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Offset Tracking Correction for Optical Recording

IP.com Disclosure Number: IPCOM000062053D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 68K

Publishing Venue

IBM

Related People

Habegger, MA: AUTHOR

Abstract

In an optical memory, an object-lens servo is used to access information on either side of a central track. As a result of the radial direction offset of the lens, the tracking servo will not center the focused spot on a track. This is due to the fact that the returning beam has an offset which produces unequal intensities on the two halves of the photodetector used in the servo sensor circuit. However, some of the actuator driving signal can be fed back into the sensor circuit with the correct phase to eliminate the offset in the sensor circuit. This can be done with a fixed resistor or for more accurate adjustment; it can be done as shown in Fig. 3, described hereinafter. A typical optical head layout for a digital audio disk (DAD) player is shown in Fig. 1.

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Offset Tracking Correction for Optical Recording

In an optical memory, an object-lens servo is used to access information on either side of a central track. As a result of the radial direction offset of the lens, the tracking servo will not center the focused spot on a track. This is due to the fact that the returning beam has an offset which produces unequal intensities on the two halves of the photodetector used in the servo sensor circuit. However, some of the actuator driving signal can be fed back into the sensor circuit with the correct phase to eliminate the offset in the sensor circuit. This can be done with a fixed resistor or for more accurate adjustment; it can be done as shown in Fig. 3, described hereinafter. A typical optical head layout for a digital audio disk (DAD) player is shown in Fig. 1. When the focused beam from the object lens is centered on a track, the two halves of the quadrant photodiode -- (a + d) and (b +
c) -- have equal intensities on them. As the focused beam moves off the track in one direction, the focused spot of light is so diffracted by the groove in the disk that one half of the detector receives more light than the other half. the tracking signal for the object-lens servo is the difference in the photo current between the two halves (a + d) - (b + c). This is the so-called differential tracking technique. This technique works fine in the DAD player where the object lens is moved in the radial direction a maximum of 20 or 30 tracks to compensate for the eccentricity in the mounting hole of the disk. In an optical memory, the object- lens servo will be used to access information perhaps as much as 0.3 mm on either side of the central track. The result of this is...