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SOUND OPTICAL SYSTEM 173
noise that accompanies the reproduction of the film. Most commercial machines will show definite tone waver; on a better-grade machine the waver should be almost imperceptible to the average Listener. It would be good practice to run a short length of about 50 ft. of the 3000-cycle flutter test film as a test every day before a machine is used. This would spare many trained ears from the horror of projectors that wow* and flutter, defects that are all too common and to a considerable degree nn necessary among many machines in everyday use.
Sound Optical System, Amplifier, and Loudspeaker
The Optical System. In most projectors the optical system is quite simple in design and consists essentially of a small helical coiled filament lamp of 6-v., 1-amp. or 4-v., %-amp. rating, the filament of which is imaged on the moving film by a simple cylindrical lens.f The light beam emerges from this lens and impinges upon the sound track as a thin line of light perpendicular to the direction of travel of the film.
The light beam is between 0.070 and 0.072 in. long and 0.001 to 0.0005 in. thick; its center is adjusted to scan the film at a distance of 0.058 in. from the unperforated edge of the film. After passing through the film and being modulated by its movement, the light beam is directed to a photoelectric cell where it causes changes in current that arc amplified by an amplifier and reproduced by a loudspeaker.
The thickness of the slit image depends upon the optical reduction ratio at which the cylindrical lens is worked. A relatively wide slit will
* Wow represents flutter of dominant rate less than approximately 5 per second.
t This simple system, called the focused filament type, is very efficient in terms of the amount of light flux delivered to the photocell compared with the amount of light flux emitted by the exciter lamp. Because of its simplicity, it does not afford much opportunity for effective color correction ; for this reason it is not unusual for the effective slit width to vary markedly from the geometrically calculated slit width. Effective operation with low distortion demands that when variable-area films are scanned, the photocell used shall be very uniform in surface sensitivity. As commercial photocells are not checked accurately for surface sensitivity uniformity, it is possible that occasional cells may show intermodulation distortion as high as 50% if by chance a non-uniformly sensitive cell is chosen. Since the light from a variable density sound track always covers substantially the same portions of the cell surface, this distortion effect does not occur with variable density films. Gross nonuniformity of surface sensitivity of a photoelectric cell may he detected by using a "snake" track test film, ASA Z."2.7 that is available through the Society of Motion Picture Engineers. An output meter connected across a dummy load substituted for the loudspeaker will reveal output level variations if a poor cell is in the machine.