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Digital Anti-Alias Filter

IP.com Disclosure Number: IPCOM000103499D
Original Publication Date: 1990-Dec-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 2 page(s) / 82K

Publishing Venue

IBM

Related People

Stephenson, J: AUTHOR

Abstract

Digital servo control on Direct Access Storage Devices (DASD) requires that an analog position error input signal be converted to a digital value at a fixed sampling interval. In sampled data systems, such as this, there is a phenomena called aliasing which results from sampling any frequency that is more than 1/2 the sample rate (called the Nyquest frequency). Any frequency that is higher than the Nyquest frequency appears as a lower frequency to the digital processor after being sampled. A normal method used to prevent aliasing from occurring is to filter the analog signal before it is sampled. There are certain applications where it may be inappropriate to use an analog filter.

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Digital Anti-Alias Filter

      Digital servo control on Direct Access Storage Devices (DASD)
requires that an analog position error input signal be converted to a
digital value at a fixed sampling interval.  In sampled data systems,
such as this, there is a phenomena called aliasing which results from
sampling any frequency that is more than 1/2 the sample rate (called
the Nyquest frequency).  Any frequency that is higher than the
Nyquest frequency appears as a lower frequency to the digital
processor after being sampled.  A normal method used to prevent
aliasing from occurring is to filter the analog signal before it is
sampled.  There are certain applications where it may be
inappropriate to use an analog filter.  An alternative to an analog
filter that might be useful in these applications is to increase the
sample rate so that the Nyquest frequency is above the bothersome
noise and then use a digital filter to remove the noise.  The output
of the digital filter could then be sampled at a lower rate with the
noise removed.

      DASDs, which use a dedicated servo surface for position
information, frequently have difficulty staying over the centerline
of the data tracks written on other disk surfaces.  The centerlines
of the data tracks can move around due to temperature variations and
disk slip.  To correct for this a reference servo track is often
written on each of the data surfaces so that position information can
be read from each data surface and a correction signal can be applied
to the dedicated servo information.

      The reference track is not written inside the data band region
so a seek to the reference track must be made periodically so its
servo information can be read and saved in the s...