GB2155627A

GB2155627A - Tracking error signal generating apparatus for an optical information recording/playback system

Info

Publication number: GB2155627A
Application number: GB08505045A
Authority: GB
Inventors: Hidehiro Ishii
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1984-02-27
Filing date: 1985-02-27
Publication date: 1985-09-25
Also published as: DE3506961A1; GB8505045D0; JPH0652577B2; JPS60179946A; GB2155627B

Abstract

The tracking error signal generating apparatus includes first and second light receiving elements 4,5 respectively for receiving first and second light beams 1,3 applied on both sides of an information reading point 2 and the first light beam being located on a forward side of a movement of the information reading point with respect to the recording surface of a recording medium. In order to compensate for a time difference between the waveforms of output signals of the first and second light receiving elements 4,5, the apparatus is further provided with a signal delay means 8,10 for delaying one of output signals of the first and second light receiving elements 4,5 by a time period corresponding to a relative speed of the movement of the information reading point with respect to the recording medium. <IMAGE>

Description

SPECIFICATION Tracking error signal generating apparatus for an optical information recording/playback system The present invention relates to a tracking error signal generating apparatus, and more specifically to a tracking error signal generating apparatus in an optical information recording/playing back system.

Optical information recording/playing back systems are generally provided with a tracking servo system so that a playback light beam for reading recorded information or a recording light beam for recording information on a recording medium accurately follows an information track on the recording medium. In those systems, it is therefore necessary to produce a tracking error signal which serves as a control signal for a servo control loop of the tracking servo system.

In a type of tracking error signal generating apparatus, known as the three-beam system, the tracking error signal is produced as a signal corresponding to a difference between output signals of a couple of light receiving elements which are respectively adapted to receive a reflected light or a transmitted light of one of two light beams, i.e. a preceding light beam and a following or pursuing light beam, other than a recording/playing back light beam, which are applied on a recording medium for the purpose of tracking operation.

However, since these two light beams are spaced for a predetermined distance on the recording medium with respect to the direction of the movement of the pickup device, there is necessarily a time difference between the waveform of the output signals of two light receiving elements. Due to this time difference, it was often the case that the RF playback signal is introduced into the tracking error signal as a noise component in the conventional arrangement of the tracking error signal generating apparatus. In addition, if there is a flaw or defect on the surface of the reocording medium, a disturbance within the pickup output signal sometimes appears in the tracking error signal also due to presence of the above mentioned time difference.

An object of the present invention is therefore to provide a tracking error signal generating apparatus in which the noise component in the tracking error signal is reduced and which ensures the stable tracking operation even under the presence of a scratch or a defect of the recording surface of the recording medium.

According to the present invention, the tracking error signal generating apparatus is constructed such that an output signal of a first light receiving element adapted to receive a reflected or a transmitted light of the preceding light beam is delayed for a predetermined time duration corresponding to a relative speed of the movement of the information detection point. The tracking error signal is produced as a signal corresponding to the difference between the delayed output signal of the first light receiving element and an output signal of a second light receiving element adapted to receive a reflected or transmitted light of the following (pursuing) light beam.

Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating a preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

FIG. 1 is a block diagram depicting the construction of a conventional tracking error signal generating apparatus; FIGS. 2A and 2B are waveform diagrams illustrating waveforms of signals A and B appearing in the circuit of the apparatus shown in FIG. 1; FIGS. 3A and 3B are waveform diagrams similar to FIGS. 2A and 2B, however illustrating the relation between the tracking error signal produced by the apparatus and the signals A and B produced by the light receiving elements; FIG. 4 is a block diagram depicting the construction of an embodiment of the tracking error signal generating apparatus according to the present invention; FIGS. 5A and 5B are waveform diagrams illustraing the waveforms of signals A' and B appearing in the circuit of the apparatus of FIG. 4; and FIGS. 6A and 6B are waveform diagrams illustrating the relation between the tracking error signal and the signals A and B in the apparatus according to the present invention and shown in Fig. 4.

Before entering into the explanatroy of the embodiments of the signal detection device according to the present invention, reference is first made to FIG. 1 which schematically shows the principle of the operation of the socalled three beam system.

In a light source (not illustrated), a laser beam from a laser source is applied to a grating, i.e., a diffraction grating, where a O order light beam and a pair of first order light beams i.e., i 1 order light beams are obtained.

As shown in the left hand side of the figure 1, the 0 order light beam and the + 1 order light beams are irradiated on the surface of recording medium in such a manner that a light spot 2 of the 0 order light beam and a pair of light spots 1 and 3 of the i 1 order light beams arranged symmetrically with respect to the light spot 1 on both sides thereof, are alind on a line L which is inclined by a predetermined angle relative to the direction of a recoring track of the recording medium.

