FOCUS SERVO DEVICE

Abstract: A focus servo device for an apparatus for reproducing recorded information which includes a pickup for reproduc­ ing information recorded on a recording disk and a tracking servo device for maintaining the pickup over a recorded track on the disk, has mean (53) in which loop gain is reduced in response to detected abnormalities (22) in a recording disk, in order to prevent interference with a tracking servo device in an optical disk player.

Description: The present invention generally relates to a focus servo device, and particularly to a focus servo device in an optical information reader for optically reading out information recorded on a recording disk. In an optical information reader for reading out information recorded on a digital audio disk, a video disk, or the like, rotational driving of a recording disk is controlled by a spindle servo device, and recorded information is read out by a pickup movable in the radial direction thereof. A focus servo device providing control for correctly converging an information detecting spotlight from the pickup onto a recording surface of the disk, and a tracking servo device provides control for causing the spotlight to track a recording track, for example, spirally formed on the recording surface of the disk. In the tracking servo device, the higher the loop gain, the higher the trackability (the capability of following the track) of the information detecting spotlight so that a malfunction such as so-called track jumping is not caused even if there are external vibrations, or if the disk has substantial eccentricity (i.e. not perfectly round). As a result, stable tracking control is possible. However, in information readers having these kinds of servo devices, it is possible that an abnormal portion such as a defect, a scar, or the like, on the recording surface of the disk can cause distortion of the error signal of the servo system, resulting in a malfunction such as track jumping or the like when the information reading spotlight from the pickup tracks the recording track at the abnormal portion. To solve this problem, it has been proposed that in a tracking servo device, when an abnormal portion is detected on the recording surface, a servo operation be performed with a reduced loop gain or with an immediately preceding error value, or alternatively that a servo operation is to be performed instead on the basis of a tracking error signal (independent of the abnormal point) obtained in another way, thereby obtaining a stable tracking operation without causing any malfunction such as track jumping due to a defect, such as a scar, on the recording surface. As to reading of the recorded information at the abnormal portion, on the other hand, when drop-outs due to a defect are detected, a drop-out compensation circuit prevents the recorded information from being produced so that it is unnecessary to perform particular control of the focus servo device for an abnormal portion on the recording surface. However, experiments by the inventors have shown that disturbance in the focus servo system due to a defect such as a scar as described above in turn disturbs the tracking servo system. That is, when a defect is detected on a recording surface of a disk, a sufficiently stable tracking operation cannot be performed unless the tracking servo system is controlled as well as the focus servo system. EP-A-93582 discloses a focus servo device, for an optical disk player of the type having a single irradiating spot, comprising a circuit for detecting information dropout in accordance with the level of an output signal of the pickup, and reducing the loop gain of a scanning servo loop over the whole frequency band. Article 497 of Neues aus der Technik, 15/2/79, discusses the problems of focus control in optical disk players and in particular discloses that usual abnormalities on the surface of a disk produce signal components in a high frequency band. FR-A-2536196 discloses a optical disk player of the type in which a pickup irradiates the disk surface with a first, information-detecting beam, and a pair of further beams, preceding and following the first beam, which are used for tracking control. An object of the present invention is to provide a focus servo device in which it is possible to improve the trackability of an information detecting spotlight even where there exists an abnormal portion on a recording surface of a disk. According to the present invention, therefore, a focus servo device, for an apparatus for reproducing recorded information which includes a pickup for reproducing information recorded on a recording disk and a tracking servo device for maintaining the pickup over a recorded track on the disk, the focus servo device having a recorded-information detecting spotlight irradiated from the pickup controlled in accordance with a focus error signal derived from said spotlight so as correctly to converge the spotlight onto the recording surface of the disk; an abnormality