On/off-track signal in optical information recording and/or reproducing apparatus

Abstract: An optical information recording and/or reproducing apparatus includes an optical head for recording and/or reproducing information by applying a light beam to an optical recording medium having a plurality of tracks, an actuator for relatively moving the light beam in a direction which intersects the tracks, an optical detector for detecting, by intersecting the tracks, a reflected or transmitted light of the light beam, which has been modulated, a peak holding circuit (2, D1, C1, R1) for holding a peak value of a signal (1) detected by the optical detector, a bottom holding circuit (3, D2, C2, R2) for holding a bottom value of the detected signal, a circuit (4, 5, R3, R4) for obtaining an intermediate value of the held peak value and bottom value, and a comparison circuit (6) for transmitting an on-track/off-track signal by making a comparison between the intermediate value and the detected signal (1).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus for recording and/or reproducing information on an optical recording medium by irradiation of a light beam, and, more particularly to an optical information recording and/or reproducing apparatus provided with a means for obtaining an on-track/off-track signal (a track count signal) from reflected light or transmitted light of the light beam. Description of the Prior Art Fig. 1 is a schematic view which illustrates an example of the structure of a conventional optical information recording/reproducing apparatus. Referring to this drawing, reference numeral 10 represents an optical disk having a surface in which spiral or concentric tracks are formed. This optical disk 10 can be rotated by a spindle motor 11 so that information can be recorded or reproduced as a result of the application of the light beam from an optical head 21. The optical head 21 includes a semiconductor laser 12 which serves as the light source. The light beam emitted from this semiconductor laser 12 is converged on the optical disk 10 after it has passed through a collimator lens 13, a beam splitter 14, and an objective lens 15. The beam reflected from the optical disk 10 is again passed through the objective lens 15, and is separated from the incident beam by the beam splitter 14. Then, the thus-separated beam is detected by a light detector 18 after it has passed through a sensor lens 17. The light receiving surface of this light detector 18 is so divided into a plurality of regions as to have a focusing signal and a tracking signal detected in an autotracking (AT) and autofocusing (AF) control circuit 19 by a known method such as the astigmatism method, the push-pull method, or the like on the basis of the output from each of the above-described divided regions. The method of detecting the control signal of the type described above has been disclosed in, for example, U.S. Patent No. 4,410, 969. The thus-detected control signal is fed back to a lens actuator 16 so that the autotracking and the autofocusing are enabled by moving the objective lens 15 in the direction of the optical axis of the light beam and in the direction which is perpendicular to the optical axis and also which intersects the track. In order to access the light beam to a desired track in the apparatus of the type described above, a structure can be employed which is arranged such that the overall body of an optical head 21 is moved by a linear motor 22 in the radial direction of the disk, that is, in the direction which intersects the track. Alternatively, another structure can be employed which is arranged such that the objective lens 15 is moved in the direction which intersects the track by inputting a track jump signal to the lens actuator 16. In order to detect the position or the moving direction of the light beam, an on-track/off-track signal is used in the apparatus of this type. The on-track/off-track signal can be obtained by processing, by an on-track/off-track detection circuit 20, the total signal transmitted from all of the regions in the light detector 18. The utilization of such an on-track/off-track signal has been disclosed in, for example, in U.S. Patent No. 4,106,058. Fig. 2 is a schematic view which illustrates an example of the conventional on-track/off-track detection circuit. An output (a total signal) 1 from the light detector 18 having the waveform as designated by a symbol S shown in Fig. 3 is subjected to a comparison made, by a comparator 6, with a predetermined reference value C which is approximated to the average of the output 1 and transmitted from a constant-voltage source 8. As a result, an on-track/off-track signal 7 as shown in Fig. 3B is transmitted. However, in the above-described conventional devices, the total signal from the light detector 18 involves, as designated by the symbol S shown in Fig. 3A, a certain offset with respect to a ground level G and becomes a signal modulated by the track when the light beam intersects the track with the focus servo actuated. Furthermore, the offset and the degree of modulation can be changed corresponding to the change in the reflectance of the disk or the change in the laser power. Therefore, the conventional devices encounter a problem in that the on-track/off-track signal which serves as the output cannot be obtained with satisfactory accuracy, be maintained or, in the worst case, the determination of the on-track and off-track cannot be performed if the change in the offset and in the modulation degree is too large since the comparison voltage is arranged to be a constant level. