American cinematographer (Feb-Dec 1929)

Record Details:

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February, 1929 AMERICAN CINEMATOGRAPHER Five that the Kerr cell is some 25 times better than a string galvanometer used in this manner. Fig. 7 shows a method of using a string galvanometer with the string vertical to produce a variable density type of record. The upper diagram represents a plan or horizontal view of the system and the lower diagram a vertical elevation. The source of light is focused by means Fig. 7 Method of using vertical string galvanometer for making variable density type of record. of the condensing lens C on the ribbon of the galvanometer. The galvanometer is imaged on the film in the vertical plane by means of a cylindrical lens. Consequently, the solid angle of the cone of light (or pyramid of light in this case) reaching the film is the product of the angles ©i and ©2. It is obvious that the value of © may be made approximately as great as the angle © in the Kerr cell arrangement of Fig. 6. The maximum value of ©i requires a little more careful consideration. If no mechanical difficulties were encountered, the galvanometer should be placed as close to the film as possible, the limit being reached when the opening subtends the same angle from the film as the lens C. In other words, except for mechanical limitations ©i may approach the value of ©2 in a well-designed system. If, as usually happens, the magnets of the galvanometer prevent it from being placed as close to the film as desired, the string can be imaged at the proper point by means of a cylindrical lens inserted in the upper diagram. Since the opening in the galvanometer, however, is already as bright as the source and since it is impossible to increase this brightness by any optical system, the limit of light-gathering power of the latter system is the same as the case just considered. Consequently we may sum up the consideration of the vertical string type of string galvanometer with the statement that by properly designing the optical system it may be used to produce records of the variable-density type but is quite useless for the production of records of the variable-width type. Recording with a String Galvanometer : String Horizontal We have already seen that the deflection obtained with a string galvanometer suitable for recording high frequencies are small, usually of the order of 0.001 inch or less. Since the width of the slit is of this order of magnitude, it is possible to make sound records of the variable-density type by means of the system shown in Fig. Method of using horizontal string galvanometer for producing variable density record. 8. The source of light is focused on the film. Each of the four lenses shown may conveniently be a microscope objective of 66 mm. focal length and 0.25 numerical aperture. Since the magnification of the source of light on the film is approximately unity, the illumination is four times as intense as that produced by the same objectives used in the Kerr cell arrangements of Fig. 6 because of the loss occasioned by the Nicol prisms. The sound produced by this system is slightly distorted in the high frequencies, when the velocity of the string approaches that of the film. Since the effect of distortion, however, is to introduce higher harmonics, it may not be serious because of their relative inaudibility. This ar rangement is as satisfactory as any for the production of sound records of the variable-density type. Records of the variable-width type are difficult to produce with a horizontal string galvanometer. Recording with an Oscillograph: Variable Density Records In the string galvanometer method of recording, a mechanical part such as a wire or ribbon is used as the movable portion of the light valve. Instead of utilizing an actual motion of this sort the equivalent effect may be produced by causing the image of some stationary aperture to move with respect to some other stationary aperture. This can be done most conveniently by the use of an oscillograph mirror. The oscillograph consists essentially of two parallel wires stretched taut in a magnetic field. These wires are so connected together that the current flows upwards, say, in the one at the same time that it flows downwards in the other. A small mirror is attached by its edges to these wires in such a manner that a current causes a rotation of the mirror, usually about a vertical axis. The optical system of the oscillograph has been made the subject of a previous investigation.'1 Fig. 9 shows an adaptation of this instrument to the production of sound records of the variable-density type. The source Oscillograph Arrangement used with oscillograph for producing variable density records. of light is focused on the oscillograph mirror which in turn is refocused on the film, Fig. 9 being a horizontal or plan view of the optical system. This figure has been drawn as though the mirror produced no deflection of the beam, but acted only as a stop or aperture in the system. In any actual case, of course, the light should fall as nearly perpendicularly on the mirror as possible and should be reflected in the same manner. At each of the objectives is placed a grid consisting of parallel wires or ribbons with alternate transparent spaces. At the oscillograph mirror is a single lens which images the first grid on the second. Since light traverses this lens twice, the effect is the same as if there were two lenses, and Fig. 9 has been so drawn. Before a program is recorded, the mirror is so adjusted that when no signal is impressed on the oscillograph, the image of the wires of the first grid half covers the spaces of the second grid. The grid should be so designed that the maximum deflection of which the mirror is capable will produce a shift of the image equal to one-half of the width of the spaces. Consequently, when a signal is impressed on the oscillograph, the amount of illumination at the film varies in accordance with the sound pressure at the microphone. This system has been shown with a close-up slit but it may be modified for use with an imaged slit as in Fig. 4. In practice, the width of the mirror in a suitable oscillograph is rarely more than 0.5 mm. Since this mirror is imaged on the film, it must obviously be magnified at least six times in order to cover a sound track 3 mm. wide. Since the illumination in the image is about inversely proportional to the square of the magnification plus one (see footnote 4), it follows that in this case the illumination can be only 1/12 as intense as with the unit magnification of the system shown in Fig. 8. This may be partially remedied by inserting a cylindrical lens with its axis perpendicular to the plane of Fig. 9. This is usually possible because the height of the oscillograph mirror is greater than the width of the recording slit so that unit magnification or less can be used in this plane. If we assume the magnification in the vertical plane to be unity and the magnification in the horizontal plane to be 6, the light-gathering power with this arrangement is roughly two-sevenths as great as with the system of Fig. 8.