The pallophotophone

The name pallophotophone has been given to a new device used both for permanently recording speech in a wavy trace on a moving photographic film, and for transforming the air vibrations of sound into exactly corresponding electrical vibrations for transmission directly, for example, to wireless broadcasting generators. The syllables “pallo” of this new word are taken from the Greek and with the following syllable indicate “dancing light.” This particular function, involving a rapidly vibrating beam of light, seems to be the most distinctive one of the combination. To explain the operations briefly perhaps it would be more illuminating to start with resemblances and differences as compared to the well known telephonic devices and talking machines. The dark wavy trace, Fig. 6a, on the films corresponds to the wavy spiral of the victrola disk. “There is the resemblance.” With proper translating means either device can reproduce the sounds recorded. In the translating mechanism is the difference between the two methods of recording speech. The victrola employs a needle set into vibration at one end of a lever, the other end of which is attached to the central point of a thin disk of mica. On the other hand, the dark wavy trace on the photographic film, moved in front of a slot, allows variable amounts of light to fall on a photo-electric cell. In turn, the photo-electric cell varies the electric current passing through it in direct proportion to the change in light. The lower diagram of Fig. 11 shows this arrangement. The human ear cannot hear the wavy light, neither can it hear the variations of current in the photo-electric cell, but if the variations of current are passed into a telephone receiver or loud speaker, either directly or through the intermediary of wireless waves, the diaphragm of the telephone transforms the electric vibrations into air vibrations of sound — “and there you are.” As compared to the simple mechanism of Edison's talking machine, the pallophotophone seems to traverse a wide detour. Each, however, has its particular uses. The pallophotophone has its marked advantages — some of which are quite evident and others hidden until explained. When the artist is available and at the broadcasting studio the pallophotophone dispenses with the record on a film and passes the energy of speech and music to light vibrations, and thence electric vibrations which pass directly to the wireless generators. Important to note, there are no microphones or other telephonic devices involved in these transformations. Nevertheless, the nearest analogous operation to the pallophotophone transmitter is in the familiar telephone transmitter. The telephone has a mouth-piece at the end of which is a diaphragm which vibrates with the sound waves. So also has the pallophotophone. However, its diaphragm is much lighter in weight. But from there on the resemblance disappears. The diaphragm of the telephone receiver pushes against loose particles of carbon and changes their ohmic resistance, thereby giving an electric current proportional to the vibrations of the diaphragm. On the other hand, the diaphragm of the pallophotophone has attached to it, by a lever, a tiny mirror, the combined weight of which is about half that of the head of an ordinary pin. The diaphragm, the lever, and the tiny mirror are the sole mechanical parts which vibrate. The mechanism of the pallophotophone transmitter, being so exceedingly light in weight, is capable of responding to the overtones of sound and music which give the fine distinctive quality and timber of different voices and instruments. While the telephone transmitter still has the practical advantage of compactness, simplicity, and a single step from sound vibration to electric vibrations, it has a greater mass and therefore is unable to respond to the distinctive overtones. While this loss of the overtones has little to do with clearness of speech it takes away from the cultivated enjoyment of musical qualities both of voices and instruments. The pallophotophone has this improving feature that will spread, still further than the telephone can, the emotional pleasures of a fine art. In the pallophotophone the light from an incandescent lamp is focused upon the tiny mirror of the transmitter. The reflected dancing beam of light, illuminating more or less the photo-electric cell, induces a current in the cell corresponding to the frequency and intensity of the sound vibrations that impinge on the diaphragm of the transmitter. Thus the photo-electric cell in the pallophotophone performs a service corresponding to the carbon granules in the telephone transmitter. (Fig. 11). In the realm of small dimensions there are features of interest in what the pallophotophone will accomplish. For example, the photo-electric cell will respond to a movement of the beam of light of one-thousandth inch (0.0024 cm.) at the cell opening, and this movement, too tiny to be visible to the naked eye, will give a fairly loud reproduction of speech or music in a pair of head-phones in the circuit of the first pliotron. Calculating back to the corresponding movement of the diaphragm, the resulting figures seem incredible. The diaphragm's movement is magnified two thousand times in the movement of the light on the photo-electric cell. Therefore, when the light has moved one-thousandth of an inch the diaphragm has moved only a half-millionth of an inch. In other terms, a movement of the diaphragm of only twenty-five times the diameter of a molecule will be reproduced in clear speech and music. If instead of projecting the dancing light on to the photo-electric cell, to translate the dance of light into electric current, the reflected beam of light is caused to fall on a moving film the sound is, so to speak, “canned” photographically on the film for future use. This combination is shown schematically in Fig. 6. The reproduction of the voice either directly or through the means of a film is so identical that hundreds in audiences, who have been asked to decide which reproduction was from the voice directly and which from the film of the same voice, have been unable to distinguish between the two. To follow briefly through the consecutive steps of the pallophotophone — first come the voice vibrations; second, the corresponding vibration of the diaphragm which carries the tiny mirror; and third, reflected from the vibrating mirror the vibration of light which falls either on the moving film to form a permanent record of the sound or, otherwise, on a photo-electric cell which gives faithfully perfect electric oscillations for wireless broadcasting or telephone receivers. The foregoing description gives the essential elements — in addition, there are of course amplifiers of current, motive power for moving films, horns, and the various mechanical parts in detail which will be found described in the text following. The apparatus for reproducing speech from a film is shown in the photograph, Fig. 10.