detecting means for detecting abnormalities on the recording surface in accordance with the level of an output signal of the pickup; and, gain setting means for setting the gain of a focus servo loop, the gain setting means reducing the loop gain within a servo zone in response to a detected output from the abnormality detecting means whereby the focus servo device prevents abnormalities from affecting the tracking servo device; is characterised in that: the pickup additionally irradiates the surface of the disk with a pair of tracking information detecting spotlights respectively preceding and following the recorded-information detecting spotlight when it moves relative to the disk; and, the abnormality detecting means detects the abnormalities in accordance with the output signals of the preceding tracking information detecting spotlight. Preferably, the abnormability detecting means holds the detection output for a predetermined time after disappearance of the output signals. Examples of apparatus according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 shows the positional relationship of three spotlights on the recording track; Figure 2 is a circuit diagram showing an embodiment of the present invention; Figure 3A is a waveform diagram showing a focus error signal in the vicinity of an abnormal portion on the disk recording surface; and, Figure 3B shows a shaped output signal thereof; Figure 4 shows a gain characteristic set by the gain setting circuit of Figure 2; Figures 5A, 5B and 5C show three modifications of the gain setting circuit of Figure 2; and, Figures 6A, 6B and 6C show gain characteristics of the corresponding circuit arrangements in Figures 5A-5C. Figure 1 shows three spotlights, which are obtained by converging respective laser beams, that is, a recorded-information detecting spotlight 1 and a pair of tracking information detecting spotlights 2 and 3 respectively preceding and following the spotlight 1 when it moves relative to a disk, irradiated on a recording track 4 on the disk. The pair of tracking information detecting spotlights 2 and 3 are positioned on respective side edges of the track 4 when the recorded information detecting spotlight 1 is positioned on a centre-line of the track 4. Therefore, when the recorded-information detecting spotlight 1 deviates from the centre-line of the track 4 in the direction perpendicular to the track 4 (in the radial direction of the disk), the difference in quantity of light between the reflected light beams of the pair of tracking information detecting spotlights 2 and 3 corresponds to the direction and quantity of the deviation of the spotlight 1. The respective reflected light beams of the spotlight 1, 2 and 3 enter photoelectric transducer elements 5, 6 and 7 which are contained in a pickup (not shown). Figure 2 shows a circuit in which the respective reflected light beams of the tracking information detecting spotlights 2 and 3 enter the respective photoelectric transducer elements 6 and 7 and are transduced into electrical signals which are in turn supplied to a subtractor 21 respectively through waveform shaping circuits 11 and 12, the subtractor comprising an operational amplifier 16, a pair of input resistors 17, 18 and resistors 19 and 20. The waveform shaping circuit 11 comprises an optional amplifier 8, a capacitor 9, and a resistor 10, and the waveform shaping circuit 15 comprises an operational amplifier 12, a capacitor 13, and a resistor 14. A signal is formed in the subtractor 21 in accordance with the difference in level between the respective outputs of the photoelectric transducer elements 6 and 7 and forms a tracking error signal. The output signal of the photoelectric transducer element 6 which receives the reflected light of the preceding spotlight 2 is supplied also to an abnormality detecting circuit 22 through the waveform shaping circuit 15. The abnormality detecting circuit 22 is provided for detecting abnormalities on a disk recording surface in accordance with the level of the output signal of the photoelectric transducer element 6 after being shaped by the waveform shaping circuit 15. The abnormality detecting circuit 22 is constituted by an AC coupling circuit 25 comprising a capacitor 23 and a resistor 24; an amplifier 29 comprising an operational amplifier 26 and resistors 27 and 28, a time constant circuit 34 comprising a capacitor 30, a diode 31, a charging resistor 32, and a discharging resistor 33, and a comparator 38 comprising an operational amplifier 35 and voltage dividing resistors 36 and 37. The reflected light of the recorded information detecting spotlight 1 (see Figure 1) enters the photoelectric transducer element 5 through a cylindrical lens (not shown). With respect to the cylindrical lens, the position on the optical axis of the lens where the luminous flux converges is different in the case where the luminous flux transmits a surface containing a generatrix of the lens and the case where the luminous flux transmits a surface