Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-07-06
2001-11-20
Young, W. R. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S053290
Reexamination Certificate
active
06320828
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to a track search servo system in an optical disk drive, and more particularly to an improved method and apparatus for detecting track crossings during CD-ROM search operations.
Optical disk drive systems operate by storing and retrieving information on an optical storage medium at various track locations within the media. The system typically employs a track search operation in which a track is quickly located to store or retrieve the desired information. The search operation typically employs a process by which the number of track crossings are counted until the targeted track is reached. Using this technique, the distance traveled and the velocity of the head can be precisely controlled.
FIG. 1
shows a simplified block diagram of a conventional optical disk drive system
10
. Optical disk
12
, such as a CD-ROM is driven by spindle motor
14
under control of spindle motor control circuity
16
. Optical pickup unit
20
retrieves or records information to/from disk
12
by means of photodiodes (not shown) properly placed above/below the desired track of the CD-ROM
12
, the location of the pickup unit
20
being precisely positioned by feed motor
22
. In addition, the pickup unit
20
provides a tracking error signal (TE) in order for the disk drive to maintain proper radial tracking.
Microprocessor
40
communicates with host computer
50
and, in turn, controls spindle motor
16
, focusing servo
26
, which in turn, controls the pickup unit
20
and tracking servo
24
, which in turn, controls feed motor
22
. The output signals from the pickup unit
20
are fed to focusing servo
26
and tracking servo
24
to aid in their operation, as well as the data processing circuity
30
for extracting data which is feed via BUS
44
to host computer
50
, and signals for use by microprocessor
40
and spindle motor control circuity
16
for their operations.
The optical drive system typically operates in a coarse tracking mode initially to locate the desired track. During coarse tracking, microprocessor
40
calculates the difference between the current track of the CD-ROM
12
and the target track and determines the direction of movement. The difference calculated is the remaining distance the pickup unit
20
must travel to arrive at the target track, the difference being loaded into a counter in the servo system. A tracking servo
24
then drives the pickup unit
20
in the desired direction. The pickup unit
20
provides the tracking error signal (TE), which, when the pickup unit
20
is traversing the disk, is a sinusoidal waveform having a zero crossing whenever the pickup unit passes a track center. One cycle of the TE signal represents crossing of one track. Using the TE signal, the tracking servo system determines when the pickup unit crosses a track and decrements a counter by one.
The tracking servo continues to drive the pickup unit until the counter decrements to zero. At this point, the pickup unit reads the current track information and commences a fine search operation to arrive at the target track. It can be appreciated that the servo system must detect track crossing accurately so that the counter will decrement correctly. If, due to erroneous track crossing detection, the track counter miscounts, the pickup unit is grossly mis-positioned and the system has to re-seek, increasing the seek time significantly.
To generate accurate track crossing counts, prior art servo systems utilize the tracking error crossing signal TX and a quadrature signal RX. The TX signal is the digitized waveform of the tracking error signal TE. The quadrature signal RX is the digitized waveform of the radio frequency ripple signal RFRP. The RFRP signal is derived from the summed output of the photodiodes (within the pickup unit
20
) which detects the reflected main laser beam.
When the pickup unit is positioned above/below a track center, the summed signal is the data signal and contains high frequency components. When the pickup unit is traversing the disk during the search operation, the summed signal becomes modulated. The modulation of this summed signal is a sinusoidal waveform 90° out of phase with the tracking error signal TE. The RFRP signal is generated by filtering the high frequency components from of the summed signal, and when digitized, yields the RX signal.
Unfortunately, both the TX and RX signals are prone to noise contamination. This is especially true for the RX signal since it is derived from a data signal containing high frequency components. Noise and glitches in the RX and TX signals can lead to erroneous track crossing detection.
One commonly encountered problem is detection of multiple track crossings. This occurs when transients in the TE signal are recognized as additional zero-crossings which are translated into additional transitions when the TE signal is digitized into TX. Thus, multiple zero crossings can be erroneously generated for each legitimate track crossing. The erroneous TX signal generated causes the counter to miscount and the pickup unit to arrive at the wrong track.
What is needed is a servo system and method for accurately detecting the number of disk track crossings.
SUMMARY OF THE INVENTION
The present invention provides a servo system having an accurate track crossing detection capability. The servo system of the present invention generates a noise free quarter track signal from noise contaminated RX and TX signals. Half and full track signals which are used to further process track information are derived from the noise free quarter track signal, thereby improving seek time and reducing the need for re-seek operations.
In one embodiment, the servo system includes an optical pickup for producing the RX and TX signals and a difference counter for providing track information based upon the RX and TX signals. The difference counter includes RX signal processing circuitry, TX signal processing circuitry, and a latch. The RX processing circuitry includes a RX z-hold (zero order) circuit, a RX delay circuit coupled thereto, and a RX XOR logic circuit coupled thereto for producing an RX processed signal. The TX processing circuitry includes a TX z-hold circuit, a TX delay circuit coupled thereto, and a TX XOR logic circuit coupled thereto for producing an TX processed signal. The RX and TX processed signals are fed into the latch which, in response, generates a noise-free quarter track signal. Half and full track signals are generated therefrom to further process the signal track information.
Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the following drawings.
REFERENCES:
patent: 5038334 (1991-08-01), Uchikoshi et al.
patent: 5121370 (1992-06-01), Yanagi
patent: 5796686 (1998-08-01), Maeda
Sun Kai C. K.
Sun Linda S. L.
Oak Technology, Inc.
Sun Linda S. L.
Townsend and Townsend / and Crew LLP
Young W. R.
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