Browse Prior Art Database

Optical Data Shredder

IP.com Disclosure Number: IPCOM000105313D
Original Publication Date: 1993-Jul-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 4 page(s) / 142K

Publishing Venue

IBM

Related People

Crocker, PE: AUTHOR [+3]

Abstract

Periodically, customers desire to quickly eradicate data stored on write-once (WORM) optical media. Although this may be done by over-writing the optical disk at a rate of 1 track at a time, this means of overwriting is very time consuming. A faster method is needed for rendering useless any unwanted data stored on WORM media. Additionally, a faster method could be used by MO and phase change media, if the data needed to be rendered useless quickly rather than taking the time to erase the rewritable media.

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Optical Data Shredder

      Periodically, customers desire to quickly eradicate data stored
on write-once (WORM) optical media.  Although this may be done by
over-writing the optical disk at a rate of 1 track at a time, this
means of overwriting is very time consuming.  A faster method is
needed for rendering useless any unwanted data stored on WORM media.
Additionally, a faster method could be used by MO and phase change
media, if the data needed to be rendered useless quickly rather than
taking the time to erase the rewritable media.

      Utilizing the fact that the ECC code can not correct for more
than 50-80 errant bytes in a 1024 byte block, and that only one
errant bit is needed to nullify a byte, we propose to overwrite the
data with a coarse pattern by seeking with the laser at full write
power through the zone of data to be eradicated until more than
50-80/1024 or 5-8% of the data is overwritten.  At this time, the
data is beyond recovery by ECC and is effectively eradicated in only
5-8% or 1/20-th to 1/12-th of the time that would be taken by
overwriting each and every track.

      If the media is spinning at a Constant Angular Velocity (CAV),
the coarse pattern we would write once the head reached its streaming
velocity is called the spiral of Archimedes [*]  and eqn.  (1).  This
overwrite pattern could spiral outward with a positive coefficient A
or spiral inwards with a negative coefficient A.  If the media were
spinning slowly, the coarse pattern would resemble an oscillating
sine wave.

R = A*THETA/(2*PI) + Ro
(1)
where R     = current radial position of the overwrite pattern
      Ro    = initial radial position of the overwrite pattern
      THEA  = angular position of the overwrite pattern in radians
      PI    = 3.14159
      A     = pitch of the overwrite pattern
              A > 0 means overwrite pattern spirals outward
              A <  0 means overwrite pattern spirals inward

     The angle PHI that overwrite pattern would form with the data
lands would be the arctangent of the ratio of the streaming velocity
of the head actuator and the tangential velocity of the disk, eqn.
(2).  Angle PHI is sufficiently steep that several thousand tracks
would be tranversed per revolution of the optical disk.

                  dR/dt
PHI = arctan ---------------
(2)
              R * d(THETA)/dt
where:  PHI   = angle between overwrite pattern
        dR/dt = radial velocity of actuator
                (time derivative of radius)

Procedure for Data Eradication - If the disk is not fully written,
the first step is to locate the beginning and end of data tracks on
the disk.  Alternatively, pseudo beginning and end of data tracks
could be selected outside of the band to be eradicated so that the
head carriage moved at its streaming velocity through the band to be
eradicated.

      The beg...