GB2165064A

GB2165064A - Tracking error suppression for disc player

Info

Publication number: GB2165064A
Application number: GB08520961A
Authority: GB
Inventors: Takashi Kikuchi
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1984-08-29
Filing date: 1985-08-21
Publication date: 1986-04-03
Also published as: GB8520961D0; JPS6159635A; BE903140A; DE3530749A1

Abstract

A servo system for controlling the position of a reading beam in a digital audio disc player or the like in which an abnormality detection signal is generated at a time when the level of a signal obtained from the recording disc by a pickup falls below a predetermined level. Starting from the time of the generation of the abnormality detection signal, the servo loop gain is maintained at a reduced value for a period longer than the rotational period of the data detection point relative to the recording disc. Accordingly, erroneous operations such as track jumping are prevented when the reading beam crosses scratches, smudges or the like on the recording medium. <IMAGE>

Description

SPECIFICATION Servo system in data reading apparatus Background of the invention The present invention relates to a servo system, and more particularly, to a servo system for use in a data reading apparatus for reading data from a recording disc.

In a data reading apparatus such as a digital audio disc player, a recording disc is rotated by a motor and a spindle servo device associated with the motor. A pickup is provided which moves only in the radial direction of the recording disc. A tracking servo system is provided to cause the data detecting point of the pickup to accurately follow a track formed spirally on the recording disc.

Figure 1 shows an example of a conventional tracking servo device. Three spot beams 2, 3 and 4, obtained by focusing a laser beam, are applied to a recording track 1 in the positional relationship shown in Figure 1. When the data detecting spot beam 2 is located on the track 1, the remaining two spot beams 3 and 4 are located on opposite edges of the track 1. Accordingly, if the detecting spot beam 2 is displaced in a direction perpendicular to the track (radially of the video disc), the difference in light quantity between the reflected beams of the spot beams 3 and 4 changes according to the direction of displacement and the amount of displacement of the detecting spot beam 2. The reflected beams of the spot beams 3 and 4 are converted into electrical signals by photoelectric conversion elements 5 and 6, respectively. These signals are applied to a differential amplifier 7.

A tracking error signal is produced by the amplifier 7 according to the difference in level between the outputs of the photoelectric conversion elements 5 and 6. After being amplified by a tracking servo amplifier 8, this signal is applied to a tracking actuator 9. A tracking servo loop is thus formed.

The tracking actuator 9 includes, for example, a coil which drives a tracking mirror (not shown).

The tracking mirror is rotated by the coil to thereby deflect the spot beams 2, 3 and 4 in the radial direction of the recording disc.

The reflected beam of the detecting spot beam 2 is radiated through a cylindrical lens (not shown) onto a light receiving surface of a photoelectric conversion element 10. The plane including the generating line of the cylindrical lens is different from the plane perpendicular thereto in the position of the optical axis where the light beam passed through the cylindrical lens is focused.

Thus, the shape of the light beam projected onto the light receiving surface of the photoelectric conversion element 10 changes according to the positional relationship between the focusing points of the laser beam. In order to detect the changes in shape of the light beam projected onto the light receiving surface of the photoelectric conversion element 10, the surface of the element 10 is divided into four light-detecting segments which are independent of one another. The light receiving surface of the photoelectric conversion element 10 is arranged at a position where, when the focusing point of the laser beam is positioned accurately on the recording surface of the recording disc, the reflected beam passed through the cylindrical lens is substantially circular.

The difference in level between the outputs of adders 11 and 12, each adder summing the outputs of opposed segments of the photoelectric conversion element 10, is indicative of the direction and amount of displacement of the focusing point of the laser beam. The outputs of the adders 11 and 12 form focus error detection signals b and c, respectively. In addition, the output of an adder 13, adding the outputs of adders 11 and 12, forms a read data signal, which is an RF signal.

