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May, 1930] WESTERN ELECTRIC RECORDING SYSTEM 523
to the film recording machine. The sound may be played back either in the monitor room or it can be reproduced by means of a loud speaker on the stage. The main purpose of the playback is not to gauge the quality of the recording from a frequency characteristic standpoint inasmuch as the playback from the wax is inferior because of the wax itself, but to give the director and the artists an opportunity to gauge the results of their efforts. If a line has been spoken incorrectly, or if there is a break in the sound which is unnatural, or if there has been some disturbance which has been inadvertently recorded, the playback will provide instantly information upon which may depend whether or not a retake is necessary. Thus the playback is an extremely important feature of a properly designed sound recording system, not so much because of the technical aspects, but because of the economical value in time and labour saving.
Film Recording Machine. — Sounds are recorded on film by exposing film running at a speed of 90 feet per minute to light variations which are linear with respect to sounds originating in the studio. The electrical energy is delivered from the main amplifier to a device in the film recording machine known as the light valve. This light valve consists of two duraluminum ribbons spaced 2 mils apart which lie in a powerful magnetic field. When current flows through these ribbons they open and close in accordance with the current flowing through them. A source of illumination is provided and the image of the aperture of the ribbon is focused on the film through an optical system having a two-to-one reduction. The normal slit image on the film will therefore be 1 mil in width.
Exposure is a product of intensity multiplied by time. In light valve recording the intensity is constant and time is the variable. With the film running at its normal speed the time of the average exposure is Vis.ooo second. This exposure with the intensity of light used is satisfactory for recording on positive stock. The tension of the ribbons in the light valve is so adjusted that they will vibrate naturally at about 7500 cycles per second. Due to their extremely low mass the deflections of the ribbons are substantially linear with the current flowing through them over the recording frequency range. The light valve despite the dimensions of the ribbon used is in reality a very rugged device, is not sensitive to jars or vibrations and when properly used does not require fussy adjusting. Light valves have*frequently been used for days and weeks without