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Mar., 1939]
DISK RECORDS FOR DIRECT PLAYBACK
249
Under normal operating conditions, the overall distortion of the combined recording and playback operations is less than 5 percent. The rms. total distortion to be expected at 400 cps. at varying degrees of modulation is indicated in Fig. 4. These observations were made at a record speed of 33.3 rpm. and a diameter of 12 inches.
15 14 13 12 II 10 9 8
DISC DIAMETER INCHES
FIG. 5. Average losses at various record diameters at 33.3 rpm.
All disk systems suffer from high-frequency transfer losses and increased distortion when the record diameter or linear surface speed becomes too small. The average losses encountered at various record diameters at 33.3 rpm. are shown in Fig. 5. These are for soft composition blanks used in direct playback work. The losses are less serious on standard records pressed from harder material.
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Z CD
25-10 a z-15
50
100
5000 10,000
FIG. 6.
500 1000
FREQUENCY CYCLES PER SECOND Reproducer characteristic with standard 12-inch pressing at 33.3 rpm.
These losses are caused by a combination of the finite size of the reproducing point, the wavelength of the recorded sound, the weight of the pick-up, and the compliance of the record stock. It is possible to compensate to some extent for the losses but it has not been found practicable to attempt full compensation above 6000 to 7000 cps. because of the serious attenuation in the upper register. Therefore, in making high-fidelity records where quality is of paramount importance, the best results are obtained by dividing the time into two or more disks and confining the recording to reasonably high linear speeds.