Sound Recording on Magnetic Tape 



By C. N. HICKMAN 



This paper describes an improved method of recording sound 

 magnetically on a steel tape, similar in principle to that of the 

 Poulsen telegraphone. In the latter a longitudinal magnetic pat- 

 tern of the voice current is imprinted on a steel wire by drawing it 

 rapidly past recording pole pieces. The high speed used by Poulsen 

 and subsequent investigators has been directly and indirectly a lim- 

 iting factor in the application of magnetic recording to commercial 

 uses. The system here described makes use of perpendicular 

 magnetization. This method makes it possible, with suitable 

 equalization, to obtain a substantially uniform frequency-response 

 characteristic up to 8000 cycles per second with a tape speed of 

 only 16 inches per second. In many cases a speed of 8 inches per 

 second is adequate for recording speech. At the same time the 

 ratio of signal to background noise has been substantially increased. 



The decrease in efficiency resulting from the use of perpendicular 

 instead of longitudinal magnetization is offset to a great extent by 

 the use of a better design and construction of the pole-pieces and a 

 more suitable recording medium. The recording medium is a 

 steel tape having a thickness of about 1.0 to 2.0 mils (0.025 to 

 0.051 mm.) and a width of about 50 mils (1.3 mm.). 



Introduction 



A SYSTEM of recording speech magnetically on a steel wire was 

 invented by Poulsen almost forty years ago. The wire was drawn 

 past a pair of pole-pieces surrounded by coils carrying a speech current. 

 A magnetic pattern corresponding to the current was thus impressed on 

 the wire. When the wire thus magnetically treated was again drawn 

 past the pole-pieces a current corresponding to the recording current 

 was induced in the surrounding coils. It was common practice to 

 place the pole-pieces on opposite sides of the wire and offset with 

 respect to each other. The magnetic pattern in the wire thus con- 

 sisted mainly of a variation in the intensity of magnetization, the 

 direction of the magnetization being substantially parallel to the axis 

 of the wire. This method of putting the record on the wire is known 

 as longitudinal magnetization. With such a system the wire must 

 travel at a very high speed if high frequencies are to be recorded and 

 reproduced. It was customary to use speeds of from six to ten feet 

 per second. By using tape instead of wire, the recording and re- 



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