Browse Prior Art Database

Automatic Tracking Balance and Threshold Control

IP.com Disclosure Number: IPCOM000037157D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Conly, DJ: AUTHOR [+2]

Abstract

An important parameter for reliable tracking in an optical disk drive is that the tracking servo operates nominally as close to the center of the linear region of the tracking error signal as possible. Small movements of laser and other optical components in the head cause shifts in the balance between the light on the two halves of the tracking detectors. Movement of the rest position of the actuator lens in the tracking direction also causes an imbalance in light on the two halves of tracking. Excessive tracking transients need to be detected during recording to prevent destroying data on adjacent tracks in the event of an inadvertent track jump. (Image Omitted)

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Automatic Tracking Balance and Threshold Control

An important parameter for reliable tracking in an optical disk drive is that the tracking servo operates nominally as close to the center of the linear region of the tracking error signal as possible. Small movements of laser and other optical components in the head cause shifts in the balance between the light on the two halves of the tracking detectors. Movement of the rest position of the actuator lens in the tracking direction also causes an imbalance in light on the two halves of tracking. Excessive tracking transients need to be detected during recording to prevent destroying data on adjacent tracks in the event of an inadvertent track jump.

(Image Omitted)

This article describes a means of dynamically measuring offset required to cancel tracking imbalances and for setting excessive transient thresholds. The spatial diagram (Fig. 1) shows the tracking error signal (TES) vs. position on a disk. This signal occurs when the head is in focus and radially crossing tracks. Unintended track crossings may occur due to disk eccentricity and to actuator lens vibrations. When the tracking servo is enabled, the lens position is servoed to keep the lens at the center of the track. This is accomplished by moving the lens such that the tracking error signal stays at zero volts. For the servo to track correctly, it must always keep the lens in the linear region of operation. If the lens moves out of the linear region, the tracking servo becomes unstable and jumps one or many tracks.

The tracking imbalance mentioned above causes the DC level of the tracking error signal to shift as shown. Since the tracking servo still tries to maintain the tracking error signal at zero volts, the linear region in the direction of the DC shift is reduced. This means that a small defect on the media or shock to the drive may cause the lens to move out of the linear region and lose tracking control. The range of the linear region is often about one micron, so the lens can move up to 0.5 micron either way without losing tracking. However, if the tracking error signal is shifted 25% of the peak-to-peak value, the lens can only move 0.25 micron off track in one direction without losing tracking control.

In the circuit diagram (Fig. 2), after the drive achieves focus, the microprocessor sets the Threshold Digital-to-Analog Converter (TDAC) and the Offset Digital-to-Analog Converter (ODAC) to the midrange value and resets the two Excessive Tracking Transient (ETT) latches Q1 and Q2. Since the tracking loop is open, the lens is moving across many tracks due to lens vibration and track eccentricity. These track crossings will set the appropriate ETT latch if they exceed the positive or negative ETT threshold. By varying the TDAC and ODAC and checking the states of the ETT latches, the microprocessor can balance the tracking error signal around zero.

The actual algorithm is given in Table 1. The ETTHI and ETTLO columns...