System for obtaining optimum focusing position in optical disc system

Abstract: A system for obtaining an optimum focusing position in an optical disc system, wherein an optical beam is impinged onto a track of an optical disc (1), and a reflection signal having information of an intensity of a optical beam reflected by an optical disc (1), is obtained. A predetermined frequency component (DRF1), which component is generated in a reflecting signal (SVa, SVb, SVc, SVd) by a modulation caused by an existence of a pit on a track of the optical disc (1), is extracted from the reflection signal (SVa, SVb, SVc, SVd), and then a focus position wherein a maximum intensity of the above extracted component is obtained, is searched by changing the focus position of the optical beam. Further, the above system is incorporated in a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, to determine an optimum offset value (VF) which is to be adjusted to compensate an error arisen from a construction of the focus servo control system, during an offset adjusting operation.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a system for obtaining an optimum focusing position in an optical disc system, and further, relates to a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, wherein the system for obtaining an optimum focusing position by an offset adjustment is incorporated. Optical discs are used as a large capacity external memories. The optical discs have a narrow track (a few micrometers in width) which is usually preformed spirally on its own surface. Data is written on the track surface by impinging a high-intensity optical (light) beam onto the track surface to form a pit, and is read by impinging a low-intensity optical beam onto the track surface, and then detecting variations of the intensity of an optical beam reflected at the track surface, caused by a pit. To carry out an effective and accurate writing and reading operation, the optical beam impinged onto the track surface is required to be focused in an optimum condition on the track surface. (2) Description of the Related Art Figure 1 shows an outline of the construction of an optical disc apparatus. In Fig. 1, reference numeral 1 denotes an optical disc, 11a denotes a rotation mechanism such as a motor, 11b denotes a rotating axle, 12 denotes a optical head, 13 denotes a track servo control portion, and 14 denotes a focus servo control portion, 20 denotes a semi-conductor laser device, 21a denotes a lens, 22 denotes a polarized beam splitter, 23 denotes a quarter wavelength plate, 24a denotes a mirror, 25 denotes an objective lens, 26 denotes a track actuator, 27 denotes a focus actuator, 21b denotes a lens, and 28 denotes a light sensing device. The optical disc 1 rotates driven by the motor 11a through the rotating axle 11b. The optical head is moved in the radial direction by an actuating device (not shown) driven by a motor (not shown), and is then positioned to write or read data on an objective track of the optical disc 1. In the optical head 12, a light emitted from the semi-conductor device 20 is lead and focused through the lens 21a, beam splitter 22, the quarter wavelength plate 23, the mirror 24, and the objective lens 25, and is then impinged on to the track surface of the optical disc 1. A light reflected by the track surface is lead through the objective lens 25, the mirror 24, the quarter wavelength plate 23, the beam splitter 22, and the lens 21b, to the light sensing device 28, and thus, is detected at the light sensing device 28. Responding to the above detected signal, the light sensing device 28 generates an electric reflection signal, the amplitude of which corresponds to the intensity of the reflected and detected signal. Generally, tracks are formed spirally or concentrically on optical discs at the pitch of a few micrometers, and the extension of a focused beam spot area on the track surface is less than one micrometer. However, some eccentricity of an amount comparable with the above pitch or more may exist in the arrangement of the above tracks on optical discs. In addition, some waviness, which causes an off-focus at the track surface of optical discs, may exist in optical discs. In spite of the above situation, the optical beam is required to be focused on the track surface, and the focused beam spot must follow the above narrow track. To fulfill the above requirement, the focus actuator (focus coil) 27, the focus servo control portion 14, the track actuator (track coil) 26, and the track servo control portion 13 are provided. The focus actuator 27 moves the objective lens 25 in the optical head 12 in the direction perpendicular to the surface of the optical disc 1 to adjust the focus position of the impinging optical beam under the control of the focus servo control portion 14, and the focus servo control portion 14 receives the output of the light sensing device 28, which consists of a plurality of receiving light signals, and generates a focus error signal (FES) to drive the focus actuator 27 in a feedback mode so that the optimum focus condition at the track surface of the optical disc 1 is maintained. The