Projection engineering (Sept 1929-Nov 1930)

Page 12 Projection Engineering, September, 1929 Energy Levels The energy level of the whole reproducing system is controlled ordinarily by the means of a fader. The "standard" positions of the fader for different records or portions of records are indicated by a schedule supplied to the operator. Varying local conditions, such as the size of the audience and so forth, often make a departure from standard schedule desirable. It is customary, therefore, to have two or more monitors seated in different parts of the house and in communication with the projection booth. These experienced individuals can thus transmit the request to raise or lower the level a point or two when circumstances require. Film Recording We now propose to consider the optical method whereby a photographic record corresponding to the fluctuating sound impulses is obtained upon a film in the studio and subsequently reproduced as sound in the theater. There are two fundamental forms which such a film record may take. The one most commonly employed has a constant transverse width and an intensity or "density" which varies from point to point along the film in accordance with the frequency and amplitude fluctuations of the corresponding sound waves. Such a record is shaded in appearance and similar in character to a photograph of a heavily-banded light spectrum. It may be obtained by varying the intensity of the light source, either directly or indirectly, or by changing the effective width of the narrow slit opening through which the film is exposed. In either case the density across the record is constant at any given point along the film, giving thus the characteristic banded effect. In the second type of a record, a constant source of illumination is employed, while the electrical impulses corresponding to the sound waves operate a mechanism that serves to vary the relative amount of the trans The ERPI photoelectric cell amplifier. Note the suspension for absorbing vibrations. verse slit which is illuminated at any given instant. This produces a blackand-white non-shaded record having a fine saw-tooth appearance. The Neon Lamp There are three distinct methods of optical recording which may be described here, all of which are fundamently adapted to produce a banded type of record. The first makes use of the neon or similar type of glow lamp which is so well known in the art of television. The intensity of the light emitted by such a lamp can be varied rapidly and easily by a fluctuating voltage applied to it. If, now, our film is driven uniformly along behind a narrow transverse slit which is illuminated on the opposite side by such a lamp, we have the means of producing a banded film record corresponding to the variations of the sound waves picked up by the studio microphones. The Light-Valve The second method utilizes a constant source of light and employs an ingenious device known as a "lightvalve." This consists of two parallel duraluminum tapes each six mils wide and three mils thick. These are so placed in the optical system that, when at rest, they form an effective slit which, viewed against the source of light, presents an opening two mils wide by one quarter of an inch long. By means of a high grade optical system an image of this slit in the light valve is thrown onto the film in the form of a transverse line of light one-eighth of an inch long and, normally, only one mil wide. The two duraluminum tapes form an electrical loop circuit and they are so located in a steady magnetic field that, when a pulsating current is passed 'through them, they move in opposite directions. In this manner the effective gap opening between the tapes, and, hence, the width of the image line on the film, is varied according to the frequency and amplitude of the electrical impulses. Such a modulation of the light gives, of course, the characteristic banded record on the moving film. We note that in this case the time during which each spot on the film is exposed to a constant light source varies, while in the other two methods described each spot on the film record is illuminated for the same time interval by a modulated light intensity. It is found desirable in practice to adjust the tension on the tapes until they have a natural frequency of about 7000 cycles per second. Under this condition a 100 percent, modulation of the light, i. e., opening the valve slit to a maximum of four mils and just closing it completely requires about ten milliwatts of power at the lower audio frequencies and about 0.1 milliwatt at the natural frequency of the tapes. The Kerr Cell A third method of optical recording utilizes an interesting device known as the Kerr cell. A beam of light of constant intensity is passed first through a Nicol's prism which polarizes the beam in a particular plane. It is then passed through a narrow gap between two electrodes and subsequently through a second Nicol's prism set at 45 degrees to the first. The gap between the electrodes in the Kerr cell is filled with nitro-benzol, a liquid which has the property of rotating the plane of polarized light passing through it when subjected to an electrostatic Illustration of the Western Electric "light valve." field to a degree proportional to the impressed voltage. Obviously, then, a modulation of the effective intensity of the light source may be produced by applying an alternating potential to the electrodes, so that, if we employ a fixed transverse slit against the film, a banded record will result. The separation of the electrodes and the length of the light path between them determine the voltage necessary to produce 100 per cent modulation, i. e., variation between full transmission and total extinction of the light. As in the case of the neon lamp, the degree of modulation of the Kerr cell is essentially independent of frequency, but in the Kerr cell the degree of modulation is proportional to a cosine function of the amplitude which, obviously, limits the usefulness of this device. In all three methods of optical recording the nominal effective time of exposure is about 1/18,000 second, corresponding to the nominal film speed of ninety feet per minute. This means that modulation of the record at a frequency of 18,000 cycles or higher would be nil with an increasingly better modulation as we go below 18,000 cycles. In the workable audio range, however, the modulation is satisfactory, or can be made so by the use of equalizers. The technique of studio recording follows along the general lines previously described. The responses of the several studio microphones, prop