Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-10-27
2004-05-25
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044340, C369S044410
Reexamination Certificate
active
06741532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of optical storage systems, and in particular, to servo circuitry in an optical storage system for counting tracks on an optical storage medium.
2. Statement of the Problem
An optical disk stores data on circular tracks on its surface. An optical disk device reads the optical disk by centering an optical head over a track, reflecting a light beam off of the surface of the disk, and detecting the reflected light beam with a four-quadrant photo-detector. The photo-detector generates a signal for each quadrant and transfers the four signals to servo circuitry. The servo circuitry uses the quadrant signals to keep the optical head centered over the track when following the track during a read function. The optical disk device is also able to seek out a particular track on the optical disk to read from that track. The optical disk device seeks by counting the number of tracks that the optical head crosses over during the seek function. The servo circuitry again uses the quadrant signals to count track crossings and locate the desired track. A problem with the current servo circuitry is the servo circuitry does not have a bandwidth that is easily adjustable. Another problem is that the servo circuitry is error prone when the optical disk contains a defect. Another problem is that the servo circuitry does not count tracks on both Compact Disks (CDs) and Digital Video Disks (DVDs) using the same track counter.
FIG. 1
shows an optical disk system
100
in the prior art. Optical disk system
100
is comprised of an optical disk device
102
coupled to a servo system
104
. Optical disk device
102
includes an optical head that contains an optical transducer and a four-quadrant photo-detector
110
. Servo system
104
is comprised of a servo detector
106
connected to a servo controller
108
. Servo detector
106
is comprised of diagonal signal generators
122
-
123
, a positive correlator
126
, a negative correlator
128
, an Adaptive Dual Arm Correlator (ADAC)
130
, and a track counter
132
. Optical disk device
102
is connected to diagonal signal generators
122
-
123
in servo detector
106
. Diagonal signal generators
122
-
123
are connected to positive correlator
126
and negative correlator
128
. Positive correlator
126
and negative correlator
128
are connected to ADAC
130
. ADAC
130
is connected to track counter
132
and servo controller
108
. Track counter
132
is connected to servo controller
108
. The servo controller
108
is connected to optical disk device
102
.
Optical disk device
102
stores data on an optical disk in the form of a series of pits arranged into tracks. The data is encoded on pit edges. The pit lengths and the distance between pits are integer channel bit periods. Run-length limited encoding determines the distance between edges. To read from the optical disk, the optical disk device
102
positions the optical head over the optical disk. The optical transducer projects a light beam onto a surface of the optical disk. The light beam reflects off of the pits and the surface of the optical disk, and onto photodetector
110
. Photodetector
110
has four quadrants and each quadrant generates a signal. For instance, quadrant B generates a signal B representing the reflection of the light beam captured by quadrant B. If the optical head is centered over a track on the optical disk and the track runs parallel to a line between quadrants A and B, then quadrants A-D will see substantially similar light power and will generate similar amplitude signals. If the optical head is off-center, quadrants A and D see substantially similar light power and quadrants B and C see substantially similar light power. In the off-track case the sum of the power at quadrants A and D is different from the sum of the power at quadrants B and C. Photo-detector
110
transfers respective signals A-D to servo system
104
.
Servo system
104
processes embedded tracking error data to center the optical head over the track with logic appreciated by one skilled in the art as follows. Servo system
104
receives signals A-D. Signals A-D include encoded user data and the embedded tracking error data. A read channel circuit (not shown) processes signals A-D to detect the encoded user data. Servo system
104
processes signals A-D to detect the embedded tracking error data. The embedded tracking error data is not physically written onto the optical disk, but is inherent to optical systems and results from the reflection of the light beam changing as the optical head moves off-track. Servo system
104
uses the embedded tracking error data to center the optical head during track following functions and to find a particular track during seek functions.
Within servo system
104
, servo detector
106
generates a Position Error Signal (PES) and a track count signal by logic appreciated by one skilled in the art as follows. The PES represents how far off track the optical head is during the track following functions. The track count signal represents track crossings during the seek functions. Diagonal signal generator
122
receives signal A and signal C from photo-detector
110
. Diagonal signal generator
122
adds signal A and signal C to generate a diagonal signal S
1
. Diagonal signal generator
122
transfers S
1
to positive correlator
126
and negative correlator
128
. Diagonal signal generator
123
receives signal B and signal D from photo-detector
110
. Diagonal signal generator
123
adds signal B and signal D to generate a diagonal signal S
2
. Diagonal signal generator
123
transfers S
2
to positive correlator
126
and negative correlator
128
.
Positive correlator
126
receives S
1
from diagonal signal generator
122
and S
2
from diagonal signal generator
123
. Positive correlator
126
correlates S
1
and S
2
by summing S
1
and S
2
over a length L bits, where L represents a correlation length. Positive correlator
126
generates a positive correlation CorrP and transfers CorrP to ADAC
130
. Negative correlator
128
also receives S
1
from diagonal signal generator
122
and S
2
from diagonal signal generator
123
. Negative correlator
128
correlates S
1
and S
2
by summing S
1
and S
2
over the length L bits. Negative correlator
126
generates a negative correlation CorrN and transfers CorrN to ADAC
130
.
ADAC
130
receives CorrP from positive correlator
126
and CorrN from negative correlator
128
. ADAC
130
generates the PES by taking the difference between CorrP and CorrN. ADAC
130
transfers the PES to servo controller
108
. ADAC
130
also receives S
1
and S
2
from diagonal signal generators
122
-
123
. ADAC
130
generates a Phase Offset Signal (POS) by determining the offset between S
1
and S
2
. ADAC
130
transfers the POS to track counter
132
. The POS is a sinusoidal signal that is cyclic with track crossings. Track counter
132
receives and filters the POS with a programmable band pass filter. Track counter
132
slices the filtered POS and counts the number of edges to generate the track count signal. Track counter
132
transfers the track count signal to servo controller
108
.
Servo controller
108
uses the PES to center the optical head during the track following functions and the track count signal to position the optical head over the particular track during the seek functions.
A problem with servo detector
106
is ADAC
130
is an adaptive algorithm that relies on a measured phase difference between the diagonal signals S
1
and S
2
to be linear between +/− pi. In practice the phase difference is more apt to be sinusoidal. ADAC
130
has problems track counting and finding a transducer gain for servo system
104
during calibration. Another problem is noise and defects easily corrupt track counter
132
. Track counter
132
uses a programmable filter to ameliorate the effects of noise. Unfortunately, the filter limits the bandwidth of track counter
132
and requires the servo detector
106
adjust a ban
Feyh German S. O.
Graba James Mark
Huang Hao
Supino Louis
Cirrus Logic Inc.
Duft Setter Ollila & Bornsen LLC
Edun Muhammad
LandOfFree
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