In the above-described conventional tracking servo system, as the loop gain is increased, the data detecting spot beam 2 follows the track more accurately, and erroneous operations, such as track jumping, are less liable to occur, even if external vibration occurs and/or there is some eccentricity of the recording disc. Accordingly, with an increment of the loop gain, more stable tracking control can be achieved. However, if there are abnormalities such as scratches or smudges on the recording disc, an increment of the loop gain causes the tracking error signals to be disturbed at the time when the data detecting spot beam 2 crosses the abnormalities on the recording track. As a result, erroneous operations such as track jumping may occur.

Accordingly, in a conventional tracking servo system, it has been necessary to employ a rather low gain value in order to prevent the erroneous operations such as track jumping caused by scratches or smudges and to attain stable tracking control. The conventional tracking servo system is therefore disadvantageous in that sufficiently stable tracking control cannot be achieved.

It has been proposed to decrease the tracking servo gain only upon detection of abnormalities.

To this effect, the abnormalities are detected by detecting disturbances in the waveform of an RF signal produced upon detecting the data detecting spot beam 2 or a tracking error signal produced upon detecting the spot beams 3 and 4. To detect disturbances in the RF signal or the tracking error signal, an integrating circuit with a time delay is required. However, a relatively long time is required to detect disturbances in the waveforms of these signals using such an approach, and it is still difficult to perfectly prevent erroneous operations such as track jumping.

Summary of the invention A primary object of the present invention is to provide a servo system which is not affected by external vibration and/or eccentricity of a recording disc, and which can stably control the position of a data detecting point.

In the servo system according to the present invention, an abnormality detection signal is generated at the time when the level of a signal obtained from the recording disc by a pickup falls below a predetermined level, and from the time of the generation of the abnormality detection signal, the loop gain is lowered over a period longer than the rotational period of the data detection point relative to the recording disc.

Brief description of the drawings The present invention will be described hereinafter with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a conventional tracking servo system; Figure 2 is a block diagram showing a preferred embodiment of a servo system of the present invention; Figure 3 shows a recording surface of a recording disc; and Figure 4 is a waveform diagram used in explaining the operation of the device of Figure 2.

Description of the preferred embodiments Preferred embodiments of the invention will now be described in detail with reference to Figures 2 and 3.

In Figure 2, three spot beams 2, 3 and 4, obtained by focusing a laser beam, are applied to a recording track 1 in the positional relationship illustrated in Figure 1. Further, photoelectric conversion elements 5, 6 and 10, a differential amplifier 7, a tracking servo amplifier 8, a tracking actuator 9, and adders 11, 12 and 13 are connected in the same manner as shown in Figure 1.

The servo system of Figure 2 differs from that of Figure 1 in the following points. The gain of the tracking servo amplifier 8 is decreased when a gain decreasing control signal e is applied thereto from the Q output terminal of a retriggerable monostable multivibrator (hereinafter referred to as an "RMMV") 15. The trigger input terminal of the RMMV 15 is supplied with the output of an abnormality detection circuit 16. The abnormality detection circuit 16 generates a high level abnormality detection signal when the level of the RF signal from the adder 13 falls below a predetermined level.

In the circuit thus constructed, when there are abnormalities A, such as scratches or smudges, on the recording disc as illustrated in Figure 3, the waveform of the RF signal will vary as shown in Figure 4A. That is, the envelope level of the RF signal decreases each time the data detecting spot beam 2 passes the abnormality A. The period of the level decreases of the RF signal is equal to the rotational period of the recording disc. The first time a periodical decrease in the level of the RF signal a occurs as the abnormality A is first encountered, the change in the level of the RF signal is small. However, each successive time the data detecting spot beam 2 passes the abnormality A, the change in the level of the RF signal becomes larger.

An abnormality detecting signal d is outputted at the same period as a rotational period T, as shown in Figure 4B. The time constant of the RMMV 15 is set to a value slightly longer than the longest value of the rotational period T. For example, for a CLV (constant linear velocity) disc, when the data detecting spot beam is positioned at the outermost peripheral track of the disc, the time constant is about 300 msec for a rotational speed of 200 r.p.m.

