Movies for TV - Early Television Foundation

MOVIE. MAKING EQUIPMENT 69
Sound Recording on Film Irrespective
of whichever method
of recording sound is chosen, the principles are the same a change
in the loudness of the sound causes a variation in the strength of
a light ray falling on the light-sensitive emulsion of the film. These
variations in light and shadow correspond to the strength of the
sound producing the changes and can be converted back to sound
by the reverse process.
Variable-Area System The variable-area sound
system,
recording
sometimes known as the RCA system, is just what the
name implies a sound track whose area varies according to the
strength of the sound. Figure 3-13 shows the two different types
of track; Figures 3-13A and 3-13B are the variable-area track.
Figure 3-13A is a negative track and Figure 3-13B is a positive
track. It will be noticed that the positive is an exact reversed
replica of the negative. This means that when more light passes
through the valve during recording, a larger clear area is formed
on the positive. And conversely, a larger dark area is formed on
the negative. Remember this for it will be referred to again.
The equipment consists of a small, powerful lamp, the light
from which is focussed through a small triangular mask opening
by means of a lens. The light then falls on a minute mirror which
is mounted on a coil of wire positioned between the poles of a
powerful magnet. This is known as a galvanometer. When the
speech currents which are picked up by the microphone and
amplified by the recording amplifier pass through this coil of
wire, they set up small magnetic fields around the wire. These act
in opposition to, and in phase with, the field from the large mag-
net. Thus the mirror vibrates with a frequency which depends
on and corresponds to, the frequency of the sound being heard.
The spot of light reflected from the mirror is focussed onto
the film by another lens and passes through a small, limiting slot.
When the spot of light falls on the film, the latter is exposed and
produces a clear area on processing. With zero modulation, i.e.,
without speech or music, etc., the light falling on the film as it
passes causes only a very thin, clear line which should have no