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical information recording and/or reproducing apparatus capable of overcoming the above-described conventional problems and in which an accurate on-track/off-track signal can always be obtained even if the reflectance of the medium or the laser power is changed. The above-described object can be achieved by an optical information recording and/or reproducing apparatus according to claim 1. Its preamble reflects said prior art. BRIEF DESCRIPTlON OF THE DRAWINGS Fig. 1 is a schematic view which illustrates an example of the structure of a conventional optical information recording and reproducing apparatus: Fig. 2 is a circuit diagram which illustrates an example of a conventional on-track/off track detection circuit; Figs. 3A and 3B are views which illustrate the waveform of a signal in the circuit shown in Fig. 2; Fig. 4 is a circuit diagram which illustrates an embodiment of the on-track/off-track detection circuit according to the present invention; Figs. 5A and 5B are views which illustrate the waveform of the signal in the circuit shown in Fig. 4; Figs. 6A and 6B are views which illustrate the waveform of the circuit shown in Fig. 4 when the medium is damaged; Fig. 7 is a circuit diagram which illustrates another embodiment of the on-track/off-track detection circuit according to the present invention; and Figs. 8A and 8B are views which illustrate the waveform of the signal in the circuit shown in Fig. 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 4 is a view which illustrates an embodiment of an on-track/off-track detection circuit for use in an apparatus according to the present invention. The apparatus according to the invention is structured similarly to the apparatus shown in Fig. 1 except for this circuit. Referring to Fig. 4, reference numeral 1 represents a total signal from a light detector; reference numerals 2 and 4 each represent a differential amplifier which form a peak holding circuit together with a diode D1, a capacitor C1 and a resistor R1; reference numerals 3 and 5 each represent a differential amplifier which form a bottom holding circuit together with a diode D2, a capacitor C2, and a resistor R2; reference characters R3 and R4 represent resistors for processing the intermediate value between the peak held value and the bottom held value; reference numeral 6 represents a comparator for obtaining a binary on-track/off-track signal 17 upon a comparison made between the intermediate value and the total signal 1 from the light detector, this intermediate value being between the peak held value and the bottom held value. The total signal 1 from the light detector is input to the differential amplifier 2, and is subjected to a peak holding process performed by diode D1, capacitor C1, and resistor R1. Then, it is transmitted through the buffer performed by the differential amplifier 4. Similarly, the total signal 1 from the light detector input to the differential amplifier 3 is subjected to a bottom holding process performed by diode D2, capacitor C2, and resistor R2. Then, it is transmitted through the buffer performed by the differential amplifier 5. On the basis of the outputs from these differential amplifiers 4 and 5, the intermediate value is calculated by the resistors R3 and R4. The thus-calculated value becomes the intermediate level of the total signal (a modulation signal by the track) from the light detector even if the offset and the degree of modulation of the optical sensor are changed. Fig. 5A illustrates this state, where symbol S represents the waveform of the total signal, PH represents the peak holding signal, BH represents the bottom holding signal, M represents the calculated intermediate value, and G represents the ground level. Therefore, the on-track/off-track signal 17 can be, as shown in Fig. 5B, always transmitted from the comparator 6 by way of performing the comparison made between the total signal 1 and the intermediate value M. As described above, the apparatus according to the invention is protected from the influence of the change in the reflectance of the medium or change in the laser power. However, the medium can be damaged when it is used. The damage of the medium generates portions with a high reflectance or low reflectance. Figs. 6A and 6B are views which illustrate the waveform of the circuit shown in Fig. 4 in this state. Referring to Fig. 6A, symbol S represents the waveform of the total signal, PH represents the peak holding signal, BH represents the bottom holding signal, M represents the calculated intermediate value, G represents the ground level, and N represents a portion with a high reflectance. The peak value at a portion N thereof as shown in Fig. 6A can be held intact. As a result, in the apparatus as shown in Fig. 4, it becomes the signal 37 is generated as shown in Fig. 6B. This leads to an inability of determining whether the light beam is on-track or off-track during the holding time constant period. Another embodiment of the on-track/off-track detection circuit in which the above-described problem is corrected is shown in Fig. 7. Referring to Fig. 7, wherein like reference numerals have been used for like elements as in the embodiment of Fig. 4, reference numeral 1 represents a total signal from the light detector; reference numerals 2 and 4 each represent a differential