Movies for TV - Early Television Foundation

72
FUNDAMENTALS
for many years. Its principle is much the same as the RCA system
inasmuch as the sound signal is used to vary the light falling on the
film. In the WE system, a recording lamp is focussed onto a very
small slit 0.001 inches by 0.256 inches. Across this slit are two
fine duraluminum ribbons about 8/1000 by 250/1000 of an inch.
The ribbons are in a very strong magnetic field, and the speech
currents flow through them (the ribbons). As this happens, the
speech currents cause the ribbons to move due to varying magnetic
fields around the ribbons. This controls the amount of light admitted
by the galvanometer. A varying light bar falls on the film, and a
characteristic of this type of recording is the piano-key-like appear-
ance of the track (Figure 3-13G). This system is not as sensitive
to film-fogging and scratches as the variable-area system. In the
latter, the amount of sound is determined by the ratio of dark to
light areas ; the greater the latter, the stronger the sound. In the case
of the former, the sound signal has the same effect, but this time
instead of controlling the amount of light by a shutter-like device,
it is controlled by the density of the emulsion between the light
source and the photocell. Noise reduction is also used in this system
current is used to bias the ribbon so that
whereby part of the speech
it causes an extremely low background noise on quiet passages.
It appears that there is little to choose between the two systems,
since each is good. In the opinion of the author, the variable area
may be more suited to mobile operation and therefore to single
system operation, while the variable-density system may be better
suited to studio work. Whichever is better, it is really just a matter
of individual preference. It is interesting
to note that most of the
portable and single system cameras and sound equipment use vari-
able area for the sound head.
When it comes to reproduction, either system works perfectly
on any projector without special adjustment. After all, they both
work on the same principle : that is, a light falling onto a photocell
causes variations in the electrical output. The rate at which this
variation occurs determines the frequency of the sound. The photo-
cell doesn't care how the variation in illumination is caused, pro-
vided one is caused.