Optical disc apparatus

Abstract: An optical disc apparatus incudes an optical pickup for reading recorded information on an optical disc, a servo controller for performing tracking servo control and/or focus servo control of the optical pickup in response to an output signal from the optical pickup, a vibration detector for detecting a vibration occurred to the body of the apparatus, and control means for performing up and down control of a servo gain for the tracking servo and/or the focus servo of the optical pickup in response to an output signal from the vibration detector.

Description: This invention relates to an optical disc apparatus. Optical disc apparatuses are provided with some countermeasure against flaws on discs, dust that adheres thereto or the like, as well as some countermeasure against vibrations or other external mechanical disturbances. In a servo circuit concerning tracking control and focus control, in general, compromises involving opposite characteristics are known: an increase in the servo gain alleviates erroneous operation caused by vibration but invites an excessive response to disc flaws, dust, etc, which results in undesired movements of the optical pickup; in contrast, a decrease in the servo gain alleviates the adverse influence of disc flaws but increases erroneous operations caused by vibrations. In this connection, Japanese patent laid open publication number Sho 60-74129, for example, or EP-A-0160095 on which the precharacterising portion of appended claim 1 is based,discloses an apparatus which refers to a distortion in the waveform of an Reference signal output from an optical pickup to decide whether the cause of the distortion is a disc flaw or dust on the one hand, or by vibration on the other, thereby to decrease the servo gain in case of the former but to increase the servo gain in case of the latter. However, it is difficult to decide reliably from the RF signal waveform distortion the reason for the distortion because an RF signal distortion caused by disc flaws or dust may give the an appearance of having been caused by vibrations. This erroneous decision causes an increase of the servo gain, which is an adverse result opposite to the desired gain control (low gain). Therefore, it has been difficult to realise an apparatus less affected both by disc flaws or dust and by vibrations. EP-A-0 306 715 discloses the use of a vibration sensor wherein the output of the sensor in processed so as to produce a compensating current for the read/write head drivers. According to an aspect of the invention, there is provided an optical disc apparatus comprising: an optical pickup for reading recorded information on an optical disc; servo control means for performing tracking servo control and/or focus servo control of said optical pickup in response to an output signal from said optical pickup; vibration detector means for detecting a vibration occurring of the body of said apparatus and control means for performing up and down control to increase or decrease a servo gain of the tracking servo and/or the focus servo of said optical pickup; wherein said control means operates in response to an output signal from said vibration detector means; characterised in that : said vibration detector means including a vibration sensor for detecting said vibration and for converting it into an electrical signal and including a level detector circuit for converting an output signal from said vibration sensor into a digitised signal and for outputting the digitised signal; wherein said level detector circuit is operative to supply a first detection signal to said servo control means when said output signal from said vibration sensor is above a predetermined reference level, and to supply a second detection signal to said servo control means when said output signal from said vibration sensor is below a predetermined reference level. As will become apparent from the following description, the invention can provide an apparatus which is less susceptible to erroneous operation and which is suitable for in-car use. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figure 1 is a block diagram showing the entire construction of an optical disc apparatus embodying the present invention; Figure 2 is a circuit block diagram showing the main section of the optical disc apparatus of figure 1; Figure 3 is a schematic perspective view of the optical disc apparatus of figure 1 in which its housing is partly cut away to show part of its interior arrangement; and Figure 4 is a schematic waveform diagram; and Figure 5 is a schematic eye pattern diagram of an RF signal. An optical disc apparatus according to the invention is described below in detail, referring to a preferred embodiment illustrated in the drawings. The illustrated embodiment is a compact disc player 1 incorporating the instant invention. In figures 1 and 3, reference numeral 2 denotes a disc and 3, an optical pickup. When a laser beam is irradiated to the disc 2, reflected light is received by a detector (not shown), and a