New approaches to AC-biased magnetic recording

Ac-biased magnetic recording is described in a brief analysis based upon the anhysteretic magnetization process. Several different models are used to illustrate particular aspects of the recording process. It is shown that, using conventional record heads, there is bound to be a conflict between the efficient recording of short and long wavelength signals. The critical requirements involved in recording 15 kc at as low a tape speed as ips are emphasized by pointing out that, on reproduction, 75 per cent of the output comes from the first 0.7 micron (28 microinches) of the coating. Various methods for improving the record process have been proposed from time to time. These methods consist of proposals for sharpening the recording field as a whole, sharpening the bias field alone, and increasing the uniformity of the recording field through the coating thickness. The design of heads to accomplish any of these objectives remains to be worked out in detail. However, several possible techniques are discussed, namely pole-shaping, eddy-current shielding and the use of poles on both sides of the tape. An entirely new recording technique is described which utilizes successive recordings by two separate gaps of different sizes. The two gaps can be contained in separate heads or in a single head, provided that the time-delay between the two recordings, if significant, is compensated for prior to recording. The principle is simple. The long gap is fed only with low-frequency signals which it can record effectively throughout the whole coating; the short gap records the high-frequency signals in the outermost layer of tape. The bias field from the short gap need not significantly reduce the level of the low-frequency recorded information. Filtering requirements and methods of time-delay compensation are discussed in the text, and a mathematical analysis is given of the interference to be expected from an uncompensated system.