A Brief History of Sound
Sound was introduced to the movies in 1927 with Al Jolson's The Jazz
Singer. In 1977, the motion picture industry celebrated the 50th anniversary
of the talkies.
1920's
The very first sound was produced in the early 1900's from a phonograph
disk running in mechanical synchronism with the picture at 33 1/3 RPM.
Obvious synchronization problems requiring the constant attention of the
projectionist led to a system which allowed the picture and sound track to be
printed together on the same piece of film.
1930's
Two photographic-sound recording systems evolved-variable-density and
variable-area. Variable-density meant that the density of the sound track
varied in accordance with the audio signal. Variable-area meant that the
width of the clear area of the track varied with the signal.
Also, there were several different types of variable-area tracks-the earliest unilateral, the improved bilateral and dual-bilateral and the special push-pull tracks. Because of the complexities of push-pull tracks, they were used for in-house operations, not released. Only on picture, the 1941 version of Walt Disney's Fantasia, was released with push-pull tracks, and then only as a special road show performance where Disney technicians had complete control.
1940's
The primary shortcoming of photographic sound tracks was (and still is)
noise. Early in their use, schemes were devised for noise reduction. Over
the years, many variations of both variable density and variable-area tracks
were developed to increase their dynamic range. This need for greater sound
level led to the abandonment of variable density in favor of the higher output
variable-area recording.
The added realism of stereophonic sound challenged engineers. In the late 1930's, Bell Labs developed a stereo system with four variable-area tracks on 35 mm film and in 1941, Fantasia was released as the first commercial stereo release.
1950's
The 1950's brought wide-screen pictures-most using multiple magnetic
tracks for stereo sound. The driving force was more realistic and exciting
theater entertainment to counter the home TV threat to their business. In late
1952, a three-camera, three-projector, ultra-wide screen format was
introduced. Its seven sound tracks were on a separate film run
synchronously with the picture. In 1953, Fox released The Robe in
CinemaScope,--a 2.35:1 wide screen picture from a standard 35 mm print
with four magnetic tracks, three for wide-band audio and a narrow track for
surround sound. Todd-AO, the company which invented 70 mm 6-track
magnetic sound tracks, revolutionized the industry with its 70 mm release of
Oklahoma in 1955. This double-width film not only gave the very best
wide-screen picture, but its six magnetic-sound tracks produced stereo sound
of superb quality. Many other wide-screen contenders offering improved
quality or lower cost came and went-CinemaScope 55, MGM Camera 65,
Cinemiracle, Technirama, and VistaVision.
1960's and 1970's
In the 1960's and early 1970,s, 70 mm 6-track magnetic sound and 35 mm
CinemaScope fared the best. However, the laws of economics did catch up
with CinemaScope. Ninety percent of these prints were released with no
magnetic tracks, only a monaural optical track. Of the remaining 10 percent
that had magnetic tracks for stereo, nearly all also had a 1/2 width optical
sound track nudged in so that the print could be played in theaters without
magnetic stereo capability. The reason was simple. The addition of
magnetic stripes and recording four tracks on each print increased their cost
from 50 to 75 percent. Also, superior magnetic sound required scrupulous
and costly maintenance of the magnetic sound reproducers.
These cost pressures caused engineers to take a close look at optical sound. If they could substantially improve the frequency response and signal-to-noise ratio of an optical track, several tracks could be recorded in the space used for one. They could produce stereo sound without the added print costs of magnetic tracks.
In mid-1965, Ray Dolby from Oregon, then living and working in England, developed a noise reduction system for magnetic reduction in magnetic recording that was adopted immediately in the music industry. In 1972, Dolby noise reduction was introduced into motion-picture sound- recording, but for monaural sound, not stereo sound.
Dolby Laboratories, spurred by a Kodak employee, Ron Uhlig's success with 2-track, 2-channel stereo sound for 16 mm film, developed a 2-track stereo variable-area system with complete compatibility. Theaters converted to decode Dolby tracks could enjoy the low noise, relatively wide-frequency range stereo reproduction and also get acceptable monaural sound when playing a standard Academy mono print.
Also, Dolby-encoded stereo prints would yield acceptable monaural reproduction on unconverted projectors on theaters not equipped for stereo. In 1974, two pictures were released with Dolby Stereo Variable-Area (SVA) tracks; in 1976, four pictures; and by 1978, 25 pictures. Over 900 theaters worldwide were equipped to reproduce Dolby-encoded SVA tracks by 1979.
