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On-Track Focus Offset Calibration

IP.com Disclosure Number: IPCOM000100704D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Fennema, A: AUTHOR [+3]

Abstract

This article describes a method of achieving an optimum on-track focus calibration (smallest spot size) in an optical disk drive. Optimum focus occurs when the amplitude of the read back signal is at its highest value. This procedure will allow the servo system to compensate for a variety of offset errors due to component variations, temperature variations, and dark currents in the system.

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This is the abbreviated version, containing approximately 87% of the total text.

On-Track Focus Offset Calibration

       This article describes a method of achieving an optimum
on-track focus calibration (smallest spot size) in an optical disk
drive.  Optimum focus occurs when the amplitude of the read back
signal is at its highest value.  This procedure will allow the servo
system to compensate for a variety of offset errors due to component
variations, temperature variations, and dark currents in the system.

      When written PWM marks are scanned by the read beam at its
optimum focus, the read back signal will have the fastest rise and
fall time (slope), and the differentiated PWM signal will have the
highest amplitude.  On the other hand, when the PPM marks written at
high densities and frequencies are read back with the best focused
beam, the effect of Intersymbol Interference (ISI) is minimized, and
this results in the highest read back signal amplitude.

      Focus calibration uses the embossed tracks with sectors
containing a continuous 1 0 0 ROM pattern in the data sector area.
These tracks are located at the inner most diameter of the disk.
Fig. 1 shows the focus offset calibration block diagram.  Multiple
read signal amplitudes are sampled by the ADC for every focus offset
value, and the optimum DC focus offset value (DAC) is calculated
using these data points.  Focus calibration takes place whenever a
new media is inserted into the drive or a large focus offset is
detected by the servo or read channel.  Fig. 2 shows a...