DeForest Achievements (c. 1920s)

Record Details:

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Phonofilm—actual size, the sound track showing to the left of the action photo- graphs. This actual record- ing shows a continuous sound —that of a male duet with a musical accompaniment. On cessation of sound the sound track shows absolutely black on the positive prints. In the case of recording on film, the electrical waves from the micro- phone are conducted into a photion light, an invention of Dr. DeForest. The photion light fluctuates in inten- sity in exact proportion to the elec- trical current which passes through it. Therefore when the electric cur- rent from the microphone passes into the photion the fluctuations in the light are really a visual translation of the sound waves which have en- tered the microphone. Exposing this light to the sound track, gives an exact photographic record of the sound vibrations. So to translate this photographic record back to sound is not a com- plicated matter. A bright light, called the exciting lamp, is placed in back of the sound box, and is focussed upon the sound track on the film as it passes in front of a photo-electric cell. The function of this cell is to translate any light that falls upon it into electric im- pulses. The beam from the exciting lamp shining through the sound track fluctuates according to the photographic record of the sound, and so from the sound box, via the photo-electric cell, comes electrical waves similar to those that comes from the microphone in the record- ing process. That is all that there is to it and this process is automatic. No human factors enter into it, except in see- ing to it that both the exciting lamp and photo-electric cell are oper- ating efficiently. From the sound box to the speakers the process is the familiar one of the family radio. The electric currents go into an am- plifier and then to the loud speakers where the electric currents are trans- lated into audio waves or sound.