International photographer (Jan-Dec 1934)

Eighteen The INTERNATIONAL PHOTOGRAPHER August, 1934 Motion Picture Sound Recording By CHARLES FELSTEAD, Associate Editor Chapter XI [HE two preceding chapters discussed audiofrequency amplification as it is applied to motion picture sound recording. The first of those chapters dealt with the theoretical phases of vacuum tube operation and audio amplification ; while the chapter last month described the amplifying equipment used in the direct recording circuit. The material presented here deals with the monitoring amplifier and associated equipment and the methods of direct and photoelectric cell monitoring. The chapter last month mentioned that the sixth branch from the bridging bus leads to the monitoring amplifier and through it to the monitoring horns. It is true that for normal monitoring the monitoring amplifier is connected directly to the bridging bus in that manner; but there is a relay (No. 1) in the input circuit of this amplifier that permits it to be switched to either of the photo-electric cell amplifiers in the film recording machines when it is desired to check if the sound is reaching the film safely. The position of this relay is apparent in Figure 1. When the relay is not actuated by the passage of a current through its winding, the monitoring amplifier is connected directly to the bridging bus, and the circuit is said to be arranged for direct monitoring. That is the normal line-up for monitoring during recording and rehearsals ; so we will discuss that arrangement first. The Monitoring Circuit The sixth branch from the bridging bus connects directly to a variable attenuator of the T-type. This attenuation network is identical with the recording machine attenuators described last month. Like them, it has a constant input and output impedance of 500 ohms ; and it is variable in ten steps of two decibels each, thus providing a maximum loss of twenty decibels. (Last month it was stated in error that these attenuators possessed twenty steps of one decibel each.) This monitoring attenuator is mounted on a panel in the center bay of the main amplifying system of the channel. The attenuator is inserted in the circuit at this point to permit balancing of the three monitoring circuits. Smaller attenuators, which will be discussed in a later chapter, are connected to the outputs of the two photoelectric cell amplifiers. By regulating these three attenuators, the transmission engineer is enabled to obtain such a perfect balance of the direct and the two photoelectric cell (PEC) monitoring paths that the monitor man is not aware of a change in the sound volume in the monitor horns when he switches from one to the other of the monitoring circuits. The only noticeable difference when the switch is made from direct monitoring is the mushiness of the sound from the photo-electric cell amplifiers, that distortion of the sound quality being produced by the modulated light beam passing through the emulsion and the celluloid base of the motion picture film in the recording machine. Since the PEC monitoring is used only for a moment during each sound "take" to verify that the speech current is reaching the light valve and operating it, this distortion of sound quality is not objectionable. The Monitoring Amplifier After leaving the direct monitoring attenuator, the circuit passes through the contacts of the No. 1 relay to the input of the monitoring amplifier. Two separate amplifiers that closely resemble the amplifiers in the recording circuit are combined to form this monitoring amplifier. The first of these amplifiers is a three-stage affair of exactly the same type as the main recording amplifier, the only variation being that the primary windings of its input transformer are connected to provide a 500-ohm input impedance that matches the impedance of the bridging bus and the monitoring attenuator. It will be remembered that the main recording amplifier has a 200-ohm input impedance. The output of this first monitoring amplifier feeds directly into the input of a single-stage push-pull amplifier that is a duplicate of the bridging amplifiers. But this second amplifier has an input impedance that matches the output impedance of the main monitoring amplifier, instead of having the high input impedance that characterizes the bridging amplifiers. Otherwise there is no difference between the two amplifiers. This push-pull power amplifier is not equipped with a gain control, all regulation of the level of the speech current at its output being provided by the twenty-two step gain control on the main monitoring amplifier. The tubes used in these two amplifiers that form the monitoring amplifier are the same as those used in the equivalent recording amplifiers. Alternative Alain Amplifiers It must be mentioned at this point that there is somewhat of a variation in the type of main recording amplifier and first monitoring amplifier used in different installations. There is an alternative type of amplifier that is frequently substituted for the three-stage amplifier previously described. This alternative amplifier has only two stages of amplification. The first tube is of the voltage amplifier type ; and the second tube is of the power amplifier type, but it is operated at lower power output, just as is the second tube in the three-stage amplifier. These two tubes are the same type as the tubes used in the three-stage amplifier ; and the circuit employed closely resembles the circuit of the first two stages of the larger amplifier. The input and output coupling devices of this twostage amplifier are transformers; and coupling between the tubes is provided by impedance coupling. A tapped potentiometer in the grid circuit of the second tube furnishes a means of regulating the amount of gain in the amplifier. Tap switches of four and six points each connected to this potentiometer provide rough and fine regulation of gain. A filament rheostat and jacks that permit measurement of the filament and plate current drawn by the tubes are included. Substitution of Amplifiers A single one of these two-stage amplifiers is frequently used in place of the three-stage amplifier as the main recording amplifier. It provides ample gain for the normal recording installation ; and its tubes are not Please mention The International Photographer when corresponding with advertisers.