signal a(E) indicative of the received light is formed into a signal b through an RF amplifier 4 and supplied to a digital signal processor 5. Reference numeral 6 designates a vibration sensor which detects a vibration occurred to the compact disc player 1 and produces an electrical detection signal c, and it may be of a type using an ultrasonic vibrator. The vibration sensor 6 is mounted on a bottom plate 1a of the disc player 1, with its detecting portion in close contact with the bottom plate 1a, as shown in figure 3. Although the sensitivity of the vibration sensor 6 varies slightly, depending on the direction of the vibration of the compact disc player 1, ie, vertical vibration or horizontal vibration, the difference, however, is small and can be disregarded. Reference numeral 7 denotes a level detector circuit which is provided for extracting the output signal c of the vibration sensor 6 in the form of a digitised signal d. As shown in figure 2, the level detector circuit 7 includes, for example, a rectifier circuit 9 connected to an amplifier 8 for amplifying a voltage generated by the vibration sensor 6 outputs a DC voltage responsive to the vibration. In this case, if the vibration sensor 6 and the amplifier 8 have flat frequency characteristics over a desired range, a DC voltage independent from the vibration frequency can be obtained. The output after rectification may be digitised by means of a smoothing capacitor 10, a hysteresis comparator 11 and a buffer 12. The digitised output is supplied via a transistor and a relay to a servo controller 13 referred to later as a signal d. More specifically, a H (high) signal is obtained when a vibration above a predetermined level is detected, an L (low) signal is obtained otherwise. Since the magnitude of the obtained DC voltage varies with the gain of the amplifier 8, the reference level (threshold value) for comparison and decision can be set at a desired value by changing the gain of the amplifier 8. Additionally, by selecting the capacitance of the smoothing capacitor 10, the output time of the H signal, ie, the time in which a constant status is held, can be set to a desired duration. Reference numeral 13 denotes a servo controller which performed focus servo control for holding an objective lens provided in the optical pickup 3 at a predetermined distant position from the disc surface and performs tracking servo control for ensuring the optical pickup 3 to precisely trace a track of the disc 2. The servo controller 13, in response to a tracking error signal e and a focus error signal f from the digital signal processor 5, supplies predetermined control signals g and h to a tracking coil and a focusing coil in the optical pickup 3 to perform servo control thereof. The servo controller 13 also controls the response characteristic of the servo system by varying the servo gain of the servo controller 13 up and down in response to the digitised signal from the level detector circuit 7. For example, when the signal d from the level detector circuit 7 is the L signal, the servo gain is set to be 0 dB in the frequency range of 1 to 2 kHz. When the signal d from the level detector circuit 7 goes to the H signal, the servo gain is increased by 6 dB as long as the H signal persists. The servo gain need not be increased by 6 dB at once when the servo controller 13 receives the H signal from the level detector circuit 7, but it may be increased stepwise by 1 to 2 dB for every predetermined period of time to finally reach the total variation (6 dB). Also when the servo gain is returned to the original value after removal of a vibration, it may be decreased stepwise. Reference numeral 14 refers to a guide shaft, 15 to a thread-moving motor for moving the entire optical pickup 3 along the guide shaft 15 in the radial direction of the disc, and 16 to a spindle motor for rotating the disc. These motors 15 and 16 are controlled by signals i and j from the servo controller 13. The operation of the compact disc player 1 described above is described below. It is noted that figure 4 shows, merely schematically, signal waveforms at various portions of the level detector circuit 7. Figure 4A refers to the waveform of the output signal of the vibration sensor 6, figure 4B to the waveform of the output signal of the amplifier 8, figure 4C to the waveform as rectified and smoothed (the broken line indicates the threshold value), and figure 4D refers to the waveform of the output signal from the hysteresis comparator 11. The waveform which is schematically shown in figure 5 represents the RF signal E detected by the optical pickup 3. As long as the vibration experienced by the compact disc player 1 is small, the output of the hysteresis comparator 11 of the level detector circuit 7 is held in the L level, and the servo controller 13 sets the servo gain for tracking servo and focus servo at a low value. Therefore, when the disc 2 has a flaw or dust attached thereto, the RF signal E output from the optical pickup 3 temporarily goes to a low level as shown by an arrow