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Tracking Acquire On an Optical Disk

IP.com Disclosure Number: IPCOM000061128D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Bonvallet, JC: AUTHOR [+3]

Abstract

For reliable tracking acquire on an optical disk, two conditions must be met before the tracking servo loop is closed: the relative velocity of the lens to the tracks must be small, and the lens must be in the correct position with respect to the lands and grooves.

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Tracking Acquire On an Optical Disk

For reliable tracking acquire on an optical disk, two conditions must be met before the tracking servo loop is closed: the relative velocity of the lens to the tracks must be small, and the lens must be in the correct position with respect to the lands and grooves.

A tracking servo loop is provided to maintain the lens' position over the center of a track. At initial tracking servo enable, and after a long track seek movement in which the servo loop is open, the drive must go from the condition of not tracking to a condition of servo tracking. This process is call tracking acquire. Since tracking acquire is a part of a track seek, it must be reliable so as not to adversely affect access time.

One factor which has a significant effect on tracking acquire reliability is the velocity of the lens with respect to the tracks at the time that the tracking servo is closed or enabled. This velocity mismatch can be caused by eccentricity of the tracks, oscillation of the tracking lens due to movement of the carriage, and by other external vibrations.

If the tracking servo is closed when the relative velocity is too great, the servo may not accelerate the lens fast enough to acquire a track without slipping several tracks. In the region between tracks, the servo is unstable because the tracking error voltage versus position changes sign (see the figure). As the servo slips tracks, it goes between stable and unstable regions, and in the unstable region the lens is accelerated in the wrong direction. The result can be that a track is never acquired.

A method for detecting when velocity mismatch is small is as follows: The tracking error signal has a zero crossing at the middle of either a land or a groove, as shown in the figure. The servo logic controlling acquisition monitors a comparator which detects the zero crossings of signal 10, and restarts a timer every time the signal changes state (or every time a transition in a specified direction occurs). The timer period is selected such that if the timer expires before a transition is detected, the relative velocity of the lens an...