Browse Prior Art Database

Overwrite Protection in a SECTOR SERVO Tape System

IP.com Disclosure Number: IPCOM000119827D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 125K

Publishing Venue

IBM

Related People

Canon, MD: AUTHOR [+2]

Abstract

Disclosed is a method to prevent overwriting SECTOR SERVO information for DIGITAL TAPE RECORDING. The use of SECTOR SERVO in a tape drive requires writing position information (PI) at periodic intervals down the tape. Unlike rigid disk technology, which can also use sector servo, care must be taken in tape drives to prevent overwriting the previously written PI due to large and unpredictable changes in tape velocity. A particularly severe disturbance is inter-layer slip, which occurs at the supply reel and results in large changes in tape velocity in a short time. In this publication we propose a velocity-independent method for measuring the exact position of the tape. Writing is inhibited prior to crossing the PI field.

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Overwrite Protection in a SECTOR SERVO Tape System

      Disclosed is a method to prevent overwriting SECTOR SERVO
information for DIGITAL TAPE RECORDING.  The use of SECTOR SERVO in a
tape drive requires writing position information (PI) at periodic
intervals down the tape.  Unlike rigid disk technology, which can
also use sector servo, care must be taken in tape drives to prevent
overwriting the previously written PI due to large and unpredictable
changes in tape velocity.  A particularly severe disturbance is
inter-layer slip, which occurs at the supply reel and results in
large changes in tape velocity in a short time.  In this publication
we propose a velocity-independent method for measuring the exact
position of the tape.  Writing is inhibited prior to crossing the PI
field.

      In most tape applications using digital recording a leading
read/write (head) module is used to write data which is then read by
a trailing read/write (head) module.  This technique, commonly called
read after write, adds an extra degree of reliability to the tape
system.  This disclosure uses the fixed separation between the
leading and trailing modules as a ruler to measure the distance of
the leading module relative to the next PI field.

      One of the simplest implementations results when the distance
between PI fields is slightly larger than the distance between
leading and trailing head modules.  To fix ideas, assume that the
distance between the leading and the trailing modules is 2 mm, and
the distance between PI marks is slightly more than 2 mm.  Then,
immediately after reading the PI field with the leading module, the
leading module writes a synchronization field and a Begin Mark (1).
As the tape moves, the trailing module will eventually read the Begin
Mark (1) field.  At this time the leading module will be precisely 2
mm beyond Begin Mark (1), independent of tape velocity.  Since the
leading module is about to cross the next PI field, writing is
inhibited.  The procedure is now repeated.

      We can generalize this result.  For example, suppose that the
distance between the leading module and the trailing module is 2 mm
and the distance between PI marks is slightly more than 4 mm.  Then,
as in the last example, after reading the PI field with the leading
module, the leading module writes a synchronization field and Begin
Mark (1).  As the tape moves, the trailing module will eventually
read Begin Mark (1), and at this time the leading module will be
precisely 2 mm beyond Begin Mark (1).  Now, the leading module must
write another Begin Mark on the tape, which we call a "Virtual Begin
Mark (2)."  When the trailing module reads the Virtual Begin Mark
(2), the leading module will be precisely 2 mm beyond the Virtual
Begin Mark (2), and 4 mm beyond Begin Mark (1).  As before, writing
is inhibited before crossing the next PI field.

      The disclosure requires that the leading head module write
"Virtu...