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Mar., 1939]
DISK RECORDS FOR DIRECT PLAYBACK
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unless the winding were kept small. This is not consistent with high output. Therefore a shunt capacity is connected across the pick-up which, by reacting broadly with the inductance, increases the response through a portion of the upper range. The reproducer has a slightly rising characteristic at the upper end, enough to offset highfrequency needle or transfer losses encountered at a mean record diameter of twelve inches at 33.3 rpm.
In making high-fidelity records, including disks for immediate playback, use is made of what is known as pre and post-equalization or complementary compensation . Because of the energy distribution in most speech and music, it is possible to accentuate the higher frequencies when making a record and attenuate them in reproduction, thereby reducing the surface or ground-noise due to the record stock.
Fig. 7 shows the recording and reproducing amplifier characteristics and the ideal overall to which must be added the characteristics of the reproducer. This method of reducing surface noise can be used successfully in most cases without adding appreciable distortion.
Tests indicate negligible wear of the diamond stylus on nonabrasive records. On shellac composition records there is sufficient wear after 5000 ten-inch faces to justify replacement of the point. This is considerably longer life than so-called permanent points of iridium or sapphire, when used on abrasive records with the same pressure of two ounces.
An improvement in pick-up tracking has been made by offsetting the head with respect to the arm. This angle, which is about ten degrees, results in two positions of tangency with the groove, one near the center of the record and the second near the outer edge. The error in tracking angle between these positions is less than 5 degrees.
REFERENCE
DIAMOND / \DRIVE LINK
FIG. 8. Construction of pick-up.
1 PIERCE, J. A., AND HUNT, F. V.: "Distortion in Sound Reproduction from Phonograph Records," /. Soc. Mot. Pict. Eng., XXXI (Aug., 1938), p. 157.