Browse Prior Art Database

Focus Hold for Optical Disk Drives

IP.com Disclosure Number: IPCOM000110981D
Original Publication Date: 1994-Jan-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Bui, N: AUTHOR [+3]

Abstract

ANSII standard magneto optical media has two general regions on each track: the Magneto Optical (MO) region where customer data is stored and then read using changes in polarization; and a pitted Read Only Memory (ROM) region where track and sector numbers are encoded and read by detecting variation in light intensity. Each of the two regions has different optical characteristics. When entering the ROM area, a false offset is generated in the Focus Error Signal (FES) (Fig. 1 top). This causes two problems: First, the Focus Servo will attempt to correct the offset, thereby introducing a real offset (Figure 1 middle); Second, transients will occur in the Focus Drive Signal (Fig. 1 bottom), which is also an undesired effect.

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Focus Hold for Optical Disk Drives

      ANSII standard magneto optical media has two general regions on
each track: the Magneto Optical (MO) region where customer data is
stored and then read using changes in polarization; and a pitted Read
Only Memory (ROM) region where track and sector numbers are encoded
and read by detecting variation in light intensity.  Each of the two
regions has different optical characteristics.  When entering the ROM
area, a false offset is generated in the Focus Error Signal (FES)
(Fig. 1 top).  This causes two problems:  First, the Focus Servo will
attempt to correct the offset, thereby introducing a real offset
(Figure 1 middle); Second, transients will occur in the Focus Drive
Signal (Fig. 1 bottom), which is also an undesired effect.

Since the ROM regions of the disk come at known intervals, the FES
signal can be sampled during the MO portion of the sector, and the
last value held at a constant level throughout the ROM region.
Because the ROM area is narrow, little deviation in focus results
when using the "Held" FES.  Once beyond the problem region, logic
circuits return the FES signal to a constant sampling mode.  The
accuracy of focus throughout the ROM region and at the beginning of
the MO region is consequently greatly improved.