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PRINCIPLES OF CINEMATOGRAPHY
INCIDENT SOUNDS CREATE VARIATIONS IN AIR PRESSURE
MICROPHONE DIAPHRAGM VIBRATES IN SYMPATHY WITH THE AIR PRESSURE WAVES
DIAPHRAGM CREATES ELECTRICAL WAVES PROPORTIONAL TO THE AIR PRESSURE WAVES
FILM IS PROCESSED AND PRINTED TO PRODUCE A POSITIVE IMAGE
I
POSITIVE FILM RUNS THROUGH REPRODUCER AT A SPEED ECUAL TO THAT OF THE ORICINAL RECORDING
LIGHT MODULATOR CREATE! A VARYING EXPOSURE ON THE PHOTOGRAPHIC FILM
LIGHT BEAM IS MODULATED BY POSITIVE SOUND TRACK
ELECTRICAL WAVES AMPLIFIED TO OPERATE A LIGHT MODULATING DEVICE
MODULATED LIGHT BEAM CAUSES PHOTO-CELL TO CREATE ELECTRICAL WAVES
LOUDSPEAKER DIAPHRAGM
RECREATES THE ORIGINAL
AIR PRESSURE WAVES
ELECTRICAL WAVES ARE AMPLIFIED AND APPLIED TO LOUDSPEAKERS
Fig. 120.— Sound recording and reproducing cycle.
Area or Variable Density exposure on a photographic film running at a constant speed past this modulation. (6) After processing and printing, a positive copy of the photographic record of the variations in air pressure is obtained. (7) This positive copy is then run at a constant speed, equal to that of the original negative film, through the sound reproducing head. (8) A constant beam of light is directed onto the sound track and, after passing through this track emerges as a beam modulated in intensity in proportion to the original variations in air pressure, created by the incident sounds. (9) This modulating light beam is directed onto the photoelectric cell which is capable of creating variations in an electrical circuit. (10) These electrical waves are finally amplified and applied to loudspeakers which will then cause a mechanical oscillation of the diaphragm. (11) The mechanical movement of the speakers recreate the air pressure waves and, therefore, the original sounds.
The action of the photo-electric cell is similar in principle to that of the thermionic valves used in radio equipment inasmuch as electrons, liberated by the Cathode, move through space and are collected by the Anode. One essential difference is, however, that whereas electrons are liberated from the Cathode in a radio valve by the action of heat, those liberated from the Cathode in a photoelectric cell are propelled by the action of light energy. At present our knowledge of the properties of light is not sufficient to give a satisfactory explanation of this phenomenon.
Four main types of photo-cells are available, firstly 'Photo