Reflected light beams, or transmitted light beams in some cases, of the i 1 order light beam from the recording medium are respectively received by a pair of light receiving elements 4 and 5. Output signals A and B of the light receiving elements 4 and 5 have mutually anti-phase relation and respectively represent a displacement of the light spot 2 from the recording track in a direction perpendicular thereto. The output signals A and B of the light ieceiving elements 4 and 5 are, in turn, applied to a differential amplifier 6 respectively as an in-phase input signal and an anti-phase input signal. In the differential amplifier 6, the output signals of the light receiving elements 4 and 5 are respectively applied to a positive input terminal and a negative input terminal of an operational amplifier 7 through input resistors R, and R2.A resistor R3 and a variable resistor VR for zero adjustment are connected between the positive input terminal and the ground. In addition, a feedback resistor R4 is connected between the negative input terminal of the operational amplifier 7 and an output terminal thereof.

By this operational amplifier 7, a signal K(A B) corresponding to the direction and the magnitude of a deviation of the position of the light spot 2, that is, an information reading point relative to the recording track is produced. In the above expression, K represents a gain of the differential amplifier 6.

In the thus constructed tracking error signal generating system, if we assume that the movement of the light spot 2 relative to the recording medium, while the information is detected normally by means of the light spot 2, is in a direction on the side of the light spot 1, then the + 1 order light beam goes first and the 1 order light beam beam follows two other light beams.

In this state, the waveforms of the signals A and B becomes as shown in FIGS. 2A and 2B, and a time difference t will be present.

This time difference t is expressed by the following equation (1): t = D/V, (1) where D represents the distance between the light spot 1 and the light spot 3, that is, the distance between the preceding light beam and the following or pursuing light beam on the recording surface, and VL represents the relative speed of the movement of the light spot 2 on a recording track, that is, the linear velocity.

In the case of the digital audio disc which is an example of the CLV (Constant Linear Velocity) disc, the playback is performed with a fixed linear velocity of 1.2 through 1.4 m/sec.

The disctance D between the preceding light beam and the following light beam in the case of the digital audio disc is determined to be 50um for instance.

Therefore, the time difference t is calculated as 35.7 x 10-6 through 41.6 x 10-6 (seconds) according to the expression (1). This value corresponds to the frequency of 24kHz through 28kHz. Generally, this frequency value is sufficiently higher than the frequency range of the servo control (around 1 kHz) and it was believed that the influence of the time difference upon the tracking servo control can be neglected.

However, due to the presence of this time difference t, the RF signal components contained in the output signals A and B, which are substantially in-phase with each other, are not cancelled in the differential amplifier 6 and enters into the tracking error signal as a noise component. Moreover, as shown in Fig.

3B, if a disorder (denoted at a) occurs in the waveform of the output signals A and B due to a flow or a defect of the recording surface, the disorder also appears in the tracking error signal as shown in Fig. 3A. Thus, it was sometimes difficult to perform a stable tracking control operation.

The embodiment of the present invention will be discussed in detail with reference to Figs. 4 through 6 of the accompanying drawing.

In FIG. 4, light spots 1 and 3 of i 1 order light beam are aligned on a line L and on both sides of a light spot 2 of the 0 order light beam which is also on the line L. This arrangement is the same as the light spots 1 through 3 of Fig. 1. As in the case of Fig. 1, an output signal of a light receiving element 5 for receiving a reflected light or a transmitted light of the light spot 3 is applied to a negative input terminal of a differential amplifier 6. On the other hand, according to the invention, an output signal of a light receiving element 4 is connected to a variable delay element 8 having a control input terminal and, for example, made up of a CCD (Charge Coupled Device) or BBD (Bucket Brigade Device). The output signal of the light receiving element 4 is also applied to one of two input terminals of a change-over switch circuit 9. To the control input terminal of the variable delay element 8, an output signal of a linear velocity detection circuit 10 is supplied. The variable delay element 8 and the linear velocity detection circuit 10 together form a signal delay means. The linear velocity detection circuit 10 includes, for instance, a rotational speed detection means such as a generator which generates a signal corresponding to a rotational speed of a disc used as the recording medium, and a radial position detection means for detecting a radial position of a slider which carries the pickup device and is movably disposed along a radial direction of a disc.With this construction, the linear velocity detection circuit 10 derives an angular velo city X by means of the rotational speed detection means and a radial position r by means of a radial position detection means and in turn calculates the linear velocity V, (V, = rx ), then produces a clock pulse signal having a repetition frequency corresponding to the thus obtained linear velocity.