perpendicular to the first-mentioned surface, so that the shape of the luminous flux projected on a light receiving surface of the photoelectric transducer element 5 varies depending on the positional relation between the recording surface of the disk and the convergent point of the laser beam. In order to make it possible to detect the variation in shape of the luminous flux projected onto the light receiving surface of the photoelectric transducer element 5, the photoelectric transducer element 5 is arranged such that the light receiving surface thereof is divided into four independent light receiving sub-elements by two perpendicular lines. The sum of the respective outputs of a pair of diagonally disposed sub-elements of the photoelectric transducer 5 and the sum of the respective outputs of another pair of diagonally disposed sub-elements of the same are supplied to a subtractor 52 comprising an operational amplifier 45, capacitors 46 and 47, and resistors 48 to 51, respectively through an amplifier 41 comprising an operational amplifier 39 and a resistor 40, and an amplifier 44 comprising an operational amplifier 42 and a resistor 43. The output signal of the subtractor 52, which represents the direction and quantity of deviation of the convergent point of the laser beam is produced as a focus error signal. Further, if the respective output signals of the amplifiers 41 and 45 are added to each other, an RF signal, which is a recorded-information read-out signal is obtained. The focus error signal is supplied to a gain setting circuit 53, for setting a gain of a focus servo loop, through a variable resistor VR. The gain setting circuit 53 comprises, for example, capacitors 54 and 55, resistors 56 to 60, and a transistor 61. The transistor 61 functions as a switch which is turned on by a high level of the detection output supplied from the foregoing abnormalitiy detecting circuit 22. When the transistor 61 is turned on, the loop gain in a high frequency band within a servo zone is reduced. The focus error signal derived from the gain setting circuit 53 is supplied to a phase compensating circuit 70 which performs phase compensation for the supplied signal. The phase compensating circuit 70 is constituted by an operational amplifier 62, capacitors 63 and 64, resistors 65 to 68, and a transistor 69. The transistor 69 functions as a switching means which is turned on by a high level of the detection output supplied from the foregoing abnormalitiy detecting circuit 22, so that phase compensation is performed in the high frequency band in which the loop gain is reduced by the gain setting circuit 53, in response to the turning-on of the transistor 69. Next, the operation of the circuits of the focus servo device will be described. First, in this embodiment, in the information reader using a so-called tracking error signal system of the three-beam type, an abnormal portion on the disk recording surface is detected only on the basis of the output of the photoelectric transducer element 6 which receives the reflected light of the preceding tracking information detecting spotlight 2. In operation the signal level of the output signal of the photoelectric transducer element 6 is reduced, as shown in Figure 3(A), in accordance with the degree of the abnormal state on the disk recording surface, and the output signal is supplied to the abnormality detecting circuit 22 after being shaped by the waveform shaping circuit 15 as shown in Figure 3(B), so that the abnormality is detected by the circuit 22 on the basis of the change in signal level. The output signal of the photoelectric transducer element 6 supplied to the abnormality circuit 22, through an AC-coupling comprising the capacitor 23 and resistor 24, is amplified by the amplifier 29 and time-extended by the time constant circuit 34. This is because, when an abnormality is detected on the basis of the preceding tracking information detecting spotlight 2, it is necessary to maintain the detected abnormal state until the succeeding tracking information detecting spotlight 3 has passed through the abnormal portion. Specifically, the charging resistor 32 is selected to have an exceedingly small resistance value R32 in comparison with a resistance value R33 of the discharging resistor 33 (R33 » R32), so that the capacitor 30 is rapidly charged through the diode 31. As a result, no delay results in the abnormality detecting operation when the capacitor 30 is charged, while the operation of discharging the capacitor is performed in accordance with the time constant determined by the capacitor 30 and the discharging resistor 33 thereby to extend the discharging time when the capacitor 30 is discharged. The level of the output signal of the time constant circuit 34 is compared with a reference level given by the voltage-divider resistors 36 and 37 in the comparator 38 and the thus obtained comparison output is used as an abnormality detection output. In the gain setting circuit 53, the transistor 61 is turned on in