Browse Prior Art Database

Enhanced Timing Based Servo Decoder Logic

IP.com Disclosure Number: IPCOM000116915D
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 100K

Publishing Venue

IBM

Related People

Adams Jr, FF: AUTHOR [+4]

Abstract

An improved method for decoding servo timing pulse to generate PES information is disclosed. The method uses predictive logic to reduce the effective error lengths caused by missing or added pulses in the raw timing information.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 54% of the total text.

Enhanced Timing Based Servo Decoder Logic

      An improved method for decoding servo timing pulse to generate
PES information is disclosed.  The method uses predictive logic to
reduce the effective error lengths caused by missing or added pulses
in the raw timing information.

      Timing-based servo systems read azimuthally recorded data from
servo tracks on the media to derive the position error signal and
maintain servo head tracking.  The readback data on the servo tracks
consists of peaks in the servo waveform which are detected to produce
pulses, and gaps which are signal-free.  These sequences of repeating
pulses/gap/pulses/gap information feed the decoding logic which uses
the information to calculate A and B values which indicate the
lateral position on the track, and are used by the controller to
calculate the PES, position error signal.  In general, the distance
between pulses must be precisely measured, as well as the pulse
relationship to the gaps, which serve as synchronization features.
The Figure shows the operation of the system.

      A missing pulse is this system not only invalidates the A/B PES
counts in the pulse train in which the error occurs, it propagates
the error so A/B counts are invalid for several subsequent sequences.
This is because the control signals which operate period measurement
counters and summing logic (OUT1,OUT2,CLR1, and CLR2) are effected by
the same error and therefore actions neccesary to clear the logic and
restart are not performed.  The improved decoder uses a GAP
Prediction state machine to operate the control signals in such a
manner as to limit propagation of errors.  The state logic text is
included below.  This state logic basically keeps track of previous
GAP types and predicts next GAP type expected.

      After reset, the LOST latch is set so the state machine
operates in non-predict mode where actual GAP values generate the
control signals.  At each GAP the state machine compares predicted to
actual GAP value.  If they agree, the FOUND counter is incremented.
Once the found counter exceeds the found threshold, the state machine
operates in predict mode.  When GAP is detected, it performs actions
based on predicted GAP values, not actual, therefore an error does
not effect it.  It then compares actual to predicted and increments a

LOST counter if they do not compare.  If they compare, the LOST
counter in not incremented.

      On each GAP a similar process takes place.  If the LOST counter
ever exceeds the LOST threshold, the LOST latch i...