At that time, it cost between $10,000 and $15,000 to add Dolby SVA to theaters already equipped to play stereo from 4-track CinemaScope or from 6-track 70 mm prints. For theaters only able to play monaural tracks, these costs increased to between $15,000 and $25,000. All for the attraction of Dolby Stereo on the marquee, but that's proved to be a substantial attraction to the many theaters who invested in Dolby.
Other contenders for this marketplace were Colortek, Todd-AO/Nuoptix, Universal with its Sensurround, 20th Century-Fox with their Fox Sound 360, and Pacific Theaters with their drive-in bilingual presentation of Star Wars.
1980's
Through it all, three formats have withstood the test of time:
By the mid-1980's, considerable interest had developed in digital sound on motion picture film. This interest was spurred to no small degree by the availability to the consumer of compact audio discs. This digital recording medium is quickly supplanting tape and long-play phonograph records for home sound systems because of its virtually flawless audio quality.
1990's and Beyond
In 1990, Cinema Digital Sound (CDS) for film became a reality. The
Cinema Digital Sound System was co-developed by Optical Radiation
Corporation and the Motion Picture and Television Products Division of
Eastman Kodak Company. CDS features six discrete channels of pure
digital sound optically encoded on the print film. CDS debuted in 1990 at
selected theaters featuring Dick Tracy in the 70 mm format in New York
City and Los Angeles.
CDS provides filmmakers with a precise ability to control the direction and movement of sound to create a more compelling illusion of reality. Five discrete channels reproduce the full tonal and frequency ranges the human ear is capable of hearing. A separate sub-woofer channel reproduces the lowest bass tones.
CDS is designed to provide consistent audio quality for the life of the
print. Wear and tear can reduce the audio quality of conventional 35 mm
optical and 70 mm magnetic sound tracks. To provide this durability of the
digital sound track, CDS features a sophisticated error collection system to
ensure that every audience will hear opening night sound quality, even
months later.
The ability to encode digital sound optically on film required a major technological breakthrough providing the key to affordability and reliability of CDS.
Theatres equipped with single channel surround speakers can easily retrofit for the dual channel surround of CDS. All it requires is installation of a digital decoder on the projector and a digital-to-analog processor in the projection booth equipment-rack. Some theaters may consider the option to upgrade speaker systems to realize the full potential that CDS offers.
CDS technology for 70 mm and 35 mm release prints is virtually the same. A decision was made to debut CDS in 70 mm format so the new audio system could be introduced in road show theaters.
Motion pictures can be released in CDS format by simply remixing the audio made for conventional prints to six discrete channels of digital optical sound.
Eventually recording and mixing techniques will evolve to take full advantage of CDS features. More original sound will be recorded and mixed digitally now that there is a way to release movies in digital sound format.
Magnetic and Photographic Sound
Sound is recorded on a motion picture print in one of two ways, either
magnetically on a metallic oxide strip coated on the film or photographically
by an optically modulated light system.
A magnetic sound track consists of a strip of metallic oxide coated along the edge of a motion picture film. Sound is recorded on this stripe by running it past a magnetic recording head that selectively magnetizes the metallic particles in the coating. Since coating formulations have been developed that are not affected by the processing chemicals, they can be applied to the film before (prestripe) or after (poststripe) processing. Seventy-millimeter and some 35 mm prints may have multiple stripes for stereophonic sound and special sound effects.
A second, much narrower stripe of the same thickness and, usually the same material is coated near the edge of the film support that is used for the sound stripe (between the perforations and the nearest edge) on 16 mm and super 8. This stripe is normally not used for magnetic recording; it balances the film mechanically to keep it from telescoping or binding against the reel flanges during projection and rewinding.
A photographic sound track is a record of sound (voice, music, etc.) printed near the edge of a motion picture film. Photographic sound tracks are usually printed on the film at the same time as the photographic image. Thus, the two can also be duplicated simultaneously, unlike magnetic sound tracks which must be recorded on each print in a separate nonphotographic operation.
A film producer who wants photographic sound sends the rough-edited workprint, the original film, the script, and the final magnetic recording to a laboratory where conforming, editing, and addition of the sound track are accomplished. The original film, or a printing master with photographic sound track, is then printed for release.