response to the abnormality detection output from the abnormality detecting circuit 22, thereby effecting a series connection of the capacitor 54 and the resistor 57 in parallel with a series connection of the capacitor 55 and the resistor 59, thereby reducing the loop gain in the high-frequency band within the servo zone. That is, in the gain setting circuit 53, although the loop gain is reduced in the high-frequency band as shown by the solid line (a) in Figure 4, it is further reduced as shown by the one-dotted chain line (b) in Figure 4 upon detection of the abnormality. At the same time, in the phase compensating circuit 70, the transistor 69 also is turned on in response to the abnormality detection output from the abnormality detecting circuit 22 thereby to connect the capacitors 64 and 63 in parallel with each other, and a compensating zone is set in the low-frequency band in accordance with the reduction in loop gain by the gain setting circuit 53, thereby performing the phase compensation suitable for the frequency band. Thus, when a disk is reproduced in which there is an abnormal portion on the recording surface, the loop gain control, etc., is performed also in the focus servo system so that disturbance in the focus servo system caused by the abnormality on the disk recording surface does not disturb the tracking servo system. Thus, it is possible to improve the trackability of an information detecting point, that is the recorded-information detecting spotlight 1, of the pickup with respect to the abnormal portion on the disk recording surface in comparison with the conventional case where the loop gain control is performed only in the tracking servo system. Further, although the loop gain is reduced in the high-frequency band within the servo zone by the circuit arrangement of the gain setting circuit 53 as shown in Figure 2 in the embodiment as described above, it is possible, alternatively, to reduce the gain in the high-frequency band with the circuit arrangement shown in Figure 5A when an abnormality on the disk recording surface is detected. Further, it is possible uniformly to reduce the gain all over the servo zone with one of such circuits shown in Figures 5B and 5C. Figures 6A-6C show gain characteristics corresponding to the circuit arrangements in Figures 5A-5C. In each of the gain characteristics shown in Figures 6A-6C, a solid line (a) represents the gain characteristic when the disk is ordinarily operated, while a one-dotted chain line (b) represents the gain characteristic when an abnormality on the disk recording surface is detected. Further, in Figures 5A-5C, switch SW corresponds to the transistor 61 of Figure 2.

Claims: A focus servo device, for an apparatus for reproducing recorded information which includes a pickup for reproducing information recorded on a recording disk and a tracking servo device for maintaining the pickup over a recorded track on the disk, the focus servo device having a recorded-information detecting spotlight (1) irradiated from the pickup and controlled in accordance with a focus error signal derived from said spotlight (1) so as correctly to converge the spotlight (1) onto the recording surface of the disk; an abnormality detecting means (22) for detecting abnormalities on the recording surface in accordance with the level of an output signal of the pickup; and, gain setting means (53) for setting the gain of a focus servo loop, the gain setting means (53) reducing the loop gain within a servo zone in response to a detected output from the abnormality detecting means (22), whereby the focus servo device prevents abnormalities from affecting the tracking servo device; characterised in that: the pickup additionally irradiates the surface of the disk with a pair of tracking information detecting spotlights (2,3) respectively preceding and following the recorded-information detecting spotlight (1) when it moves relative to the disk; and, the abnormality detecting means (22) detects the abnormalities in accordance with the output signals of the preceding tracking information detecting spotlight (2). A device according to claim 1, wherein the abnormality detecting means (22) holds the detection output for a predetermined time after disappearance of the output signals. A device according to claim 2, further comprising means (11,15,21) responsive to reflected outputs of the preceding (2) and succeeding (3) tracking information detecting spotlights, for outputting a tracking error signal. A device according to any of claims 1 to 3, further comprising phase compensating means (70) for performing phase compensation of the focus error signal, the phase compensating means (70) changing over a compensating band in accordance with the band where the loop gain is reduced by the gain setting means (53) in response to the detected output.

Inventors: YOSHIO, JUNICHI C/O PIONEER ELECTRONIC CORPORATION
Applicants: PIONEER ELECTRONIC CORPORATION
Application year: 1985
Main classification: G11B7
IPC codes: G02B7, G11B7