The Reproduction of Magnetically Recorded Signals

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. 2 MEASUREMENTS OF SPACING LOSS R. C. Mathes, A. C. Norwine, and K. H. Davis, "Cathode-Ray Sound Spectroscope," Jl. Acous. Soc. Am., 21, 527 (1949). Plating was done by the Brush Development Company. 1145 1 2 1146 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 The ring-type record-reproduce head shown in Fig. 1 was lapped slightly to obtain a reasonably good fit with the surface of the disc. A single-frequency recording was made with the head in contact with the disc using a-c. bias in the usual way. Then the open circuit reproduced signal level was measured, first with the head in contact, and then after introducing paper shims of various thickness between the reproducing head and the medium. Thus the effect of spacing was measured at a particular frequency and recording speed. The signal was then erased and the process was repeated for other recorded frequencies and for several record-reproduce speeds. Measurements were also made for cases in which the recording and reproducing speeds were different.