Radio mirror (Jan-Oct 1923)

September 15, 1923 RADIO DIGES T— Illustrated 13 How Your Voice Looks "Taken" on a Movie Film A Description of the Famous Pallophotophone By Peter J. M. Chile TO TRANSLATE sound In terms of light and then through light to re• produce the sound seems to be almost impossible of accomplishment by science. The photographing of sound waves themselves is nothing new. Figure 1 shows three separate lines, representing various sounds photographed on a single strip of film. The wavy lines, however, that represent the voice convey through the eye no sound impression. The next logi;al step, then, is to make these wavy lines do something that will make a telephone receiver diaphragm move ,'n synchronism with it. An extraordinary device or machine which accomplishes this purpose is the pallophotophone, which has re:ently been brought to a state of perfection by Charles A. Hoxie, an engineer engaged in special development work at the laboratories of the General Electric Company. The outfit derives its name from the Greek words that signify "shaking light." Movement of the Diaphragm Let us first consider what makes the diaphragm vibrate in a telephone receiver. It is actuated by an electromagnet, the pulling power of which is being affected by an electric current, whose variations are in accordance with the vibrations of another diaphragm moving under the influence of a speaker's voice. Although, peril a p s , separated by miles, both diaphragms vibrate in unison, and speech is thus transmitted. The telephone transmitter is in reality a valve, regulating the amount of current flow. If there was a valve wherein light and shadow would affect the current, a positive connecting link between light and sound would be established. Such a device has been known to scientists for years and is called a "photo-electric cell." It is a device which in the dark does not permit ordinary current to pass through it, but if a ray of light falls upon its sensitive structure, current flow is started. Successive light and shadow cause current variations, following instantly and in strict accordance with every change in the light that impinges upon it. If a film carrying a sound record is passed before the lightcontrolled current valve, there is immediately produced a fluctuating current through the cell, which, if passed in turn through a telephone receiver, will actuate the diaphragm. Figure 1 There are two distinct devices In the pallophotophone, one for recording sound and one for reproducing the sound, either of which may be used independently. Recording Sound The recording device consists^essentially of a tiny mirror, scarcely larger than a pin head, on which is reflected a beam of light. This mirror is attached to a most delicately adjusted vibrating diaphragm. When sound waves, originating from any source, reach the diaphragm, causing it to vibrate, the mirror oscillates and the reflected beam of light moves with it. This moving ray of light falls upon a strip of photographic film, which passes in front of the mirror in a continuous motion. The film when developed in the usual way shows a succession of delicate, dark up-and-down markings on a clear background. These markings constitute the sound record and represent the oscillations of the reflected beam of light. The record produced on the film is a marvelously faithful reproduction of the sound because of the extremely small size of the mirror and diaphragm and their low inertia. These characteristics enable the "shaking light" to produce a sound record on the film that catches all the fine overtones, the delicate shadings of speech, in fact, every feature that distinguishes different voices. In this particular, the sound record is more nearly exact than any record thus far obtained with devices of this nature. The recording device operates on mechanical principles, inasmuch as there is no essentially electrical process involved in the procedure. Reproducing the Voice The reproducing device, as previously mentioned, is an entirely different contrivance, and the two devices are so independent of one another that either can be removed from the apparatus while the other is in use. This procedure has been followed repeatedly by the inventor in his demonstrations of the pallophotophone. The reproducing system is electrical in its nature of operation in contrast to the mechanical features involved in the recording device. The film carrying the sound record is wound on a reel, so that it passes in front of an extremely sensitive electrical apparatus. 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If the other electrode is made positive, with respect to the potassium, and that circuit in amounts directly proportional to the intensities of the light passing to the cell through the film, and at frequencies corresponding to the frequency of the sound vibrations recorded on the film. The remainder of the circuit and its action are quite similar to those of the standard Radio equipment Amplification Figure 2 MENT-+ 1[ \j~^ PLATE -J,\,\A > TO AMPLIFYING SYSTEM. a beam of light allowed to shine on the potassium, there will be a passage of electrons from the potassium to the positive electrode, the flow of electrons being proportional to the intensity of the light. Referring to the simple schematic circuit diagram in Figure 2, any change in the resistance of the photo-electric cell will produce a corresponding change in the electromotive force impressed on the grid of the vacuum tube. 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