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For use as part of the talking picture, the sound
to accompany the photograph action must be recorded for reproduction when the
picture is shown.
The most desirable material for this purpose has been found to be the
photographic film.
The finished film employed for reproduction in the motion picture
theater therefore
carries not only the picture of the action that is to be seen,
as it did in the silent picture days, but also the photographic record of the
sound that is to be heard.
The sound and picture
are usually photographed on separate films which are called negatives.
These negatives are combined by ordinary photographic printing
to produce the positive print which is used in the theater
to reproduce the original picture and sound.
Since the photographic film is affected only by light,
it is necessary for the recording of sound
to convert the sound waves to corresponding changes in light.
As I speak, the sound waves produced by my voice
are transmitted through the air to the microphone
where the sound waves are converted to
changes in an electric current.
These variations in the electric current are
then amplified
and used to control the light falling on the photographic film.
The light that falls upon the photographic film
is in the form of a fine bright line
extending transversely up the direction in which the film moves.
The part of the film on which the line of light falls to make the
photographic sound record
is called the sound track.
When the line of light is so placed
as normally to extend part way across the sound track
and the sound controlled variations cause the position at one end of the line
to move transversely at the track, a record
known as a unilateral variable area sound track
is produced.
When both ends of the line of light
are caused to move transversely at the track, a record known as a bilateral
variable area sound track is produced.
When the line of light extends completely
across the sound track,
and the thickness of the line is buried by the sound controlled current
variations,
a record known as a variable density
sound track is produced.
All of these sound records contain photographic records
of light variations
corresponding to the pressure variations in the original sound.
If either a variable area,
or a variable density sound record,
is run past a narrow line of light from
a fixed source
and the light after passing through the film
is visually observed, the amount of light reaching the observer is
constantly varying from instant to instant.
If the eyes were capable of
quantitatively measuring the light,
it would see that the amount of light reaching it from either sound record
varies in exactly the same way.
There are in common use today
electrical devices which will quantitatively measure
what the eye can only approximate. The photoelectric cell is one such device
and it's universally used in the reproduction photographic sound
records.
The reproducers system to which you are now listening
contains such a photoelectric cell. This varying beam of light
falling on the photoelectric cell produces variations in the electric current
which are directly proportional to the variations in the light beam.
As the varying electrical current in the photoelectric cell is small
a vacuum tube amplifier is required
to increase it to the point where it will operate a loudspeaker.
The loudspeaker converts the varying electrical current into sound waves
which are the counterpart of the sound waves falling on the microphone
in the recording studio.
To review, in the recording process the original sound waves are converted to
variyng electrical current. The varying electrical current is converted to
variations in a light beam,
and the varying light beam produces a photographic sound record.
In the reproducing process, the photographic sound record is used to
produce a varying beam of light.
The varying beam is converted into varying electrical current,
and the varying electrical current is then converted back
into sound waves.
Since the advent of successful talking pictures in the late twenties,
nearly all variable density photographic sound recording
has been done by means of a device known as the light valve.
The light valve contains two very thin metallic ribbons
tightly stretched and spaced very close together
to form a life transmitting slot.
These ribbons lie in a strong magnetic field
and the amplified sound current flows through the ribbons.
The electric current reacts with the magnetic field
causing the ribbons to move like shutters, displacing them
in direct proportion to the amplitude of the current.
In order to ensure a high-quality sound recording,
the vibrating length of these thin ribbons is limited by bridges,
over which the ribbons are tightly stretched.
The tension of the ribbons is adjusted by tensioning means.
The ribbons are so delicate, and their separation and movements so slight,
that the action of the ribbons can be observed only with the aid of a
microscope.
The separation between the ribbons is actually less than a hair's breadth.
A steady light, concentrated by a lens,
passes through apertures in the magnetic pole pieces.
This line is buried by the slot defined by the ribbons
and it's concentrated by another lens system
to form a very fine line of light on the photographic film.
Since the ribbons act like controlling shutters,
any change in the sound current flowing through the ribbons
will cause a corresponding change in the size of the light transmitting slot
defined by their closely spaced edges,
and therefore in the amount of light falling on the film.
The divide does acts like a valve controlled by the sound current
for controlling light. In the explanation of the light valve action that follows,
attention will be directed particularly to the portions of the ribbons that
control the light,
and these portions and their movement will be shown schematically.
In this schematic representation it will be understood that the relative
dimensions of the ribbons
and the arrangement of their physical supports are illustrative
rather than actual dimensions and arrangements.
As has been explained, the ribbons lie in a magnetic field,
and amplified sound current flows through the ribbons.
The electric current reacts with the magnetic field
causing the ribbons to move like shutters, displacing them
in direct proportion to the amplitude of the current.
A steady light is concentrated on the ribbons,
and these ribbons acting as shutters cause variations in the light
corresponding to the pressure variations in the original sound.
This varying light causes variation in the exposure of the film.
The exposed film, when developed and printed,
is the sound record.
The sound track used for the reproduction of music will be shown on the screen
and the sound track will be changed from a variable area
to a variable density track simultaneously on both the picture
and sound record. The photoelectric cell
and the entire reproducing system is the same in all cases.