Sound Recording on Magnetic Tape
C. Hickman · Bell System Technical Journal · 1937
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 pattern 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 limiting 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.).