Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-10-09
2001-05-15
Edun, Muhammad (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044250, C369S044280
Reexamination Certificate
active
06233210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to retrieving information from an optical disk. More specifically, the invention relates to a new method for generating a tracking error signal for any ablative, phase pit or phase change type of media such as compact disk (CD) or digital video disk (DVD). Still more specifically, the invention generates a tracking error signal by using a differential amplitude reading from a quad photodetector, or multiple pixel detector.
2. Description of Related Art
The use of optical disks for storing information has become popular in recent years and a number of products are commercially available, including compact audio disks, and digital video disks. Originally, these devices were read only memory devices, such as the music and personal computer CD players, but more recently, recordable compact disks have emerged in the marketplace. Despite the many different formats and options available in optical disk technology, there are considerable pressures to have standards which enable optical disk players to work with all data formats, and all manufacturers' products widely interchangable.
With reference now to the figures and in particular with reference to
FIG. 1
is a block diagram schematic which illustrates the components and connections of a known optical disk player. This configuration is well known in the prior art and is included here to motivate and frame the present invention. An optical disk system
100
includes semiconductor diode laser
102
that is typically used to generate laser beam
104
. Miniature optics
106
collimates and steers laser beam
104
. Generally, a movable, controllable lens
108
focuses laser beam
104
to a focused spot, also referred to as beam spot, and position on a spinning optical storage disk
110
, which is connected to a variable speed motor
112
. Light reflected from optical storage disk
110
is routed to both detector
114
and a position sensitive detector, such as a quad photodetector
116
. Detector
114
and quad photodetector
116
may be one single unit instead of the two units shown in
FIG. 1
Electronic data for the data channel is routed through read channel
118
into a servo controller/decoder
120
. The electronic signals from quad photodetector are routed through preamplifier
122
, and a servo preprocessor
124
, into servo controller/decoder
120
. These electronics generate an error signal which, through a power amplifier
126
and servo control motor
128
adjust the position of lens
108
in the vertical and horizontal directions in order to keep laser beam
104
aligned in focus and on track.
Thus, these optical disk memory devices operate by bouncing light emitted from a semiconductor diode laser off an inhomogeneous reflective surface of a spinning optical disk. The reflected light is then routed to a detector, which outputs an electronic signal that is processed to recover the stored digital data. The laser beam is nominally stationary, however, the laser assembly is slid along the radius of the disk being read. Furthermore, fine adjustments of the beam focal spot and position are made based on a feedback signal that drives an electromechanical armature. The data is encoded on the disk in a variety of ways, including ablative, phase pit or phase change type of coding.
In general, the optical disk rotates, often at high speeds, and this gives rise to a time signal, or time vector of digital data. A single speed compact disk rotates at several hundred revolutions per minute (rpm). Because it is desirable to provide a constant time spacing between data readings, a constant linear velocity is maintained as the data is read from the disk. Thus the precise rotational speed depends on the portion of the disk being read. For example, typical rotational speeds for a single speed compact disk are 200 rpm at the outermost track and 450 rpm at the innermost track. Faster microprocessors, as well as an overall improvement in control technology, have enabled disk drives that operate at multiples of the original single speed CD player. Thus, a 6× (six times single speed) CD player rotates at speeds up to 2700 rpm when reading the innermost track. Advantages of higher speeds include shorter data acquisition time, and error checking for misread data points. At these faster operating speeds, optical beam alignment tolerances, and their error correction become more challenging. Further, it is highly desirable for any processing protocols, and error correction methods, to be general across all data formats.
In early optical disk readers, tracking was accomplished by advantageously coded spot patterns on the disk. This scheme is not widely used because it is an overly specific format, and valuable disk area is used for tracking instead of data storage.
One existing method for generating a tracking error signal for audio compact disks is the three spot tracking technique. Here, a tracking error signal generator includes three light receptors for receiving three optical signals reflected from a recorded medium and for generating respective three electrical output signals. The three light beams impinge upon the recorded medium along a line having a predetermined angle with respect to a recorded track. Thus, the outer two spots sandwich the inner, data reading spot, and line up with the inside and outside edges of the data track. The difference in power between the outer beams is compared to generate the tracking error signal. The tracking error signal is set to the difference between the first and third output signals. To be widely useful, it is desirable that digital video disk players be able to play compact disks. However, because there is a difference between the track pitch of the CD and that of the DVD, the three spot tracking technique is no longer suitable for generating error tracking signals which are independent of data format.
Differential phase tracking error overcomes the limitation of data format. To generate a differential phase tracking error signal which can be used to align the laser beam onto the data track of an optical disk, the quad photodetector
116
of
FIG. 1
is used in the following manner. If the pit is left of the beam spot, the beam spot appears on quadrant A of quad photodetector
116
first and then on quadrant D. In this situation, the pit leaves quadrant A and then leaves quadrant D. The modulation of the output from these elements in quad photodetector
116
will have (quadrant A+quadrant C) ahead of (quadrant B+quadrant D) in the time domain. Measuring this phase difference provides a differential phase tracking error signal that may be used to drive servo control motor
128
in
FIG. 1
to realign the laser beam onto the optical disk data track. A limitation of differential phase tracking error signal generation is that it is dependent on the specific data pattern, and hence proves to be more effective with certain formats than with others.
Therefore, it would be advantageous to have a method and apparatus for generating a tracking error signal that is robust across different data formats and is independent of data pattern.
SUMMARY OF THE INVENTION
The present invention uses a photodetector with at least four active areas to sense the reflected laser beam. A differential amplitude tracking error signal is generated by comparing the signal strength in the different active area. In a preferred embodiment of the present invention, a quad photodetector is used to detect reflections from a spot generated by a laser beam directed onto a surface containing optical data. In this example, the signals from the diagonal elements of the quad cell are summed. These sums are then fed through differential amplifier circuitry. Sample and hold circuitry is used to form a suitable tracking error signal. Additional circuitry and filtering algorithms can be used to extend and improve the technique where necessary and justified. For example, a position photodetector with more than four elements will give information o
Edun Muhammad
LSI Logic Corporation
LandOfFree
Optical drive error tracking method and apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical drive error tracking method and apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical drive error tracking method and apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2514467