By the operation of the linear velocity detection circuit 10, the signal delay time of the variable delay element 8 is controlled correspondingly to the linear velocity V,. The output signal A of the light receiving element which is delayed by the variable delay element 8 is then applied to the other input terminal of the change-over switch circuit 9.

To a control input terminal of the change-over switch circuit 9, there is provided a changeover control signal from a control circuit which is provided for controlling various parts of the information playback system.

With this change-over control signal, the change-over switch 9 selectively outputs the signal applied to one of the input terminals, that is the signal A, at the time of the starting, the searching operation, and the jump operation, that is, the starting of the movement of the information detecting point to follow a target track. On the other hand, during a normal operation period other than the period of the above operations, the change-over switch circuit 9 selectively outputs the signal applied to the other one of the input terminal, that is, the signal A'. The output signal of this change-over switch circuit 9 is applied to a differential amplifier 6 at a positive input terminal thereof.

With this construction, the differential amplifier 6 is selectively applied with an output signal A' of the variable delay element 8 at the positive input terminal thereof during the normal operating period other than the starting period, a search mode period, and the period of the jump operation. The waveforms of the output signal A' and the output signal which is applied to the inverting input terminal of the differential amplifier 6 during this time period are such as illustrated in Figs. 5A and 5B. As will be appreciated from these figures, there is no time difference between the output signal A' and the output signal B.

Accordingly, as shown in Fig. 6B, if there is a disorder a of the waveform of the same phase is present in the output signals A' and the output signal B due to a scratch on the recording surface for instance, the level of a disorder a' appearing in the output signal K(A'-B) of the differential amplifier 6 which is output as the tracking error signal, will become sufficiently small as illustrated in Fig.

6A. This is because the disorder of the wave form is cancelled by the operation of the differential amplifier 6. In addition, the level of the noise due to the mixing of the RF signal into the input signals also becomes sufficiently small.

Above, a preferred embodiment of the present invention has been described. It should be understood, however, that the foregoing description is for illustrative purpose only, and is not intended to limit the scope of the invention. Rather, there are numerous equivalents to the preferred embodiment, and such are intended to be covered by the appended

Claims

claims. As an example, the signal delay means is made up of the variable delay element 8 and a linear velocity detection circuit 10 in the above described embodiment. However, in the case of the playback of information from a CLV disc, a digital audio disc for instance, the signal delay means may be provided using a phase delay circuit made of an operational amplifier or a high-order low frequency filter formed by a multistage LC circuit, which have a constant delay time. In that case, it is suitable to set the delay time around 35#sec. for minimizing the effect of the flaw or defect on the recording surface and for reducing the noise component. Thus, according to the present invention, the tracking error signal generation apparatus is constructed to generate a tracking error signal as a signal corresponding to a difference between an output signal of a light receiving element which is corresponding to a following light beam and a delay signal which is obtained by delaying the output signal of a light receiving element corresponding to a preceding light beam by a time duration corresponding to the relative speed of the movement of the information reading point. Therefore, a very stable tracking control operation is enabled by the cancelation of the disorders of waveform appearing in the output signals of two light receiving elements, such disorders are the result of the flaw or the defect of the recording surface and have substantially the same phase relation with each other. CLAIMS

1. A tracking error signal generating apparatus for an information recording/playing back system using an information recording medium, comprising: a light beam irradiating means for irradiating first and second light beams on a recording surface of the recording medium so that the first and second light beams are positioned on both sides of an information reading point and aligned together with said information reading point on a line which is inclined by a predetermined angle with respect to direction of a recording track, said first light beam being located on a forward side of a relative movement of said information reading point with respect to said recording surface of the recording medium, on a recording track; a first light receiving element for receiving said first light beam via said recording me dium and producing a first output signal;; a second light receiving element for receiving said second light beam via said recording medium and producing a second output signal; a signal delay means connected to said first light receiving element for delaying said first output signal by a time period corresponding to a speed of said relative movement of said information reading point; and a differential signal generating means connected to said second light receiving element and said signal delay means, for generating, as a tracking error signal, a signal corresponding to a difference between said first output signal of said first light receiving element delayed by said signal delay means and said second output signal of said second light receiving element.

2. A tracking error signal generating apparatus as set forth in claim 1, wherein said signal delay means comprises a linear velocity detection means for detecting a linear velocity of said information reading point with respect to said recording track and producing a detection signal, and a variable delay means for varying a signal delay time according to said detection signal of said linear velocity detection means.

3. A tracking error signal generating apparatus as set forth in claim 1, further comprising a signal change-over means responsive to said first output signal and said output signal of said signal delay means and connected to said difference signal generating means, for selecting said first output signal from said first light receiving element upon starting of a movement of said information reading point toward a target recording track and selecting said output signal of signal delay means for time periods other than a time of said movement of information reading point.