RECORDING/REPRODUCING APPARATUS USING MAGNETO-OPTICAL DISK

Abstract: A recording/reproducing apparatus comprising a digital modulator circuit (18) which receives digital signals that are to be recorded, means (17) for controlling a laser beam that is guided from a laser source (10) to an optical disk (1) depend­ ing upon the output of said digital modulator circuit (18), first optical means (9,8,7,6,4) which guide the laser beam from said laser source (10) to said optical disc (1), second optical means (4,6,11,14,15) which guide the laser beam returning from said optical disk to an optical detector (16), an actuator (5) which is served with a servo error signal obtained from said optical detector (16) and which adusts the laser beam for said optical disk, a low-pass filter (19) which is served with the output of said digital modulator circuit, and removing means (20,21) that remove low-pass component output signal that has passed through said low-pass filter (19) from said servo error signal. In recording the data, low-pass components are extracted from the digitally modulated output to adjust the gain and, thereby to cancel the low-pass components con­ tained in the servo-error signal. Namely, digitally modulated low-pass components are removed from both the tracking and focussing servo-error signals, making it possible to stably apply a servo signal. In reproducing the data, the power of laser beam is smaller than in recording the data, and the low-­ pass components in the recorded signals are not so intense as to adversely affect the servo signal. Therefore, the modulation system has a mild gain making it possible to highly densely record the data.

Description: This inventions relates to a magneto-optical disc recording and reproducing apparatus. Especially it deals with problems arising when tracking a laser beam along a track on the disc. EP-A-78 673 describes a magneto-optical disc recording and reproducing apparatus comprising: a digital modulating circuit supplied with a digital signal to be recorded; means for controlling a laser beam directed from a laser light source to a magneto-optical disc in response to an output signal of said digital modulating circuit; first optical means for directing the laser beam emitted from said laser light source to said magneto-optical disc; second optical means for directing the laser beam reflected from said magneto-optical disc to a photodetector; and an actuator supplied with a servo error signal obtained from said photodetector, for adjusting said laser beam relative to said magneto-optical disc. One of the necessary conditions for good performance of such a system is, that the energy of the servo frequency band in the low frequency range is small within the spectrum of the digital modulation signal. If the energy of the servo frequency band in the low frequency range is high this affects signals in a servo loop as an external disturbance, obstructing actuate tracking properties of the servo mechanism. This condition is also taken into consideration for a disc such as a compact dics that is used only for play-back. It is the object of the present invention to provide a magneto-optical disc recording and reproducing apparatus with stable tracking properties. The magneto-optical disc recording and reproducing apparatus comprises the components as listed above for the known apparatus, and additionally comprises: a low pass filter supplied with the output signal of said digital modulating circuit; and a removing means for removing from said servo error signal a low frequency component output signal derived from said low pass filter. The magneto-optical disc recording and reproducing apparatus according to the present invention is constructed such that upon recording, the output signal of the digital modulation circuit is supplied to a low pass filter which extracts the low frequency components within the modulated signal, and the low frequency components are removed from a servo error by subtracting the output signal of the low pass filter from the servo error signal. Upon recording, a digital signal is supplied to the digital modulation circuit as raw data, in which circuit it is modulated according to a predetermined modulation system. The modulated signal is supplied to the low pass filter and thereby the low frequency component contained in the modulated signal is extracted. In the preferred embodiment, the gain of the extracted low frequency component is adjusted by a gain adjusting circuit when subtracted from the servo error signal. As a result, there is obtained only a true servo error signal without low frequency components. Thus, the servo is effected by the true servo error signal. The single figure shows a block diagram of a magneto-optical disc recording and reproducing apparatus comprising a low pass filter and removing means for removing low frequency components from a servo error signal. One embodiment of this invention, applied to an optical magnetic recording and reproducing apparatus, by way of example, will hereinafter be described in detail with reference to the attached drawing. Reference numeral 1 designates a disc, 2 a winding for generating a recording and erasing auxiliary magnetic field, 3 a lens support member, 4 an objective lens attached to one side of the lens support member in opposing relation to the disc 1, 5 a focusing/tracking actuator, 6 a semitransparent mirror, 7 a grating, 8 a beam correcting prism, 9 a collimator lens, and 10 a laser diode. The beam from the laser diode 10 is collimated as a parallel beam by the collimator lens 9, corrected for beam-shape by the beam correction prism 8 and projected onto the disc 1 through the grating 7, the semitransparent mirror 6 and the objective lens 4. Another semitransparent mirror 11 is provided for the mirror 6. For the mirror 11, there are provided an analyzer 12 and a photo detector 13 to detect an inherent signal component. while, for the mirror 11, there are provided a spacing lens 14 for correcting a magnification, a cylindrical lens 15 for convergent and a photo detector 16 to thereby detect a tracking error signal. There is provided a laser driving circuit 17 which is used to drive the laser diode 10 upon recording. At the input side of the laser driving circuit 17, provided is the digital modulating circuit 18. The digital modulating circuit 18 is supplied with a digital signal not yet modulated as a raw data, in which the raw data is modulated on the basis of a predetermined modulation system so as to become a channel modulation code. This channel modulation code is converted to a drive current by the laser driving circuit 17 and then supplied to the laser diode 10. The laser diode 10 is turned on when the channel modulation code is "H" and turned off when it is "L". The on and off modulated laser light emitted from the laser diode 10 travels through the above mentioned optical path and is converged on the face of the disc 1 by the objective lens 4 and thereby recorded in an optical magnetic recording fashion. Accordingly, a light source of the light inputted to the photo detector 16 which detects the tracking error signal is the laser light modulated as mentioned above so that the tracking error signal contains the large low frequency component dependent on the modulation system employed by the digital modulation circuit 18. At the output side of the digital modulation circuit 18, there is provided the low pass filter 19 whose pass band is equivalent to the servo band. This low pass filter extracts the low frequency component contained in the modulated signal. The low frequency component extracted is adjusted in gain by the gain adjusting circuit 20, inverted in polarity and then supplied to one input terminal of an adder 21. To the other input terminal of this adder 21, there is supplied the tracking error signal from the photo detector 16. This tracking error signal contains the low frequency component of the modulated signal but if it is supplied to the adder 21 and then added with the low frequency component derived from the gain adjusting circuit 20, the low frequency component of the modulated signal contained therein is removed. Thus, only the true tracking error signal is generated at the output side of the adder 21. In other words, of the digital modulation components, the low frequency component affecting the servo is removed and the tracking error signal becomes the inherent and accurate tracking error signal. This tracking error signal is supplied through a servo circuit 22 to a tracking winding which forms the actuator 5 whereby the objective lens is moved in the radial direction of the disc, so that the accurate tracking is carried out. While in the above the tracking servo system is described for simplicity of explanation, from the focusing error signal derived from the photo detector, there is removed the low frequency component contained in the modulated signal derived from the digital modulation circuit through the low pass filter and the gain adjusting circuit, to thereby produce the focusing error signal which is supplied through the servo circuit to the focusing winding of the actuator 5 so that the accurate focusing can be carried out. In the reproducing mode, the laser diode 12 is driven by a constant current smaller than that in the recording mode and the circuit section of the digital modulation circuit 18 is not used. Hence, the modulated component is not contained in the laser light anymore. In addition, in the case of optical magnetic recording, the so-called KERR effect is not accompanied with change of the light amount so that the inherent signal component does not appear on the photo detector 16 for tracking servo, and thus the influence of the modulated component can be neglected. As described above, according to this embodiment, in either case of the recording and reproducing modes, the servo system of the optical magnetic recording and reproducing apparatus is free from the influence exerted by the low frequency component contained in the digital modulation. Accordingly, a modulation system having much low frequency components can be used, the selection range of the modulation system can be widened and the recording density can be increased. While in the above mentioned embodiment the adder 21 is used to add the tracking error signal containing the low frequency component of the modulated signal from the photo detector 16 with the low frequency component with the polarity inverted from the gain adjusting circuit 20 to thereby remove the low frequency component of the modulated signal, it is alternatively possible that instead of the adder 21, there is used a subtracter to subtract the low frequency component (however, the polarity thereof is not inverted) derived from the gain adjusting circuit 16 from the tracking error signal of the photo detector 16. Further, alternatively, the tracking error signal from the photo detector 16 can be divided by the low frequency component (however, the polarity thereof is not inverted) from the gain adjusting circuit 20 by using a divider instead of the adder 21. While the above mentioned embodiment is a case of the optical magnetic recording, this invention can also be applied similarly to the disc on which the digital data can be recorded one time.