track actuator 26 moves the objective lens 25 in the optical head 12 in the radial direction of the optical disc 1 to adjust the position of the impinging optical beam in the radial direction under the control of the track servo control portion 13, and the track servo control portion 13 receives the output of the light sensing device 28, which consists of a plurality of receiving light signals, and generates a track error signal (TES) to drive the track actuator 26 in a feedback mode so that the position of the impinging optical beam is maintained at the center of the track width of the optical disc 1. The principle of the focus servo control is explained with reference to Figures 2 to 4. In Fig. 2, "f" denotes an on-focus condition wherein a focus position of an impinging light beam is just on the track surface of the optical disc 1, "f1" denotes an off-focus condition wherein a focus position of an impinging light beam is under the track surface of the optical disc 1, and "f2" denotes an off-focus condition wherein a focus position of an impinging light beam is above the track surface of the optical disc 1. Figures 3A, 3B, and 3C show distributions of intensity of a received light on a light input surface of the light sensing device 28. Fig. 3A shows the distribution in the above off-focus condition "f1", Fig. 3B shows the distribution in the above on-focus condition "f", and Fig. 3C shows the distribution in the above off-focus condition "f2". The light sensing device 28 consists of four light sensing detectors, each corresponds to one of four quadrants, and each of which is denoted by "a", "b", "c", and "d", respectively. The aforementioned focus error signal (FES) is defined as FES = (a+b)-(c+d), where "FES" denotes an intensity of the focus error signal (FES), "a" denotes an intensity of the light received by the light sensing detector "a", "b" denotes an intensity of the light received by the light sensing detector "b", "c" denotes an intensity of the light received by the light sensing detector "c", and "d" denotes an intensity of the light received by the light sensing detector "d", respectively, and the focus error signal (FES) is obtained in the focus servo control portion 14. Figure 4 shows a variation of the amplitude of the above focus error signal (FES) as a function of the focus position regarding the track surface. Since the value of the focus error signal (FES) in the above on-focus condition "f" is zero, the focus servo control portion 14 controls the focus actuator 27 according to the above obtained value of the focus error signal (FES) so as to maintain the FES value near zero, and thereby the focusing position is maintained on the track surface in the resolution of a sub-micron order, even when waviness exists in the optical disc 1. The principle of the track servo control is explained with reference to Figures 5 to 7. In Fig. 5, "P" denotes a condition wherein a spot area of an impinging light beam is at the center of the track width of the optical disc 1, "P1" denotes an off-track condition wherein a spot area of an impinging light beam is in on one side of the center of the track width of the optical disc 1, and "P2" denotes an off-track condition wherein a spot area of an impinging light beam is on the other side of the center of the track width of the optical disc 1. Figures 6A, 6B, and 6C show distributions of intensity of a received light on a light input surface of the light sensing device 28. Fig. 6A shows the distribution in the above off-track condition "P1", Fig. 6B shows the distribution in the above condition "P", and Fig. 6C shows the distribution in the above off-track condition "P2". The aforementioned track error signal (TES) is defined as TES = (a+d)-(c+b), where "TES" denotes an intensity of the track error signal (TES), and the track error signal (TES) is obtained in the track servo control portion 13. Figure 7 shows a variation of the amplitude of the above track error signal (TES) as a function of the position of the spot area of the impinging light beam with respect to the center of the track width. Since the value of the track error signal (TES) in the above on-track condition "P" is zero, the track servo control portion 13 controls the track actuator 26 according to the above obtained value of the track error signal (TES) so as to maintain the TES value near zero, and thereby the position of the spot area of the impinging light beam is maintained at the center of the track width in the optical disc 1, even when an eccentricity exists in the optical disc 1. In an actual focus servo control system, however, the situation wherein the above value of the focus error signal (FES) obtained from the intensity of light detected at the light sensing device 28 is zero, does not necessarily correspond to the true on-focus condition wherein the focusing position is actually on the track surface of the optical disc 1, due to an off-center positioning of the light sensing device 28, or a level offset which arises in the internal circuit realizing the focus servo control system (portion 14). Therefore, it is necessary to adjust an offset value in an