology, but now with the increasing use of digital systems, many various new methods of recording the accompanying sound have been devised.

In 1906, a Frenchman, Eugene Lauste, who had worked in Thomas Edison’s laboratory, became interested in adding synchronized sound to the then very new motion pictures. Lauste’s method was to photograph a representation of the sound signal onto the picture negative. Inside the camera, he fitted an exposure lamp and a solenoid coil with a slit diaphragm to which he connected a telephone microphone. Speech signals from the microphone vibrated the diaphragm assembly, which he called a light valve, causing variations in the intensity of light falling on the film. Lauste also had to construct a special projector to reproduce this track, but since the audio amplifier had not yet been invented, he was unable to reach a wide audience.

In 1922, a U.S. scientist, Theodore Case, used an oxyacetylene flame to show the action of sound waves. He photographed the modulations of the flame caused by sound waves but soon abandoned the system as it was commercially unviable. In 1923, after inventing a photoelectric cell, he demonstrated talking films using a gas-discharge tube for recording, and his light-sensitive photocell coupled to an Audion amplifier and loudspeaker. Unfortunately, at that time film laboratories were unable to give consistent results, and the surface noise from the film was extremely high.

Al Jolson’s The Jazz Singer, which appeared in 1927, was the first movie with a synchronized soundtrack. Early talkies either had the sound recorded on the film photographically or on a separate synchronized disc. The discs were 10 in. (25 cm) in diameter, played at 3314 rpm, and contained a start mark on the inside groove, because they played from inside to outside. There were 96 grooves to the inch, so each disc lasted for 12 minutes—sufficient time to accompany 1,000 ft. (305 m) of film running at 24 frames per second. The discs were made from shellac, and very few now survive.


Optical sound

Cameras for producing optical soundtracks are available in 35 mm, 16 mm, and Super 8 (8 mm) formats. The fundamental design consists of a light-tight film chamber, a film transport system with flywheel and sound drum, a detachable magazine for holding exposed and unexposed film, and an exposure lamp with optical system and modulator, to expose a track that varies in step with sound vibrations.

Two types of modulators are used, a mirror galvanometer and a light valve. Both produce a variable-area sound negative, in which a white band in the center of a black strip varies in width. The galvanometer reflects light from the exposure lamp, via a V-shaped mask, through a narrow horizontal slit. The light valve passes light directly through to the slit. In both cases, an image of the slit is focused onto the film as it passes around the sound drum. A noise-reduction bias is applied to the modulator when there is little or no sound so that it passes the minimum amount of light and masks unwanted background noise.

The optical soundtrack is printed onto the picture film in such a way that it is in advance of its corresponding picture frame by 20 frames on 35 mm, 26 frames on 16 mm, and 22 frames on Super 8, because each frame of the film is held motionless for a fraction of a second as it passes through the projection gate. The film must be running as smoothly as possible for accurate sound reproduction, so the film is passed around a series of loops and rollers and a sound drum. The sound drum is quite heavy so that its inertia

-2146-

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How It Works: Science and Technology - Vol. 15
Table of contents

Table of contents

  • How It Works® Science and Technology 2017
  • Title Page 2019
  • Contents 2020
  • Salvage, Marine 2021
  • Satellite, Artificial 2024
  • Schlieren Techniques 2030
  • Screw Manufacture 2032
  • Seaplane and Amphibian 2035
  • Sea Rescue 2038
  • Security System 2042
  • Seismology 2046
  • Self-Righting Boat 2050
  • Semiconductor 2052
  • Servomechanism 2055
  • Sewing Machine 2058
  • Sextant 2062
  • Sheet Metal 2064
  • Ship 2067
  • Shutter 2074
  • Silicon 2076
  • Silicone 2078
  • Silver 2079
  • Sine Wave 2081
  • Siphon 2083
  • Ski and Snowboard 2084
  • Skin 2087
  • Skyscraper 2090
  • Slaughterhouse 2096
  • Sleep 2099
  • Smell and Taste 2103
  • Soap Manufacture 2107
  • Soft-Drink Dispenser 2109
  • Soil Research 2110
  • Solar Energy 2114
  • Solar System 2118
  • Solenoid 2124
  • Sonar 2126
  • Sorption 2129
  • Sound 2131
  • Sound Effects and Sampling 2136
  • Sound Mixing 2138
  • Soundproofing 2140
  • Sound Reproduction 2142
  • Soundtrack 2146
  • Space Debris 2150
  • Space Photography 2152
  • Space Probe 2156
  • Index i
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