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350 CINEMATOGRAPHIC ANNUAL ,
microphones into a single complex electrical wave which passes through the amplifier, increasing in strength at each stage. These amplifiers end in what is termed a power stage. This stage is so designated because, up to this point, we have been amplifying voltage and at this point we use that voltage to control vacuum tubes whose capacity in watts, the unit of power, is large enough to drive the mechanisms which do the work.
The output of this power stage is split into two channels. One channel consists of a monitoring circuit in which the electrical current is delivered to a dynamic cone and converted back into sound. If there were no losses anywhere in the system, it would be then possible for an operator listening to this loud speaker to hear exactly what is happening on a set or in the studio with approximately the same loudness as the original sounds. Unfortunately, however, each portion of the system so far described introduces various forms of distortion so that the sound one finally hears from the monitoring speaker is only approximately the quality of the original. These distortions are quite small, however. As a matter of fact, if it were possible to pour reproduced sound into a theatre or auditorium with the fidelity of the sound from a monitor speaker, I am sure motion picture audiences would feel much more kindly toward talking pictures.
Returning to the amplifier, we find that the rest of the power delivered is used to actuate the device which does the actual recording. Up to this point, all methods of recording are identical. From here on, each system makes use of a different recording device.
A side view of the RCA Photophone recorder mounted for Operation is shown in Fig. 1.
In Fig. 2 the adjustments for the optical system are shown in more detail.
The RCA system recorder is called a vibrator. It consists of a flat wire .0005" thick and .005" wide, stretched over two ivory bridges spaced 7/16 of an inch apart. This loop is put under tension by means of a spring attached to a tiny ivory pulley at the closed end of the loop. The ends of this wire, which are spaced 0".01 apart, are attached to binding posts and connected to the output of the amplifier. At a point half way between the ivory bridges, there is a very tiny glass mirror ce