appropriate stage in the focus servo control portion 14. To adjust the above offset value, a plurality of systems are proposed in the prior art. U. S. Patent No. 4,707, 648 described the technique wherein the offset value which yields the maximum amplitude of the track error signal (TES) is obtained as the optimum offset value. The measurement of the amplitude of the track error signal (TES) is carried out by rotating the optical disc without a track servo operation. A high frequency track error signal (TES) is required to be used in the measurement for obtaining a precise amplitude of the track error signal (TES). The high frequency of the track error signal (TES) means a large amount of eccentricity of the tracks on the optical disc because the variation of the level of the track error signal (TES) is caused when the impinging light beam traverses grooves, which are concaved portions located between adjacent tracks as shown in Fig. 5, and the phase difference of the light wave reflected at the groove from the phase of the light wave reflected at the track surface causes an interference between the light waves and decreases the intensity of the total reflected light. However, recently, it is known that the optimum offset level obtained from the amplitude of the track error signal (TES) including the track error signal (TES) during the traverse of the grooves, does not necessarily give the actual optimum on-focus condition which enables the most precise operation of writing or reading data, i. e., the most precise operations of forming a pit on the track surface or detecting a pit on the track surface because the above traversal of the grooves shifts the optimum focusing position from the above actual optimum on-focus condition at the pits. In another technique in the prior art, the offset value which yields the maximum direct current level of the intensity of the total reflected light from the surface of the optical disc, i. e., the output of the light sensing device 28, is obtained as the optimum offset value. However, the optical system for impinging a light beam onto the track surface of the optical disc includes an astigmatic error due to a distortion of the optical components by temperature change or aging. Concretely, the astigmatism causes an elongated spot shape of the impinging light beam on the track surface of the optical disc. The above direct current level of the intensity of the total reflected light from the surface of the optical disc, becomes a maximum when an elongated beam spot lies within the track surface area as shown in Figure 8. Since data is written in the form of a series of pits, and is read whether or not a pit exists at positions having predetermined (angular) intervals with each other, on a track, the beam spot elongated in the direction of the track may cause a serious error in writing or reading operations. For an example of an optimum focusing servo system with optimum focussing position searching means using the output of means filtering the reflection signal reference can be made to AU-B-530869. An example of a prior art focus servo control system with offset error compensation corresponding to the preamble of claim 1 can be found in EP-A-0 225 258. SUMMARY OF THE INVENTION An object of the present invention is to provide a system for obtaining an optimum focusing position to write or read information in accordance with the presence or absence of a pit on a track surface of an optical disc without being affected by astigmatism in an optical system. Another object of the present invention is to provide a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, and having an offset adjustment system to compensate an error arising from an electric or geometric construction of the focus servo control system, wherein an optimum offset value for maintaining an optimum focusing position to write or read information in the form of whether or not a pit exists on a track surface of an optical disc, can be obtained without being affected by astigmatism in an optical system. According to the present invention, there is provided a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, comprising : an optical beam impinging means for impinging an optical beam on a track of an optical disc; a reflection signal obtaining means for obtaining a reflection signal having information of an intensity of an optical beam reflected by an optical disc; a focus position changing means for changing said focusing position of an optical system; means for obtaining a focus error signal from the reflected beam; a servo control means for controlling said focus position changing means according to said reflection signal, said servo control means being responsive to said focus error signal; and an offset adjusting means for adjusting an offset which is used to compensate an error arising from a construction of said focus servo control system; characterized in that it further comprises a filtering means for extracting a predetermined frequency component from said reflection signal, which component is generated in a reflecting signal by a modulation caused by a pit located on a track of said optical disc; and an optimum offset value obtaining means for obtaining an optimum value of said offset, said optimum offset value obtaining means incorporating means for detecting the maximum intensity of the output of said filtering means during a scanning of the focus position, said maximum intensity determining the position of maximum focus, said offset value being determined on the basis of said maximum focus position. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 shows an outline of the construction of an optical disc apparatus; Figure 2 shows on-focus and off-focus conditions of an impinging light beam on the track surface of the optical disc; Figures 3A, 3B, and 3C show distributions of intensity of a received light on a light input surface of the light sensing device; Figure 4 shows a variation of the amplitude of the focus error signal (FES) as a function of the focus position regarding the track surface; Figure 5 shows on-track and off-track conditions of an impinging light beam on the track surface of the optical disc; Figures 6A, 6B, and 6C show distributions of intensity of a received light on a light input surface of the light sensing device; Figure 7 shows a variation of the amplitude of the track error signal (TES) as a function of the position of the spot area of the impinging light beam with respect to the center of the track width; Figure 8 shows an elongated beam spot lying within the track surface area; Figure 9 shows an outline of the construction of an servo control system in an optical disc system as an embodiment of the present invention; Figure 10 shows the construction of the focus servo control portion 4; Figure 11 shows the construction of the track servo control portion 3; Figures 12A and 12B show the construction of the RF signal amplitude detecting portion 5; Figure 13 shows an example of the waveform of the RF (sum) signal RF1; Figure 14 shows the content of the ID code, which is in accordance with the ISO standard; Figure 15 shows a flow of the operation by the MPU 6 in Fig. 9 to obtain the optimum focus offset value; and Figure 16 shows the timing and the signal levels regarding the operation in the embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiment of the present invention, first, the basic principle of the present invention is explained below. As described in the summary of the invention, in a system for obtaining an optimum focusing position in an optical disc system, according to the first aspect of the present invention, a predetermined frequency component of a reflection signal, which component is generated by a modulation caused by an existence of a pit on a track of the optical disc in a reflecting signal, is extracted by a filtering means, and then a focus position wherein a maximum intensity of the output of the filtering means is obtained, is searched by changing the focus position of the optical beam using the focus position changing means. Thus, according to the first aspect of the present invention, the optimum focusing position at which the optical system is most sensitive regarding a pit on a track, can be obtained regardless of the astigmatism in the optical system. In addition, when the above system further comprises a track servo control means for maintaining a position of the surface of the optical disc where the optical beam is impinging onto, in an optimum position on a track, and the track servo control means is operated during the operation of the optimum focusing position searching means, the operation for obtaining the optimum focus position can be carried out in an on-track condition through the operation. Therefore, the operation is not adversely affected by the off-track condition, and thus, a better result of the optimum focus position can be expected. Further, in a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, according to the second aspect of the present invention, the above principle of the first aspect of the present invention is applied to determine an optimum offset value which is used to compensate an error arising from a construction of the focus servo control system, i. e., an optimum value of the offset which gives a maximum intensity of the output of the filtering means is obtained by scanning the offset, during an offset adjusting operation. As described in the summary of the invention, the focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operation, according to the second aspect of the present invention, comprises: a focus position actuating means, a focus position driving means, a reflection signal obtaining means, a servo control means, a filtering means, an offset adjusting means for adjusting an offset which is used to compensate an error arising from a construction of the focus servo control system, and an optimum offset value obtaining means. The focus servo control for maintaining an optimum focusing position, is basically carried out by the optical beam impinging means, the reflection signal obtaining means, the focus position changing means, and the servo control means. Namely, an optical beam is impinged on a track of an optical disc by the optical beam impinging means; and then