An Introduction to Direct Access Storage Devices by Hugh M. Sierra

By Hugh M. Sierra

This booklet provides an exposition of the expertise, layout, association, and constitution of direct entry garage units (disk drives). It incorporates a dialogue of the evolution of the know-how (magnetic recording) and an evaluation of alternative garage applied sciences, together with optical recording. Examples of codes utilized in prior implementations of disk drives in addition to an software of disk force utilization dictated through reliability concerns also are integrated. The presentation assumes a minimal wisdom of magnetic recording, servomechanism layout, and coding.

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26. : In both figures, the write transitions have been placed at such distance that the isolated readback pulses do not interfere with each other at all (neither bit shift nor crowding). It is possible to place the write transitions much closer together in which case the readback waveform will look like the one shown in Fig. 27. For this case, the fundamental (or first harmonic) of all the Fourier 52 MAGNETIC RECORDING FOR DIRECT ACCESS STORAGE DEVICES Write waveform Readback waveform Fig. 27. Formation of the 2F read waveform.

5 \\ (b) \ \ ^ ' -A 2 0 + t 2 r = Wi Fig. 15. Approximation to the read pulse, (a) Graph of sin(f) vs. /. (b) Graph of \[\ + cos(0] vs. /. a bit shift λ. The effect of this bit shift is an out-of-step condition with the clock signal, which in turn may cause reading errors. It was stated before that the readback process can be considered linear. Therefore, if each individual readback pulse can be narrowed down (or slimmed) to the point where its effect upon neighboring pulses is negligible, then the bit shift λ could be eliminated.

Approximation to the write transition, (a) Graph of Arctan(f) vs. t. (b) First derivative. 4. Equalization 37 Therefore, to eliminate the negative values it becomes necessary to add 1 to the function (thus the name "raised" cosine) and to normalize the approximation to a maximum value of 1. The whole expression is divided by 2, thus obtaining Readback pulse « ^[1 + cos(f)]. 12) This is shown in Fig. 15. 12) Readback pulse - 1 - ( ^ ) , > + (j^^ (c) Gaussian pulse The Gaussian pulse exp(—t2) has been used to approximate the isolated readback pulse, that is Readback pulse ~ exp(—t2).

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