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BELL SYSTEM TECHNICAL JOURNAL 



intensity than those of lower frequency. If, however, we assume an 

 amplitude of .0001 in. at 5,000 cycles and assume a linear record 

 speed of 70 ft. per minute, then the minimum radius of curvature 

 of the undulation of the groove is .00193 in.** With the foregoing 

 assumption, the radius of curvature of the undulation of the groove 

 becomes equal to that of the needle point at about 7,000 cycles. 

 Taking into account the lower intensities of sounds encountered at 

 these high frequencies, it is obvious that present commercial needle 

 points are quite capable of following the high frequency undulations 

 of the groove up to frequencies of at least 10,000 cycles. The limita- 

 tions of high frequency reproduction commonly found in the past are 

 associated with limitations in the design of the pickup or reproducer 

 and relate either to inability of the record groove to drive the needle 

 point, with resultant chatter, or inability of the pickup structure to 

 transmit high frequency motions from the needle point to the armature. 



Electric Pickup 

 Large advances have been made within the last two or three years 

 in designing electric reproducing structures. The mechanical im- 



5 



500 1000 



FREQUENCY 



Fig. 10 — Response of a 2-a pickup driven by constant velocity pressings. 



pedance at the needle point has been reduced so that the needle 

 point truthfully follows the undulations in the groove without necessi- 

 tating excessive and somewhat destructive bearing pressures. At the 

 * The minimum radius of curvature is computed by the formula 



where : 



^' IOOttM/^ • 



Re = minimum radius of curvature in inches. 

 V = linear speed in feet per minute. 

 A = amplitude of vibration in inches. 

 / = frequency in c.p.s. 



