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248 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
bilateral form as shown in Figure 59. The general arrangement of an optical system to accomplish this is shown in Figure 60.
The Slit and Its Effects. Ideally, sound is recorded and reproduced with a pencil of light that has — figuratively speaking — a very fine point. Ideally also, the film upon which the light beam acts is made up of sensitized particles that are also very fine. In addition, the light slit or pencil edge (similar to the flat edge of a carpenter's pencil) is located exactly at right angles with respect to the direction of travel of the film both in recording and in reproducing.
Unfortunately, these idealized conditions are not met in practice. The light slit or aperture has finite size and is not infinitely small. It is not located exactly at right angles to the direction of travel of the film in either the recording machine or in the sound projector. Also, the film structure is not infinitely fine, since the sensitized particles are finite in size.
In recording it is, of course, necessary to transmit sufficient light through the optical system to provide adequate exposure of the film. As the effective slit size is made smaller, the amount of light that can be transmitted through the optical system becomes smaller. Thus, the design problem is a compromise between the amount of exposure available and the slit width; to achieve good exposure with a sufficiently small width of slit requires design ingenuity — and increased cost. In recording, a relatively wide slit (such as 0.001 in.) causes both high-frequency loss and serious harmonic distortion; in practice, the distortion is the limiting factor because of the very objectionable quality of the sound that results.
In reproducing, a relatively wide slit causes primarily high-frequency loss and some harmonic distortion. Generally speaking, most 16-mm sound projectors have used slits that are too wide to permit good sound reproduction.
Slit tilt (or azimuth error as it is often called — deviation of the slit from exact perpendicularity to the direction of travel of the film) introduces really serious harmonic distortion in both recording and reproduction. For a particular slit tilt, the percentage of distortion climbs rapidly as the recorded frequency increases. In general, the azimuth error in recording machines is of a much lower order than the error in sound projectors; in modern recording machines, such as RCA, Western Electric, and Maurer, azimuth errors are quite small. In some sound projectors it is not uncommon to find a slit tilt of the order of one degree