I in figure 5, for example. However, since the servo gain is set low, the response of the servo system to the error signal is low thereby preventing a tracking deviation or the like. When any significant external disturbance vibration occurs to the apparatus and is detected by the vibration sensor 6 and the level detector circuit 7, the output of the hysteresis comparator 11 goes high (H), and the servo controller 13 increases the servo gain for the period of time that the H signal persists. The RF signal E at this time represents a waveform as shown by an arrow S in figure 5, for example, in which the modulation level is small. However, since the servo gain is set high, the response of the servo system is increased. As a result, the servo system represents a sufficiently sensitive response to the vibrations as well, and ensures that the laser beam of the optical pickup 3 follows a recording track on the disc 2. In the compact disc player 1 referred to above, the vibration sensor 6 detects vibration of the apparatus, and the value of the servo gain is varied in response to the digitised signal which is fed from the level detector circuit 7 to the servo controller 13. In other words, the servo gain is held in a low value as long as the vibration is small so that the apparatus is less affected by flaws or dust on the disc, and only when a large vibration is detected by the vibration sensor 6, is the servo gain increased to improve the tracking properties of the servo system. As a result, even if the RF signal represents the waveform shown by an arrow S in figure 5, affected by a flaw or dust on the disc 2, an erroneous operation such as erroneous increase of the servo gain does not occur. Thus, a stable gain control can be realised and the playback performance can be improved. The embodiment described above is a mere example of the invention. The invention may be adapted to any one of various kinds of optical reading apparatuses such as laser disc apparatus, magneto-optical disc apparatus and others. Further, the servo gain control method may be any one of various kinds of methods not limited to the described embodiment.

Claims: An optical disc apparatus (1) comprising: an optical pickup (3) for reading recorded information on an optical disc (2); servo control means (13) for performing tracking servo control and/or focus servo control of said optical pickup (3) in response to an output signal from said optical pickup (3); vibration detector means (6) for detecting a vibration occurring of the body of said apparatus; and control means (13) for performing up and down control to increase or decrease a servo gain of the tracking servo and/or the focus servo of said optical pickup (3), wherein said control means operates in response to an output signal from said vibration detector means; characterised in that : said vibration detector means including a vibration sensor (6) for detecting said vibration and for converting it into an electrical signal and including a level detector circuit (7) for converting an output signal from said vibration sensor (6) into a digitised signal and for outputting the digitised signal; wherein said level detector circuit (7) is operative to supply a first detection signal to said servo control means (13) when said output signal from said vibration sensor (6) is above a predetermined reference level, and to supply a second detection signal to said servo control means (13) when said output signal from said vibration sensor (6) is below a predetermined reference level. The optical disc apparatus according to claim 1, wherein said control means (13) is operative to increase said servo gain by a predetermined amount when said vibration detector means (6) detects a vibration above a predetermined reference level, and to decrease said servo gain by a predetermined amount when said vibration detector means (6) detects a vibration below a predetermined reference level or detects no vibration. The optical disc apparatus according to claim 1 or 2 wherein said control means (13) is operative to increase or decrease said servo gain in stepwise fashion by a predetermined amount at predetermined time intervals. The optical disc apparatus according to claim 1, 2 or 3, wherein said servo control means (13) is operative to increase said servo gain by a predetermined amount upon receipt of said first detection signal and to decrease said servo gain by a predetermined amount upon receipt of said second detection signal. The optical disc apparatus according to any preceding claim, wherein said predetermined reference level can be varied in level for deciding the presence or absence of said vibration. The optical disc apparatus according to any one of the preceding claims, wherein said vibration sensor (6) is an ultrasonic vibrator. The optical disc apparatus according to any one of the preceding claims, wherein said vibration sensor (6) is attached to a bottom plate of said apparatus.

Inventors: SHINADA AKIRA, SHINADA, AKIRA
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B33, G11B7