Browse Prior Art Database

Reflectivity-Coded Phase-Sensing Servo for Optical Memories

IP.com Disclosure Number: IPCOM000035316D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 17K

Publishing Venue

IBM

Related People

Levenson, MD: AUTHOR

Abstract

This invention consists of a disc in which the walls separating tracks are made to encode an oscillating reflectivity. The walls on each side of a given track differ either in the frequency or phase of the oscillation. The oscillation is detected by the same device used to read the data, but the frequency band is below that employed to encode the data. The servo discriminant is either the difference in phase of the detected oscillation from the phase characteristic of the desired track, or the difference in Fourier amplitude of two frequency components. A particular track is defined either by a phase or an average frequency, and the servo system is capable of correcting track-following errors larger than the width of one track, and possibly as large as several dozen tracks.

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Reflectivity-Coded Phase-Sensing Servo for Optical Memories

This invention consists of a disc in which the walls separating tracks are made to encode an oscillating reflectivity. The walls on each side of a given track differ either in the frequency or phase of the oscillation. The oscillation is detected by the same device used to read the data, but the frequency band is below that employed to encode the data. The servo discriminant is either the difference in phase of the detected oscillation from the phase characteristic of the desired track, or the difference in Fourier amplitude of two frequency components. A particular track is defined either by a phase or an average frequency, and the servo system is capable of correcting track-following errors larger than the width of one track, and possibly as large as several dozen tracks. The voice coil actuator is operated open-loop to access a specific track, with this servo system used to correct any error. The servo detectors, beam splitters, and the critical assembly problem associated with them are eliminated.

One key requirement in optical memory technology is that the head be able to find and follow the track where data is to be written or retrieved. Most designs for track-following servos acquire a discriminant by optically sensing the edge of the track, which is usually defined by a slope or reflectivity change pre-formatted into the disc. Ordinarily, the optical signals reflected by these slopes are converted into electronic currents by two or more servo detectors, which are separate from whatever detectors are used to extract the information stored. Aligning these servo detectors properly is one of the manufacturing tasks requiring the highest precision. Moreover, beam splitters and other optical elements required by the track following servo system significantly increases the complexity of the head.

Typical track following servos operate over a range of locations no wider than the width of a track. They can position the read/write beam at the center of the track with high precision, but if the voice coil puts the head at the wrong track, the servo itself cannot correct the error. In systems where data is written both on the lands and grooves of a pre-formatted disc, the servo signal can be ambiguous enough to require digital processing.

This invention solves these problems by (1) eliminating the separate track- following servo detectors, (2) providing a unique signature for tracks in the vicinity of the one being addressed so that considerable off-track errors can be detected, and (3) providing a servo discriminant that is the same for lands and grooves.

This invention assumes an optical storage system in which the data is run- length-coded such that it fills a band of frequencies that does not include zero frequency. In a specific example, the data band might run from 5 MHz to 50 MHz. The servo information will be encoded on a carrier at much lower frequency, in this examp...