Magneto-optical disk reproduction apparatus

Abstract: An apparatus for reproducing a magneto-optical disk (1) which is adapted to intermittently irradiate a magneto-­ optical disk with a light beam emitted from a laser light source (5) at a predetermined sampling period, detect a polarized component of the light beam emitted from the laser light source (5) and reflected from the disk (1) by detecting elements (9,8,10,11,12,13A,13B), derive the reproduced output of an information signal recorded on the magneto-optical disk by taking the difference between the detected outputs from the respective detecting elements, and in which there is provided a random noise removing circuit (21A, 21B)at the rear stage of the detecting elements for removing photo-electric converting noises generated in the detecting elements.

Description: The present invention relates to an apparatus for reproducing a magneto-optical disk having a magneto-optical medium which permits information signals to be optically rewritten. Such an apparatus is described in an article published by M. Hartmann et al. under the title "Erasable magneto-optical recording" published in Philips Technical Review, Vol. 42, No. 2, August 1985, pages 37 to 46. It comprises the following features: a laser light source for emitting a light beam to said magneto-optical disk at a predetermined sampling period; a first and a second detector for detecting each one of two differently polarized components of said light beam emitted from said laser light source and reflected from said magneto-optical disk; and a differential amplifier means for obtaining the difference between the output signals of said two detectors. The differential detector used in this apparatus provides for a good C/N (carrier to noise) ratio. However, the prior art apparatus does not reduce any noise component. The document JP-A-618753 describes an apparatus for reproducing a magneto-optical disc, which apparatus comprises a feedback circuit far controlling the gain of one of two differently polarized components of a light beam emitted from a laser light source and reflected from a magneto-optical disk. This feedback circuit comprises two low pass filters receiving detector signals associated to said two differently polarized components, a differential amplifier means for obtaining the difference between said two detector signals, and an integrating means for integrating the output signal of said differential amplifier means. In case that the two detector signals comprise noise signals, although the difference between these signals will contain noise. This noise can not be eliminated by the integrating means, but it can only be averaged. It is the object of the present invention to provide an apparatus for reproducing a magneto-optical disk, which apparatus has an improved C/N ratio due to a reduction of random noise signals. The apparatus of the present invention comprises the above listed features of the prior art apparatus, and it is characterized by a first and a second integrating means (21A, 21B; 32A, 32B) having low pass characteristics concerning the frequency of the information signals but having integrating characteristics concerning the frequency of certain noise signals. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, throughout which like reference numerals designate like elements and parts. In the drawings: Fig. 1 is a schematic circuit diagram showing a first embodiment of the present invention; Fig. 2 is a schematic diagram showing a second embodiment of the present invention; and Figs. 3A to 3F are waveform charts used to explain operations of the second embodiment. In Fig. 1, output signals S A and S B from photo-diodes 13A and 13B, corresponding to the two detectors of the prior art apparatus, are supplied to integrating circuits 21A and 21B, each formed of an operational amplifier, through current-to-voltage converting circuits 20A and 20B, respectively. The integrating circuits 21A and 21B have their constituting elements selected so as to provide a low pass characteristic such that data reproduced from the disk 1 can pass therethough. For example, if a maximal frequency of data is 7.5 MHz, the cut-off frequency thereof is chosen to be 15 MHz. The output signals S A ' and S B ' from the integrating cirucits 21A and 21B are supplied to the differential amplifier 15 at its input terminals. The rest of the first embodiment is constructed in the same manner as the prior art apparatus. In the first embodiment, when the disk 1 is intermittently irradiated with the laser light at the sampling period, the photo-diodes 13A and 13B output the output signals S A and S B , both include random shot noise. The output signals S A ' and S B ' from the integrating circuits 21A and 21B include similar DC noises converted from the shot noise. Thus, the differential amplifier 15 delivers at the output terminal 16 the signal So without the DC noise. In the first embodiment of the present invention as described above, the shot noise is converted to the DC noise by the integrating circuits 21A and 21B and removed by the differential amplifier 15, so that deterioration of the C/N ratio due to the shot noise is suppressed to thereby largely improve the C/N ratio. Although the shot noise is proportional to the square root of the incident light amount Io, the present embodiment enables the C/N ratio to be improved in proportion to the incident light amount Io by removing the shot noise. Next, a second embodiment of the present invention will be explained with reference to Fig. 2 in which the parts corresponding to those in Fig. 1 are designated with the same reference numerals and the detailed explanation thereof will be omitted. As is illustrated in Fig. 2, the anodes of the respective photo-diodes 13A and 13B are grounded through connecting switches 31A and 31B, respectively. A connecting point P A of the photo-diode 13A and the connecting switch 31A is grounded through a capacitor 32A. A signal S A '' derived at the connecting point P A is supplied through a buffer amplifier 33A to the differential amplifier 15 at one of its input terminals. Further, a connecting point P B of the photo-diode 13B and the connecting switch 31B is grounded through a capacitor 32B. A signal S B '' derived at the connecting point P B is supplied through a buffer amplifier 33B to the differential amplifier 15 at the other input terminal. The rest of the second embodiment is constructed similarly to the prior art apparatus. In the second embodiment, when the disk 1 is intermittently irradiated with the laser light at the sampling period, the photo-diodes 13A and 13B output the output signals S A and S B as shown in Figs. 3A and 3B, respectively. These output signals S A and S B each include the random shot noise N SH . The connecting switches 31A and 31B are on-off controlled at the sampling period with the timing as shown in Fig. 3C. When the connecting switches 31A and 31B are opened, the respective capacitors 32A and 32B are in a charging condition. On the contrary, when the connecting switches 31A and 31B are closed, the respective capacitors 32A and 32B are in a discharging condition. Therefore, the signals S A '' and S B '' derived at the connecting points P A and P B are as shown in Figs. 3D and 3E, respectively. Hatched portions in Figs. 3D and 3E indicate the DC noise converted from the shot noise N SH which are respectively included similarly in the signals S A '' and S B ''. Thus, the differential amplifier 15 derives to the output terminal 16 the signal So free of the DC noise, as shown in Fig. 3F. As described above, since the second embodiment is adapted to convert the shot noise to the DC noise and remove the converted DC noise in the same manner as the first embodiment shown in Fig. 1, the same effects as the first embodiment can be produced. Further, although the above-mentioned embodiments have been described for the specific case of removing the shot noise N SH , the present invention can be similarly adapted to remove other random noise. According to the present invention as described above, since the integrating means are provided at the rear stage of the respective detectors to average the random noise such as shot noise (convert it to DC noise) or the like, the random noise can be easily removed, so that the C/N ratio can be largely improved.

