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CHARACTERISTICS OP SOUND FILM 181
sity factor of exposure. From the photographic standpoint, therefore, the general problem of Sound recording has at least three distinct phases, since, if the best possible results are to be obtained, each of the methods mentioned above requires a photographic material having characteristics and requiring processing treatments differing radically from each of the other two.
Regardless of which method of sound recording is being used, the energy required for modulating the photographic record is derived from the plate current flowing from the last tube in the recorder amplifier. In all cases it is desired that the sound record positive, printed from the sound negative, shall carry a distribution of density which will control the intensity of the radiation incident upon the photo-electric cell of the reproducing mechanism in such a manner that the instantaneous intensity of this radiation is directly proportional to the instantaneous sound pressure on the microphone diaphragm.
In case of the variable density record this end is accomplished by a variation of density from point to point along the length, that is, in the direction of travel, of the record, with no variation of density in the transverse direction, that is, along a line perpendicular to the direction of travel. When such a record is moved at a uniform linear velocity past the scanning slit, or the optical image thereof, the intensity of the radiation transmitted by this slit is directly proportional to the mean transmission of the photographic image covering the slit at any instant. It is evident, therefore, that if it be desired to cause the intensity incident upon the photo-electric cell to vary, let us say, according to a sine function of time, the transmission of the photographic image must vary as a sine function of distance measured along the length of the record. A micro-photometric analysis of such a record should therefore show a gradual increase and decrease in transmission along the length of the record which, when plotted as a function of linear displacement, should give a sine curve. Assuming that the characteristic of the recording system (microphone, amplifier, light valve, etc.) is linear, the exposure (I.t) incident on any point of the photographic recording material is directly proportional to the instantaneous sound pressure on the microphone. If this condition is fulfilled then the photographic problem resolves itself into that of obtaining a positive in which the distribution of transmission along the direction of travel is directly proportional to the distribution of exposure on the negative. This problem is fundamentally identical,