Journal of the Society of Motion Picture Engineers (1930-1949)

density than normal can be employed because of the cancellation of the crossmodulation products in push-pull reproduction, thus producing results that equal the cancellation achieved by the normal negative and print process. The nonlinear exposure versus light-transmission characteristics of the film enters into the problem of making a variabledensity direct-positive. In order to determine the nonlinear characteristics, samples were exposed by sending direct current in fixed steps through the light valve in a standard recording machine. These strips were processed using the M-G-M standard release development procedure. The resulting strips were measured by inserting a 400-cycle chopper in the light beam of a standard filmreproducing machine and measuring the audio signal at the output of the photoelectric cell amplifier. By this procedure, the test included all the variables encountered in the recording and reproducing systems. The resulting characteristic is shown in Fig. 1 . The above tests were repeated over a period of time in order to check the stability of the variables involved. The results proved that this method of making a variable-density direct-positive was practical. Figure 2 shows the schematic of an amplifier, the characteristics of which are the reciprocal of Fig. 1 . The first stage of the amplifier has a practically linear characteristic and is used as a voltage amplifier directly coupled to the second and third stages or sections. The noisereduction control voltage is fed to the input grid in series with the secondary winding of the input transformer. The second stage controls the shaping of the middle and upper end of the curve by being biased negative almost to cutoff; the signal received from the cathode resistance of the first stage is such that it is linear for the lower half of the range, and becomes nonlinear as the driving signal increases. The third stage controls the shaping of the extreme upper end of the curve. This is accomplished by biasing the grids negative beyond cutoff and driving them with a signal voltage that will cause the tubes to conduct only on the positive peaks of the signal. Figure 3 shows the method of obtaining this curvature by using a number of tubes in parallel, the grids of which are biased to operate at various points in the nonlinear portion of their grid volts versus plate-current curves. The overall shape is obtained by adjusting the balance between the grid and signal voltage, and the number of tubes used. The light valve must be directly coupled to this amplifier because the resulting distorted signal is composed of direct current; the signal fundamental and a large amount of harmonics of the signal, and in addition the noise-reduction signal must be altered by this circuit. Figure 3 shows the contributions of all three stages and the overall characteristic of the nonlinear amplifier. A Western Electric RA-1238, 200-mil pushpull variable-density light valve was used in these studies and in recording the demonstration film which was run at the close of the paper. It is necessary to employ one amplifier of the type shown in Fig. 2 for each component of the pushpull valve. This results in a classical type of push-pull reproduction, and a higher degree of an overall linearity is obtained than when using a standard single track. However, good quality is obtained from a single track provided care is taken in the setting of the operating parameters. A direct current is applied to the light valve, in opposition to the cathode current, for adjusting the static opening of the valve for zero signal input. A noisereduction bias current is applied to each component of the light valve through its associated amplifier. Since the resulting sound track is in effect a positive, the ribbons are either mechanically or electrically biased open, rather than closed as in a normal negative-positive recording. The action of the input noise O. L. Dupy: Direct-Positive Variable-Density Recording 103