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1949
MAGNETIC RECORDING
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near the region of maximum energy in the spectral-energy-distribution curve. The second harmonic is so low in level that it may be neglected. This figure for overload has some justification, since it has been found that excellent recordings of all types of material can be made with this as the peak recording level. On this basis, a signal-to-noise ratio of more than 55 decibels can be obtained when the frequency characteristic given by curve B (Fig. 11) is employed in addition to the usual 40-cycle high-pass filter. With pre and postequalization, the effective ratio lies between 58 and 62 decibels.
MAGNETIC RE-RECORDER
The magnetic re-recorder, like the recorder, is made by applying a number of conversion parts to a standard film machine. The Western Electric RA-1251B re-recorder is the basic unit, to which are added mechanical and electrical items needed to reproduce the magnetic sound track. One method of scanning the track is accomplished at the scanner drum in a manner analogous to that utilized in the recorder. In the film path, the conversion involves removing the lens-and-prism assembly, replacing the scanner drum
and installing the reproduc
Fig. 12 — Magnetic-re-recorder film path.
ing head and its movable, damped support and its shielding structure. The resulting film path is shown in Fig. 12.
In another version of the re-recorder, the reproducing head replaces the roller in the film path just beyond the scanner drum. Thus, in this model optical facilities are retained, and the machine can be used to play either photographic or magnetic film.
Removing the circuit items not needed in magnetic reproducing, such as the phototube amplifier and the lamp-control equipment, leaves ample room for the magnetic-transmission system. This system, shown in block schematic in Fig. 13, includes RA-1214-D amplifiers, high and low-frequency equalizers, and keys for switching