Claims: An apparatus for reproducing a magneto-optical disk (1) having a magneto-optical medium (3) which permits information signals to be optically rewritten, said apparatus comprising: a laser light source (5) for emitting a light beam to said magneto-optical disk at a predetermined sampling period; a first and a second detector (13A, 13B) for detecting each one of two differently polarized components of said light beam emitted from said laser light source and reflected from said magneto-optical disk; and a differential amplifier means (15) for obtaining the difference between the output signals from said detectors; characterized by a first and a second integrating means (21A, 21B; 32A, 32B) having low pass characteristics concerning the frequency of the information signals but having integrating characteristics concerning the frequency of certain noise signals, each one allocated to a respective one of said first and second detectors (13A, 13B), for integrating the output signal of the respective detector and for supplying the integrated signals to said differential amplifier means. An apparatus according to claim 1, characterized in that at least one of said integrating means is a parallel connection of a switch (31A; 31B), switched with said sampling period, and a capacitor (32A; 32B), this parallel circuit being connected in series with said detector (13A; 13B) between a voltage source (+B) and ground.

Inventors: HORIMAI HIDEYOSHI, OGAWA HIROSHI, HORIMAI, HIDEYOSHI, OGAWA, HIROSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1988
Main classification: G11B11
IPC codes: G11B11