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Inspection Method for Drop-outs in Magnetic Recording

IP.com Disclosure Number: IPCOM000113797D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 268K

Publishing Venue

IBM

Related People

Argyle, BE: AUTHOR

Abstract

Asperities can cause recording heads to lift-off magnetic media during the recording of data, leaving behind a region in which many written bits are missing or written only weakly. This region can cause errors to be generated during the read-back process. The length of track exhibiting this problem, in magnetic tape storage for example, is called a "drop-out". In testing of files, a drop-out event detected as diminished read-back amplitude from a standard test pattern, is followed by stopping the media transport when diminished amplitude from a group of adjacent bits is detected, and then inspecting the tape optically for the possible cause.

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Inspection Method for Drop-outs in Magnetic Recording

      Asperities can cause recording heads to lift-off magnetic media
during the recording of data, leaving behind a region in which many
written bits are missing or written only weakly.  This region can
cause errors to be generated during the read-back process.  The
length of track exhibiting this problem, in magnetic tape storage for
example, is called a "drop-out".  In testing of files, a drop-out
event detected as diminished read-back amplitude from a standard test
pattern, is followed by stopping the media transport when diminished
amplitude from a group of adjacent bits is detected, and then
inspecting the tape optically for the possible cause.

      In early media with tracks written wide, for example on IBM
3480 tape, the drop-out is made strikingly visible by developing the
tape with a Bitter solution.  The magnetic particles suspended in the
Bitter fluid are attracted by stray fields emanating from the
transitions between bits, so a drop-out is easily seen where the
darkening is diminished or absent.

      This method is impractical for todays high-density recorded
media however, such as metal particle tape, because the tracks are
written much narrower (<50 microns), and the fly height (<0.3 micron)
and bit length are much smaller (<1 micron).  Particles as small as
0.2 - 0.5 micron can lift the head beyond a threshold for
full-strength bit recording.  The fields emanating from the bit
transitions which are sensed during read-back, decay away from the
media at distances comparable with the gap length (<  0.3 micron) of
the head.  Such parameters make smaller asperities more significant
and optical inspections more difficult.  The Bitter method is also
considered destructive because the residue is difficult to remove.
The method described here is non-destructive.

      The direct viewing method for 0.2 micron asperities requires an
objective lens with very high numerical aperture.  Such objectives
have a very small working distance and there  therefore can interact
physically with the larger asperities in a distribution.  Its field
of view is also quite small.  Therefore, direct viewing optics is
generally impractical.  The narrower field causes a longer inspection
time to cover the length of overshoot from spooling following the
electrical drop-out and stop signals.  The small working distance and
excess search time increase wear or damage the tape and sometimes
also the recording head.  Additionally, presence of other kinds of
defects not causing magnetic skips can confuse the identification as
to the specific cause of the lift-off.  Thus, whether the particle or
asperity causing the lift off is magnetic or non-magnetic would also
be useful.  This information can sometimes be determined before
removing the particle from the tape and together with other
characteristics, is helpful in finding the possible causes of
drop-outs in manufacturing a...