As the RMMV 15 has been in the reset state (0=0) during the time interval from the occurrence to the disappearence of the abnormality detecting signal d, a high level gain decrease indicating signal e is outputted, as illustrated in Figure 4C. As a result, when the data detecting spot beam 2 is deflected in the radial direction of the disc, the gain of the tracking servo amplifier 8 is reduced over a period between the first time the data detecting spot beam 2 touches the abnormality A until it finally leaves therefrom. Therefore, since the loop gain is maintained reduced from the time the data detecting spot beam 2 first touches the abnormality A (i.e., from the time when the tracking error signal is less affected by scratches or smudges on the recording disc), erroneous operations such as track jumping are prevented.

In the foregoing embodiment, although the abnormality detecting circuit 16 is employed to detect abnormalities on the surface of the disc by the level of the RF signal a, it is possible to detect the presence of abnormalities by a level change of the tracking error signal. Although the time constant of the RMMV 15 is described as being constant, it can be changed according to the rotational speed of the disc. Moreover, although the loop gain is described as changing intermittently in response to the gain decrease indicating signal, it can be changed sequentially according to the size or degree of the abnormality.Although the RMMV 15 is employed as an instruction generator, if a microcomputer is employed in the control circuit which controls the various components of the data reading apparatus, a program stored in ROM in the microcomputer may be changed so that the microcomputer is operated as the instruction generator.

The foregoing description relates to a tracking servo system based on a beam system in an optical data reading apparatus. However, the invention is applicable also to other servo systems such as a focus servo system or a tracking servo system in an electrostatic capacity type data reading apparatus.

As described above, the servo system of the invention is designed so that abnormalities on a recording surface are detected by a signal obtained by a pickup, and the loop gain is held at a reduced value from the time when an abnormality is detected for a period longer than the rotational period of the disc. Therefore, when the data detecting point passes parts where the error signal may be significantly affected, because the loop gain is lowered in advance, the occurrence of erroneous operations such as track jumping is prevented.

Furthermore, the servo system according to the invention is simple in construction and no complicated devices, such as one in which the loop gain is lowered at the moment when a data detecting point of a pickup reaches a position of scratches or smudges, are necessary.

Also, the loop gain can be set sufficiently high during the time when data is read out from normal parts that the servo system of the invention prevents external vibration or eccentricities of the recording disc from adversely affecting the reproduced signals.

Claims

1. A servo system for a data reading apparatus for controlling a position of a data detecting point of a pickup means for reading data from a rotating recording disc, comprising: a feedback control loop for maintaining said pickup means in alignment with a track on said disc; abnormality detecting means for generating an abnormality detecting signal when said pickup is receiving information from an abnormality on said disc; and means for reducing a loop gain of said control loop in response to said abnormality detecting signal.

2. The servo system of claim 1, wherein said detecting means comprises means for generating said abnormality detecting signal when a level of a signal read out from said recording disc by said pickup means falls below a predetermined level.

3. The servo system of claim 2, wherein said signal read out from said recording disc is an RF signal outputted from an adder.

4. The servo system of claim 3, wherein said abnormality detecting means comprises means for generating said abnormality detecting signal when a level of a tracking error signal produced by said pickup means changes in a predetermined manner.

5. The servo system of claim 1, wherein said abnormality detecting signal generating means comprises an RMMV circuit.

6. The servo system of claim 5, wherein said gain reducing means comprises means for reducing said gain from a time of generation of said abnormality detecting signal to a time longer than a rotational period of said disc.

7. The servo system of claim 6, wherein a time constant of said RMMV circuit is slightly longer time than a longest rotational period of said disc.

8. The servo system of claim 6, further comprising means for varying a time constant of said RMMV circuit according to a rotational speed of said disc.