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Correction for Phase Error in an Anti-Alias Filter

IP.com Disclosure Number: IPCOM000122523D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 3 page(s) / 120K

Publishing Venue

IBM

Related People

Hagen, MD: AUTHOR [+2]

Abstract

One method of TMR (track mis-registration) compensation used in direct access storage device (DASD) products is to implement a "reference track". The reference track is an area of servo pattern, usually at the inner radius or outer radius of the data area on each data surface. The reference track is read to determine the offsets between the position of data heads relative to a dedicated servo head. The reference track position is stored and used subsequently whenever that head is being used to read data.

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Correction for Phase Error in an Anti-Alias Filter

      One method of TMR (track mis-registration) compensation
used in direct access storage device (DASD) products is to implement
a "reference track".  The reference track is an area of servo
pattern, usually at the inner radius or outer radius of the data area
on each data surface.  The reference track is read to determine the
offsets between the position of data heads relative to a dedicated
servo head.  The reference track position is stored and used
subsequently whenever that head is being used to read data.

      Because storage space is limited in a typical servo control
system, the bandwidth of the reference track position is limited and
aliasing of high frequency noise can produce DC TMR.  A digital anti-
alias filter solves this problem by sampling at a high rate and
storing the decimated output (storing every nth sample).  However,
the anti-alias filter introduces problems of its own.  The filter
introduces phase error that can distort the shape of the reference
track signal and adds delay to the position signal.  Both errors
introduce TMR into the actuator control system.

      The effects of phase delay and distortion due to the digital
anti-alias filter can be compensated for by choosing a filter
transfer function that has linear phase and then storing the
decimated filter output as if it occurred earlier an amount equal to
the amount of phase delay in the filter.

      All sampled data systems are susceptible to disturbances at
frequencies greater the the Nyquist frequency (1/2 of the sample
rate).  These high frequency disturbances alias down to look like low
fre quency disturbances.  For a servo control system as used in DASD,
these low frequency disturbances are a significant problem.  For
example, if the disturbance frequency is at the sample rate, it will
appear as a DC shift after the disturbed signal is sampled.

      A solution to the aliasing problem is an anti-alias filter.  A
digital anti-alias filter works by sampling at a rate much higher
than the control system sample rate and then low-pass filtering the
data with a cutoff frequency below the Nyquist frequency.

      The problem with the anti-alias filter is that it introduces
waveform distorting phase error at frequencies around the cutoff
frequency.  Also, the filter introduces a delay that can be
frequency- dependent.  To minimize these effects, the system sample
rate must be increased, but if the waveform is to be stored, this
costs additional memory space.

      In a reference track application, a small number of position
samples from the reference track...