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Phased Pattern Servo Detection

IP.com Disclosure Number: IPCOM000099756D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 158K

Publishing Venue

IBM

Related People

Cunningham, EA: AUTHOR [+2]

Abstract

A detector of a phased servo pattern recognizes eight track types intrinsic to the pattern. This is in marked contrast to a conventional detector that recognizes only four track types. Another implementation for this improved detector uses a pattern change to recognize four track types, with a doubling of the phase change per track.

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This is the abbreviated version, containing approximately 45% of the total text.

Phased Pattern Servo Detection

       A detector of a phased servo pattern recognizes eight
track types intrinsic to the pattern.  This is in marked contrast to
a conventional detector that recognizes only four track types.
Another implementation for this improved detector uses a pattern
change to recognize four track types, with a doubling of the phase
change per track.

      The figure illustrates a standard phased servo pattern made up
of a write recovery field, a servo identification pattern, and fields
A1, B and A2 containing phase information.  (Only the last portion of
the recovery field is illustrated.)  The pattern is typically written
with widths half that of the data pitch.  The on-track positions of
10 tracks are indicated by the numbers at the left where the
read/write head passes over one of the written half-tracks and a
portion of each adjoining half-track.

      The recovery field is written with the same phase across all
the tracks with a fixed frequency.  The servo identification field is
written with the same phase across all tracks, but has a complex
pattern such that it may be used to indicate the starting position
for the servo pattern.  Each of the three fields, A1, B, and A2, are
written with the same frequency, but the phase is changed, typically
by 22.5 electrical degrees, or 1/16 of a period, for each half track.
 Pattern A1 is written progressively later in time for higher number
tracks.  Pattern B is written progressively earlier.  Pattern A2,
like A1, is written later.

      A standard detection system measures the difference in phase
between the patterns by time measurements.  The average of the timing
difference between the phases of A1 to B and from B to A2 are
averaged to eliminate speed difference effects.  Multiple edges of
the pattern from each region are averaged to reduce the noise
effects.  This is accomplished by accumulating the timing differences
from the transition edges of the pattern from the edges of a clock
running at essentially the same frequency.

      With the standard pattern, a full customer track offset
produces a phase change in each section of 45 degrees, or a 90-degree
difference between two oppositely tilted patterns. For track 0, the
phase difference is 0.0 degrees; for track 1, 90 degrees; for track
2, 180 degrees; for track 3, 270 degrees; for track 4, 360 degrees,
which is not distinguishable from 0.0 degrees.  Thus, there are only
4 distinguishable track types.  Tracks 0, 4, 8, etc., are all termed
"track type 0"; tracks 1, 5, 9, etc., are termed "track type 1"; and
track types 2 and 3 are defined in a similar modulo 4 manner.

      The disclosed detector uses the standard detector described
above, but adds an additional detector of the phase change from the
recovery field to Field A1.  This requires that the recovery field
and the servo fields are of the same frequency to allow the
comparisons to the timing clock of the servo system....