TRACKING ERROR DETECTING SYSTEM FOR
REMOVING AN OFFSET IN AN OPTICAL DISK
FIELD OF THE INVENTION
The present invention relates to an optical pick-up system for an optical disk player, and more particularly to a system for detecting a tracking error signal.
BACKGROUND OF THE INVENTION
The essential function of the optical pick-up system is to detect an audio signal recorded on the disk and produce an electric signal corresponding to the recorded information. The system generates a control signal for 15 producing the signal. Further, the system has an optical system for detecting a focus error signal and a tracking error signal.
FIGS. 9a and 9b show a beam for detecting a tracking error signal using a push-pull method. A laser light from a laser diode (not shown) is focused onto a disk 1 through an objective 2 and a reflected light 3 is focused on a photo detector 4. The reflected light 3 includes an image detecting light and a tracking error detecting light. In a conventional system, the photodetector 4 for detecting tracking error signal is divided into two detectors PD1 and PD2. Therefore the reflected light from the disk 1 is divided into two parts on the photodetectors PD1 and PD2, respectively, for comparing the intensities of the divided light with each other. If there is no error in tracking, the intensities of the divided light are the same. Thus, the spots of beams on the photodetectors PD1 and PD2 are symmetrical about the optical axis between the photodetectors as indicated by a dashed line of FIG. 9b.
On the other hand, the optical axis XI of the tracking error detecting light 3a and the optical axis XII of the image detecting light 3b may deflect by a difference a because of the movement of a tracking actuator (not shown). Consequently, the illuminated spot 3A on the photodetector PD1 is different from the spot 3B on the photodetector PD1 in area. As a result, an offset is included in the tracking error signal.
More particularly, if the output powers of the photodetectors PD1 and PD2 are 3Aoand 3Bo when no offset occur, the tracking error signal TE is
If the offset quantity is a, and the output powers are 50 3Aj and 3Bi when an offset occurs, the tracking error signal TE is
Accordingly, the pickup is moved by a power corresponding to the tracking error 2a so that the tracking error may become zero. Namely, the pickup malfunctions because there is no tracking error.
Further, when the disk 1 is tilted relative to the beam 60 as the disk rotates as shown is FIG. 9c, an offset also occurs in the tracking error signal.
In order to reduce the offset in the tracking error signal, the diameter of the spot on the photodetector is reduced so as to be smaller than the diameter of the 65 reflected beam for preventing the influence of the offset. By this system, about half of the tracking error signal is suppressed.
However, in this system, since a small part of the reflected light is used as the effective signal, S/N ratios of both of the tracking error signal and an RF signal reduce. The reduction of the S/N ratio deteriorates characteristic of signals such as the servo signal and RF signal to be reproduced.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a tracking error detecting system which may improve the accuracy of the signal to be reproduced.
According to the present invention, there is provided a tracking error detecting system for an optical disk player comprising, a photodetector comprising a pair of first segments a total area of which is larger than an area of a spot reflected from the optical disk onto the first segments, a pair of second segments provided within the first segments and a total area of the second segments being smaller than the spot, and a detecting circuit for producing a tracking error signal by removing a difference between outputs of the first segments and a difference between outputs of the second segments.
In an aspect of the invention, the first and second segments are divided by a common line. The detecting circuit includes circuit means for removing a difference between a difference based on all outputs of the first and second segments and a difference based on outputs of the second segments.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a tracking error detecting circuit for detecting a tracking error signal according to the present invention;
FIGS, la and lb are schematic plan views showing a photodetector of the system;
FIG. 3 is a schematic illustration explanatorily showing the operation of the circuit of FIG. 1;
FIG. 4 shows a circuit of a second embodiment of the present invention;
FIG. 5 shows a circuit of a third embodiment;
FIG. 6 is a schematic illustration showing the operation of the third embodiment;
FIG. 7 is a schematic diagram showing a fourth embodiment;
FIG. 8a is a plan view showing a focus error detector of the fourth embodiment;
FIG. fib is a plan view showing spots on a tracking error detector of the fourth embodiment; and
FIGS. 9a to 9d are illustrations for explaining an offset included in a tracking error signal in a conventional tracking error detecting system.
DETAILED DESCRIPTION OF THE
Referring to FIG. 1, a tracking error detecting system of the present invention has two operational amplifiers 10 and 11.
Referring to FIGS, la and lb showing a photodetector 5, a surface of the photodetector 5 for receiving a reflected light is laterally divided into four sections 5o, Sb, 5c and Sd about the optical axis by a common line 7. When the beam is reflected on the photodetector 5, a spot 6 of the beam is formed on the surface of the photodetector 5. Each of the sections 5a and Sd has an area larger than a semicircular spot B (C). The photodetec3