DVD radial runout cancellation with self-calibration

Details DVD radial runout cancellation with self-calibration DVD radial runout cancellation with self-calibration

1999-06-01

2003-07-08

Young, W. R. (Department: 2655)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S053140

Reexamination Certificate

active

06590843

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to control of DVD drives, particularly to cancellation of radial runout of an optical disc by applying a precompensating predicted radial runout signal to the DVD optical disc tracking apparatus.
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 fully capable of extracting the data from, or “playing”, discs formatted in the 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 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
216
. The photo-detector array
216
converts the received light energies into electric signals. The entire apparatus 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
Data on an optical disc is arranged in a concentric or spiral fashion around the disk and a laser beam is positioned such that its beam spot is at the center of the target track. Fine positioning on a particular track of data is achieved by controlling the radial coil of the pickup to move the focusing lens
208
in the direction of the radial axis of the optical disc. However, the tracks of an optical disc are generally not perfectly concentric or do not follow a perfect spiral from the inner to outer diameters of the disc. A combination of factors such as disc decenter (non-concentricity), disk tilt, mounting errors, differential thermal expansion, and variations between disks can cause a radial tracking uncertainty typically exceeding 50 micrometers. At a typical pitch of 0.74 micrometers per track, this uncertainty averages ±67 tracks.
Background: Servos
The function of a servo system is to minimize the pickup position error with respect to the track on the optical disc currently being read. Open-loop servos achieve radial tracking by using a stepper motor. The pickup is moved a calculated distance toward or away from the center of the data media along its radial axis. The relatively large space between the tracks on data media such as floppy disc drives and low-capacity Winchester drives allows for tracking using the coarse adjustment of stepper motors. However, the tight track tolerance of optical disc media makes open-loop tracking impractical. Therefore, optical drives use closed-loop tracking servos. Closed-loop servos are designed to compensate for unpredictable positioning errors in order to achieve accurate focusing and tracking of optical disc media. In a closed-loop positioning servo, an optical sensor samples the light reflected from the surface of the optical disc and generates a signal proportional to the tracking error. The signal is amplified to a level that can drive a motor that holds the focusing lens
208
. The motor moves the lens in the direction that reduces the error signal, thereby improving the tracking.
FIG. 4
depicts a block diagram of a closed-loop servo system. A signal, y
0
, proportional to the location of the pickup subtracted at node
104
from the radial location of the track being read
102
, is generated by a controller
106
which acts as a tracking sensor. If the ideal pickup location does not coincide exactly with the zero-crossing of the controller's
106
s-curve, an offset bias
408
is added at node
108
to the signal, y
0
, to create signal y
2
. The filter and amplifier circuits
404
, translate y
2
into a signal to control the motion of the pickup. The radial actuator
114
applies this signal to the pickup, reducing the residual error. The radial actuator
114
also supplies the signal
120
indicating the location of the pickup to node
104
.
The performance of the closed-loop servo can be enhanced by several digital control techniques. Most notably, since tracking errors due to disc decenter or disc tilt are periodic, they can be sampled and fed back to the servo on a time-varying bias.
Background: Radial Runout
Radial runout is defined as the peak-to-peak radial motion of a track relative to the rotation axis. Changes in track radii of optical discs are difficult to control. Such changes can occur due to thermal expansion, centrifugal strain, or plastic deformation. These changes create positional uncertainties in optical discs which are just as great as those in ordinary disk drives. However, unlike ordinary disk drives, in optical drives these changes are not smaller than the track pitch. As a result of this characteristic, the track address for each track must be read directly from the headers of the tracks themselves. Consequently, the absolute radial position of tracks on an optical disc need not be controlled very precisely. This arrangement allows for changes in the track radii of an optical disc to be tolerated as long as the changes themselves are radially symmetric. Most radial runout is due to track decenter, or non-concentricity. Track decenter includes imprecise centering of the track pattern on the optical disc. (The largest contributor to decentering error is centering error between the disc and the spindle.) Generally, tracks on the

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