Claims: A magneto-optical disc recording and reproducing apparatus comprising: a digital modulating circuit (18) supplied with a digital signal to be recorded; means (17) for controlling a laser beam directed from a laser light source (10) to the magneto-optical disc (1) in response to an output signal of said digital modulating circuit; first optical means (4, 6, 7, 8, 9) for directing the laser beam emitted from said laser light source (10) to said magneto-optical disc (1); second optical means (4, 6, 11, 14, 15) for directing the laser beam reflected from said magneto-optical disc (1) to a photodetector (16); and an actuator (5) supplied with a servo error signal obtained from said photodetector (16), for adjusting said laser beam relative to said magneto-optical disc (1); characterized by a low pass filter (19) supplied with the output signal of said digital modulating circuit (18); and a removing means (20, 21) for removing from said servo error signal a low frequency component output signal derived from said low pass filter (19). A magneto-optical disc recording and reproducing apparatus according to claim 1, characterized in that said removing means includes a gain adjusting circuit (20) which adjusts a gain of said low frequency component output signal derived from said low pass filter (19). A magneto-optical disc recording and reproducing apparatus according to claim 2, characterized in that said gain adjusting circuit (20) inverts the polarity of said low frequency component output signal and supplies said inverted signal to an adder (21) included in said removing means, where the inverted signal and the signal from said photo detector (16) are added.

Inventors: HENMI, FUMIAKI, YAMAGAMI, TAMOTSU
Applicants: SONY CORPORATION
Application year: 1985
Main classification: G11B7
IPC codes: G11B7, G02B7, G11B11