amplifier which form a peak holding circuit together with a diode D1, a capacitor C1 and resistors R1 and R5; reference numerals 3 and 5 each represent a differential amplifier which form a bottom holding circuit together with a diode D2, a capacitor C2, and resistors R2 and R6; reference characters R3 and R4 represent resistors for processing the intermediate value between the peak held value and the bottom held value; reference numeral 6 represents a comparator for obtaining a binary on-track/off-track signal 27 upon a comparison made between the intermediate value of the peak held value and the bottom held value and the total signal 1 from the light detector, and reference numeral 8 represents a constant-voltage source for transmitting voltage which becomes the converged value of the holding circuit. The total signal 1 from the light detector is input to the differential amplifiers 2 and 3, and is subjected to peak holding and bottom holding processes. At this time, even if a signal of respectively high level in terms of pulse characteristics is input to the total signal 1 from the light detector, it cannot be immediately responded to by resistor R5, while even if a signal of respectively low level in terms of pulse characteristics is input to the same, it cannot also be immediately responded by resistor R6, causing the rising time to be delayed. As a result, the peak and bottom of the pulse cannot be held. This state is shown in Fig. 8A, where symbol N represents a portion with a high reflectance in terms of pulse characteristics, S represents the waveform of the total signal, PH represents the peak holding signal, BH represents the bottom holding signal, M represents the calculated intermediate value, and G represents the ground level. The voltage whose peak has been held is converged to a converged voltage output from the constant-voltage source 8 by a time constant determined by capacitor C1 and resistor R1, while the voltage whose bottom has been held is similarly processed by a time constant determined by capacitor C2 and resistor R2. The voltage whose peak and bottom have been held then passes through the buffers served by the differential amplifiers 4 and 5. Then, the intermediate voltage is obtained by resistors R3 and R4, and the thus-obtained intermediate value is, by the comparator 6, compared to the total signal 1 from the optical sensor. As a result, it becomes the binary on-track/off-track signal 27 is generated as shown in Fig. 8B. According to this embodiment, the on-track state and the off-track state can be correctly determined even if the tracking servo-loop is closed by way of setting the converged peak held/bottom held voltage which has been transmitted from the constant-voltage source 8 to the average value (intermediate value determined on consideration of the dispersion of the peak value and the bottom value of the modulation signal due to the track). The invention can be applied to a variety of applications in addition to the above-described embodiments. For example, although the reflected light from the medium is detected according to the embodiments, a structure can be formed in a case of a light transmissible type medium such that the transmitted light is arranged to be detected. In addition, any shape of the medium may be employed such as a card-like shape and a tape-like shape. The invention includes all of the applications within the scope of the claims.

Claims: An optical information recording and/or reproducing apparatus comprising: an optical system (21) for irradiating an optical recording medium (10) with a light beam, the recording medium (10) being provided with a plurality of tracks; means (22) for moving the light beam relative to the medium (10) in a direction which intersects the tracks; a light sensor means (18) having a plurality of light receiving portions for detecting reflected or transmitted light from said light beam which has been modulated by intersecting said tracks; and means for obtaining an on-track/off-track signal (17) of said reflected or transmitted light beam by comparing the output of said light sensor means (18) with a reference value (C), said optical information recording and/or reproducing apparatus being characterised in further comprising: a generating means (6) of sum signals (17) where a unique peak value and a unique bottom value of the detected light signal (1) corresponding to intersections of said tracks are generated by adding the output of the plurality of the light receiving portions of said light sensor means (18); means (2,D1,C1,R1) for holding a peak value (PH) of said sum signals; means (3,D2,C2,R2) for holding a bottom value (BH) of said sum signals; and means (4,5,R3,R4) for calculating an intermediate value (M) from said held peak (PH) and bottom (BH) values, said calculated intermediate value (M) being used as said reference value (C). An optical information recording and/or reproducing apparatus according to claim 1, further comprising means (R6) for delaying a rising response of said peak value holding means (2,D1,C1,R1) and said bottom value holding means (3,D2,C2,R2). An optical information recording and/or reproducing apparatus according to claim 1, wherein a converged value held by said peak value holding means (2,D1,C1,R1) and said bottom value holding means (3,D2,C2,R2) is arranged to be an average value of the sum signal (17).

Inventors: ANDO HIROTAKE, ANDO, HIROTAKE
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B7
IPC codes: G11B7