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Externally Referenced Coherent Servo Signal Detection

IP.com Disclosure Number: IPCOM000049436D
Original Publication Date: 1982-Apr-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Klaassen, KB: AUTHOR

Abstract

A buried servo magnetic recording system involves a moving head c magnetic recording system in which the head position is controlled by means of a prerecorded servo signal that resides in the same portion of the recording medium as the stored data. For continuous servoing from this servo signal, the system must be able to separate the servo signal from the recorded data. This is achieved with electronic filters, as indicated in Fig. 1.

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Externally Referenced Coherent Servo Signal Detection

A buried servo magnetic recording system involves a moving head c magnetic recording system in which the head position is controlled by means of a prerecorded servo signal that resides in the same portion of the recording medium as the stored data. For continuous servoing from this servo signal, the system must be able to separate the servo signal from the recorded data. This is achieved with electronic filters, as indicated in Fig. 1.

In the data-read mode, such a prior-art system needs at least two filters: a filter that blocks out the data in the servo channel and a filter that suppresses the servo signal in the data channel. The first difficulty with this prior-art system arises from the fact that these filters have to be so narrow as to block out only the data in the servo channel and only the servo signal in the data channel. To that end the filters should block out or let through only the servo signal with its modulation side bands (caused by head position variations). On the other hand, the filters should be so wide that they allow for speed variations of the disk. This latter requirement leads to a wider band width than allowed by the former requirement. This unnecessarily degrades the signal to a coherent noise ratio of the two channels. The present technique provides a solution that avoids these two filters. This technique uses an external reference signal (ERS) for the coherent detection of the buried servo signal out of the composite data and servo signal coming from the read/write head. As shown in Fig. 2, this ERS is used directly to coherently demodulate the servo signal out of the composite signal without any prefiltering. Additionally, a linear demodulator is needed, i.e., a demodulator whose transfer characteristic does not depend on the magnitude of the input signal but only on the magnitude of the reference signal.

The important property of the ERS is that it provides the demodulator with a phase-stable timing signal necessary for the coherent demodulation of the servo signal, without having to retrieve this reference signal internally by means of narrow filters out of the composite data and servo and reference signals from the read/write head. In addition to this, the ERS is inherently following disk speed variations, thereby eliminating the need for wider filters to accommodate disk speed variations.

The ERS is a signal (for instance, a sine wave or a square wave) that is derived from the rotating disk by means of a pick-up transducer. There are various ways to implement this transducer: 1. It can be a fixed magnetic recording head (inductive or magneto resistive) flying over a dedicated reference track on one of the disks of a stack of disks. 2. It can also be a capacitive pick- up consisting of a fixed electrode that reads capacitance variations due to small indentations knurled in the outer rim of the disk. 3. It can consist of an opto- electronic devi...