Photographic sound prints can be made from original films with magnetic sound stripes or from original films and separate magnetic tracks. A photographic sound track will last the life of the film and cannot be easily damaged through cleaning or other maintenance of the film. There is also no danger of accidentally erasing the track. However, the reproduction fidelity of photographic sound tracks can be degraded by dust particles and scratches. Also, changes cannot be made in a photographic sound track after it has been printed on the film.
Magnetic tracks, on the other hand, are less susceptible to dust and dirt distortion and are degraded very little by scratches. The magnetic stripe offers other advantages. The additional height of the magnetic stripe raises the emulsion (image) off the base side of the next convolution of film on a reel, protecting the picture area from frictional damage, emulsion-to-base sticking, etc. The stripe may also have higher fidelity sound (greater frequency response and better signal-to-noise ratio).
Photographic Tracks
A photographic sound-track negative consists of an exposed area whose
width and area vary with the volume and frequency of sound recorded. The
track looks like one or more narrow, jagged, black-and-white patterns along
the edge of the film. For optimum quality on a variable-area sound track, the
clear portions should be as transparent as possible, and the dark portions
should have a density at wavelengths from 800 to 1000 mm between 1.0 and
1.8. Consequently, emulsions and processes that produce high contrast are
generally used to record variable-area sound-track negatives.
Basics of Photographic Sound
The reproduction of sound requires that the sound waves be converted into
electrical signals which are then recorded. The record can then be played
back, generating electrical signals, which can be converted back to sound
waves by the speakers. In photographic sound reproduction, the actual
sound record on the print is a silver, dye, or dye-plus-silver image along the
edge of the film.
Figures 54A, B, and C show the components which convert the
photographic sound track into electrical sound signals. The light energy
from the lamp is formed into a narrow beam by a lens and aperture. The
beam is transmitted through the sound-track area of the film and then strikes
a photocell.
 Schematic of optical sound reproduction. Figure 54A |
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 A sound track as seen through the aperture. Figure 55 |
 Light attenuation by a sound track. Figure 54B |
Bilateral Sound Track. |
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Dual Bilateral Sound Track. Figure 56 |
 Response of a photocell. Figure 54C |
As the film moves, the sound track itself varies, or modulates, the amount of light that reaches the photocell from the sound lamp.
The photocell then converts the light energy into electrical energy. The electrical current produced by the photocell is directly proportional to the intensity of the light that reaches it.
Photocells are made out of various photosensitive materials, each having a different spectral sensitivity. Virtually all 16 mm and 35 mm projectors have S-1 or silicon-type photocells, sensitive primarily in the infrared area. Therefore all 16 mm and 35 mm sound tracks must be able to modulate infrared radiation, which silver and to a lesser extent, silver sulfide are capable of doing. A sound track made of dye alone will not modulate the infrared radiation as effectively, reducing the signal-to-noise ratio significantly.
As the film moves past the sound aperture, the variation in the width of the track determines the amplitude of the signal generated, and the speed of the variation detertmines the frequency of the signal.
There are several types of variable-area recordings. A unilateral track consists of modulations that are generated perpendicularly to the longitudinal dividing edge between the opaque and clear portions of the track. A bilateral track, Figure 56, uses modulations that are symmetrical about the longitudinal center line of the track. A dual bilateral track, Figure 56, has two bilateral images laid side by side; a multilateral track employs several bilateral images. The dual bilateral track is the most widely used because it minimizes distortion or signal loss resulting from any uneven illumination of the optical slit at the reproduction heads.
Photographic Sound-Track Reproduction
The effectiveness with which a photographic sound track is reproduced is a
function of the spectral energy distribution of the illuminant, the spectral
absorption of the soundtrack image, and the spectral response of the
photoreceptor. The illuminant is usually a tungsten lamp having a
comparatively low color temperature that provides relatively more energy in
the red and infrared regions of the spectrum.
Due to the multilayer construction of most color films, the color of the
light that exposes the sound-track image influences the trace characteristics
and, therefore, is generally specified for the particular film concerned.
Silver and silver-plus-dye sound-track images are normally suitable for use
with any projector and are printed from a negative sound track. Silver
sulfide sound-track images have somewhat lower quality. They are
produced on reversal color films only and are themselves reversal images
that are printed from a positive sound-track original.