Optical disc apparatus with lens position detection and control

Details Optical disc apparatus with lens position detection and control Optical disc apparatus with lens position detection and control

1999-01-27

2001-09-04

Tran, Thang V. (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044410

Reexamination Certificate

active

06285636

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to detection of lens position in an optical disc apparatus, particularly to the generation of a lens position detection and control signal regulated by the total light reflected from the optical disc.
Background: DVD
DVD is an optical disc format known alternately as Digital Video Disc and Digital Versatile Disc. The optical disc employed with the DVD format is the same size as the traditional compact disc used in audio CD and CD-ROM applications. However, the capacity of the DVD optical disc is, at a minimum, more than seven times that of an audio CD or CD-ROM. Additionally, the data transfer rate of the DVD format is approximately six times that of the audio CD format. This increase in capacity and transfer rate allows the DVD format to provide for a wide variety of applications.
The DVD format can be used for video, data storage, audio applications, and interactive videos, games, etc. Moreover, the DVD format allows each of these applications a greater flexibility than they would otherwise have under conventional video or audio CD formats or CD-ROM. In video, the resolution offered by the DVD format is much closer to the source video, at 720 pixels per horizontal line, than the VHS standard of 320 pixels per horizontal line. The DVD format also offers Dolby Pro Logic or AC-3 and MPEG-2 audio formats on up to eight separate soundtracks. Other features offered by the DVD format are multiple subtitle tracks and multiple video aspect ratios all on the same disc.
Additionally, DVD-ROM drives used in computers and home DVD players are compatible with current audio CD and CD-ROM formats. DVD-ROM drives require increased seeking and disc rotation speeds for finding and reading the data stored on the optical disc more quickly than is required for home DVD player use. As stated above, the data transfer rate standard for DVD players is only six times that of audio CDs. However, CD-ROM drives currently read data at over twenty-four times the data transfer rate of audio CDs. To be competitive in the computer market, DVD-ROM drives must be able to access and transfer data at least as quickly as their CD-ROM drive competition. Home DVD players do not currently have the same demands as they are not put to as versatile uses as DVD-ROM drives.
Background: Optical Disc Apparatus
FIG. 2
illustrates a typical optical disc apparatus
214
for reading information from an optical disc. Laser light emitted from a laser diode
202
passes through a beamsplitter prism
204
, a collimating lens
206
, and a focusing lens
208
that focuses the laser light onto a particular track of the optical disc
212
. Laser light reflected from the surface of the disc passes through the focusing lens
208
and the collimating lens
206
. The light then reflects off the surface of the beamsplitter prism
204
through a cylindrical lens
210
and finally illuminates a photo-detector array
102
. The photo-detector array
102
converts the received light energies into electric signals. The entire apparatus
214
is known as the “pickup”. The focusing lens
208
is typically held in a lens holder attached by a spring mechanism to the pickup body. Lens position is controlled by permanent magnets mounted to the pickup body. Wire coils attached to the lens holder carry electric currents that generate magnetic force interacting with the magnets and moving the lens holder relative to the pickup body. Both a focusing coil and a radial coil are used to effect and control focus and radial movement, respectively. The position of the focusing lens is controlled with the lens holder being driven in accordance with the control signals flowing into the focusing and tracking coils.
Background: Tracking
In an optical disc apparatus, the pickup is movable along the radius of the optical disc. During access, the pickup is moved to the location where desired data is stored. In a high-speed optical disc apparatus, such as that in a DVD-ROM drive, the pickup can be moved so quickly during the gross positioning of the pickup, that vibration of the lens holder relative to the pickup occurs. Such vibration can become great enough that the lens continues vibrating even after the pickup has reached the approximate target location on the optical disc and fine position positioning of the pickup is occurring. The typical vibration frequency can be between 20 and 60 Hz. Such vibration results in poor pulling operation for tracking. That is, once the target location is reached and data can be read, the light beam passing through the object lens does not follow a certain track, takes a long time to follow the track, or takes a different time to follow each time the track is accessed. However, even when fine position control is activated to keep the beam spot at the target location, the beam spot moves away due to residual vibration caused by the movement of the pickup. This movement results in more time having to be spent to move the beam spot back to the target location. Thus, there is need for a method of lens position control which eliminates vibration during pickup movement.
Center Error
FIG. 5
depicts a laser beam spot
502
reflected from an optical disc onto a photo-detector array
102
. The example array is a four-point tracker array. That is, the array is divided into four quadrants: a, b, c, and d; each with its own photodiode and photo-detector output. The laser beam spot
502
depicted is at its peak as it is not being scattered by patterns. However, the beam spot
502
is being biased towards one side of the dividing line parallel to the track direction of the pickup. Track direction is the vector in the image plane of the photo-detector array which corresponds to the direction in which patterns are read off the surface of the DVD medium. The bias is due to the lens axis being shifted away from the optical axis in the radial direction of the disc. This is the type of bias that can occur due to vibration caused by pickup movement.
This bias (or lens shift) in the reflection of the beam spot causes the divisions of the photo-detector array to be illuminated to different degrees. This varying degree of illumination of the photo-detector is known as “center error”. Presence of center error indicates that the beam spot is not in the proper position to read data from the optical disc. Ideally, each of the divisions of the photo-detector array are illuminated equally. Equal illumination of the array divisions indicates that the focusing lens is in the proper position. When the focusing lens is not properly positioned, data may not be read correctly from the disc.
Background: Prior Art
In U.S. Pat. No. 4,744,068 there is disclosed a method of lens position detection by sampling the difference signal between suitable pairs of photo-detector output signals. This difference signal is directed through a low-pass filter to remove the high-frequency noise from the signal caused by a sample-hold circuit and then provided to a servo controller. The servo controller can then control the radial position of the lens during pickup movement.
The lens position detection method used in U.S. Pat. No. 4,744,068 suffers from several shortcomings. First, reflectivity changes can take place on the optical disc. Such changes in reflectivity can be due to fingerprint smudges or other foreign matter on the surface of the disc. Changes in disc reflectivity can cause changes in the output levels of the photo-detector array. The changes, in turn, can result in lens position detection signal level changes. The lens position detection method described in U.S. Pat. No. 4,744,068 does not provide for the prevention of modulation of the lens position detection signal due to changes in disc reflectivity. Modulation in the lens position detection signal may result in a misleading indication that the lens has shifted. This misleading indication leads to uncertainty in the relationship of the lens position detection signal and the actual position of the lens.
Second, the lens position detection signal is s

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