The Reproduction of Magnetically Recorded Signals 



R. L. WALLACE. JR. 



(Manuscript Received July P, 1951) 



For certain speech studies at the Bell Telephone Laboratories, it has been 

 necessary to design some rather specialized magnetic recording equipment. 



In connection with this work, it has been found experimentally and theo- 

 retically that introducing a spacing of d inches between the reproducing head 

 and the recording medium decreases the reproduced voltage by 54.6 {d/\) 

 decibels when the recorded wavelength is X inches. For short wavelengths 

 this loss is many decibels even when the effective spacing is only a few ten- 

 thousandths of an inch. On this basis it is argued that imperfect magnetic con- 

 tact between reproducing head and recording medium may account for much 

 of the high-frequency loss which is experimentally observed. 



Introduction 



WITHIN the last few years there has been increasing use of magnetic 

 recording in various telephone research applications (examples are 

 various versions of the sound spectrograph used in studies of speech and 

 noise). Some of these uses^ have required a reproducing head spaced slightly 

 out of contact with the recording medium. Experimental studies were made 

 to determine the effect of such spacing and the results were found to be 

 expressible in an unexpectedly simple form. The general equation derived 

 is believed to be fundamental to the recording problem and to account for 

 much of the high-frequency loss that is found in both in- and out-of-contact 

 systems. 



This paper discusses results of the experimental study and presents for 

 comparison some theoretical calculations based on an idealized model. 



Measurements of Spacing Loss 



In order to measure the effect of spacing between the reproducing head 

 and the medium, an experiment was set up as indicated in Fig. 1. The 

 recording medium used was a 0.0003 inch plating of cobalt-nickel alloy- 

 on the flat surface of a brass disc approximately 13 inches in diameter by 

 J inch thick. 



This disc was made with considerable care to insure that the recording 

 surface was as nearly plane and smooth as possible and that it would turn 

 reasonably true in its bearings. Speeds of 25 and 78 rpm were provided. 



iR. C. Mathes, A. C. Norwine, and K. H. Davis, "Cathode-Ray Sound Spectro- 

 scope," //. Acous. Soc. Am., 21, 527 (1949). 



2 Plating was done by the Brush Development Company. 



1145 



