Journal of the Society of Motion Picture and Television Engineers (1950-1954)

beam a certain amount of noise which is undesirable. Between the tungsten coil | and the first optic, a blue filter is placed. This filter serves two purposes. The first is to limit the wavelength which will pass through the crystal structure. This is done since retardation is a function of wavelength. Second, the filter acts as a heat-absorbing filter. For | tunately, both of these requirements contribute only second-order effects to the output, and with no filter the increase in distortion is quite slight. However, since there is ample light available for •our application, a blue filter has been included. The wavelength to be transmitted was determined by the spectral response of the motion picture film used. The lamp filament, which is of the tight-coil variety, is imaged slightly oversized and out of focus on a slit whose dimensions are 0.1 in. X 0.0006 in. The filament image is run slightly out of focus to give a more uniform light distribution across the slit since the coils of the filament, even though they are wound quite closely, give some illumination variation. The slit now becomes the light source and the next optic picks up this aperture and collimates the light beam for passage through the crystal structure. The laminar structure including the crystal has already been described. The placement of the slit pickup optic just mentioned is such that the angular field requirement is met. The final optic or projection optic gathers the collimated beam from the crystal and focuses the image of the slit onto the film. This optic is adjustable so that the image may be sharply focused on the film for different film distances. The magnification of the optical system is unity and therefore the modulation slit height remains 0.0006 in. high. This allows 9,000 to 10,000 cycles to be impressed on the motion picture film without amplitude loss when it travels at 90 fpm. The light transmission efficiency of the entire modulator is 16%. Electrical Requirements It was pointed out in an earlier section that the magnitude of the voltage for a given retardation is a function of wavelength. Since a filter has been inserted between the tungsten lamp and the crystal, the wavelength of the transmitted light is known. For 4500 A the required voltage is 7.4 kv per half-wave of retardation. The retardation required for 75% modulation is something less than a quarter-wave and requires between 1600 and 1800 v rms drive. It is interesting to note for zinc sulfide the driving potential is approximately 500 v. The electrical drive requirements for the ADP crystal structure can be determined from the driving point impedance of the structure. The driving point impedance of these crystal structures is capacitive, and has a dissipation factor of approximately 0.07 which is substantially constant to 50 kc. The capacity measured at the crystal terminals is 64 /z/xf and is also substantially constant with frequency. The slight variations from a constant as a function of frequency of these factors does not affect the design of the driving system and does not affect sound quality. The amplifier used to drive the modulator is a 50-w high-quality amplifier working into a special output transformer designed to drive this load over the frequency range desired. This design was accomplished, giving an overall system response essentially uniform from 30 cycles to 10,000 cycles. Studies at this laboratory indicate that frequency components above 8500 cycles are very often distorted during film processing and, in addition, many theaters have equipment which would only serve to distort frequency components above this figure. As a result, a wave filter is included in the electrical system to limit the nominal frequency passband from 75 cycles to 8500 cycles. The low end has been attenuated to filter out hum which may exist in some Dressier and Chesnes: Electrooptic Recording 213