a reflection signal having information of an intensity of an optical beam reflected by an optical disc is obtained by the reflection signal obtaining means. The servo control means controls the focus position changing means to change the focusing position of an optical system (comprising the above optical beam impinging means and the reflection signal obtaining means) according to the reflection signal. The offset adjusting operation is carried out by the filtering means, the offset adjusting means, and the optimum offset value obtaining means, together with the above optical beam impinging means, the reflection signal obtaining means, the focus position changing means, and the servo control means. The offset adjusting means is provided for adjusting an offset which is used to compensate an error arising from a construction of the focus servo control system. The above-mentioned basic principle of the first aspect of the present invention is applied to obtain an optimum value of the above offset. Namely, a predetermined frequency component of a reflection signal, which component is generated by a modulation caused by an existence of a pit on a track of the optical disc in a reflecting signal, is extracted by a filtering means, and then an optimum offset value whereby a maximum intensity of the output of the filtering means is obtained, is searched by changing the focus position of the optical beam using the focus position changing means, by the optimum offset value obtaining means during an offset adjusting operation. Thus, according to the second aspect of the present invention, the optimum offset value at which the optical system is most sensitive regarding a pit on a track, can be obtained regardless of the astigmatism in the optical system. In addition, when the above system further comprises a track servo control means for maintaining a position of the surface of the optical disc where the optical beam is impinging onto, in an optimum position on a track, and the track servo control means is operated during the operation of the optimum offset value obtaining means, the operation for obtaining the optimum focus offset can be carried out in an on-track condition through the operation. Therefore, the operation is not adversely affected by the off-track condition, and thus, a better result of the optimum focus offset can be expected. Hereinafter, the preferred embodiment of the present invention is explained with reference to the drawings. Figure 9 shows an outline of the construction of a servo control system in an optical disc system as an embodiment of the present invention. In Fig. 9, reference numeral 3 denotes a track servo control portion, 4 denotes a focus servo control portion, 5 denotes an RF signal amplitude detecting portion, 6 denotes an MPU (microcomputer unit) as a control portion, and 2 denotes the optical head. Each of the track servo control portion 3, the focus servo control portion 4, and the RF signal amplitude detecting portion 5 receives four outputs (reflection signal) SVa, SVb, SVc, and SVd of the aforementioned four light sensing detectors "a", "b", "c", and "d". The control portion 6 receives a track zero cross signal TZC and an off-track signal TOS from the track servo control portion 3, focus zero cross signal FZC and an off-focus signal FOS from the focus servo control portion 4, and a digital RF amplitude signal SEDRF and a sector mark detection signal from the RF signal amplitude detecting portion 5, and outputs a track servo-ON signal TSV to the track servo control portion 3, a focus servo-ON signal FSV and a focus offset value VF to the focus servo control portion 4 to control both the track servo control portion 3 and the focus servo control portion 4. Further, the control portion 6 controls the motor to move the optical head 2. A memory 6a holds the focus offset value VF, and a work area (memory) 6b holds amplitudes of RF signals for a plurality of sectors for each offset value VF. The control portion 6 calculates the optimum offset value using the data held in the work area 6b. Figure 10 shows the construction of the focus servo control portion 4. In Fig. 10, reference numeral 40 denotes an FES generation circuit, 42 denotes an AGC circuit, 43a denotes a zero cross detection circuit, 43b denotes an off-focus detection circuit, 44 denotes a phase compensating circuit, 45 denotes a servo switch, 46 denotes a focus offset adding circuit, and 47 denotes a power amplifier. The FES generation circuit 40 receives the above-mentioned four outputs (reflection signal) SVa, SVb, SVc, and SVd, and generates a focus error signal (FES) in accordance with the equation, FES = (SVa+SVc)-(SVb+SVd). The FES generation circuit 40 shown in Fig. 10, comprises: an addition amplifier 400 which outputs a sum of the above SVa and SVc; an addition amplifier 401 which outputs a sum of the above SVb and SVd; and a subtraction amplifier 402 which outputs the above focus error signal (FES) = (SVa+SVc)-(SVb+SVd). The AGC circuit 42 obtains an AGC output by dividing the value of the above focus error signal (FES) by the value of a total reflection signal (DSC). The total reflection signal (DSC) is defined as the equation, DSC = (SVa+SVb)+(SVc+SVd), and is generated in a total signal generation circuit 31, which is shown in Fig. 11. The total reflection signal DSC indicates a total reflection level of the reflected light beam. Thus, corrections for variations of the intensity of the impinging light beam and the reflection rate are made for the focus error signal (FES). The AGC circuit 42 shown in Fig. 10, comprises: a first amplifier 420 which amplifies the above focus error signal (FES); a first FET (field effect transistor) 421 which controls the input of the first amplifier 420 by dividing the voltage of the input according to the output level of the first amplifier 420 and a second amplifier 422; the second amplifier 422 which receives the total reflection signal (DSC) and controls the first FET 421 according to the total reflection signal (DSC); and a second FET 423 which controls the input of the second amplifier 422 by dividing the voltage of the input of the second amplifier 422 according to the output level of the second amplifier 422 to compensate a non-linearity of the output of the first amplifier 420 and give a linear characteristic to the output. Accordingly, the gain of the first amplifier 420 is controlled by the total reflection signal (DSC) through the output of the second amplifier 422, and an AGC-controlled focus error signal (FES) is obtained as the output of the first amplifier. The zero cross detection circuit 43a outputs a zero cross signal TZC to the MPU 6 when a zero cross point of the focus error signal (FES) level is detected. The zero cross detection circuit 43a shown in Fig. 10, comprises a comparator 430 which compares the level of the output of the above AGC circuit 42 with the zero level, and outputs the above zero cross signal when a zero cross point of the focus error signal (FES) level is detected. The off-focus detection circuit 43b outputs an off-focus signal FOS to the MPU 6 when the level of the focus error signal (FES) exceeds a predetermined upper limit value +V₀ or a predetermined lower limit value -V₀. The off-focus detection circuit 43b shown in Fig. 10, comprises: a first comparator 431 which compares the level of the output of the above AGC circuit 42 with the above level +V₀, and outputs "H" level when FES > +V₀; and a second comparator 432 which compares the level of the output of the above AGC circuit 42 with the above level -V₀, and outputs "H" level when FES < -V₀. The logical OR of the outputs of the first and second comparators 431 and 432 is obtained as the above off-focus signal FOS. The phase compensation circuit 44 advances the phase of high frequency components of the focus error signal (FES) by adding a differential of the focus error signal (FES) and a component proportional to the focus error signal (FES). The servo switch 45 closes the focus servo loop when receiving an active servo-ON signal FSV from the MPU 6, and opens the focus servo loop when receiving a non-active servo-ON signal from the MPU 6. The focus offset adding circuit 46 receives the digital focus offset value VF supplied from the MPU 6 at the register 461, converts the digital value to analog in the digital to analog converter 462, and adds the analog focus offset value VF to the output of the above servo switch 45. The power amplifier (PA) 47 amplifies the output of the above focus offset adding circuit 46 to generate a focus actuator driving current FVD. The focus actuator driving current FVD is supplied to the focus actuator 27. Figure 11 shows the construction of the track servo control portion 3. In Fig. 11, reference numeral 30 denotes a TES generation circuit, 31 denotes a total signal generation circuit, 32 denotes an automatic gain control (AGC) circuit, 34a denotes a zero cross detection circuit, 34b denotes an off-track detection circuit, 37 denotes a phase compensating circuit, 38 denotes a servo switch, 39a denotes an inversion amplifier, and 39 denotes a power amplifier (PA). The TES generation circuit 30 receives the above-mentioned four outputs (reflection signal) SVa, SVb, SVc, and SVd, and generates a track error signal (TES) in accordance with the equation, TES = (SVa+SVb)-(SVc+SVd). The TES generation circuit 30 shown in Fig. 11, comprises: an addition amplifier 300 which outputs a sum of the above SVa and SVb; an addition amplifier 301 which outputs.a sum of the above SVc and SVd; and a subtraction amplifier 302 which outputs the above focus track error signal (TES)=(SVa+SVb)-(SVc+SVd). The total signal generation circuit 31 shown in Fig. 11, comprises an addition amplifier 310 which outputs a sum of the above SVa, SVb, SVc and SVd, and generates a total reflection signal (DSC) in accordance with the equation, DSC = (SVa+SVb)+(SVc+SVd). The total reflection signal DSC indicates a total reflection level of the reflected light beam. The AGC circuit 32 obtains an AGC output by dividing the value of the above track error signal (TES) by the value of the above total reflection signal (DSC). Thus, corrections for variations of the intensity of the impinging light beam and the reflection rate are made for the track error signal (TES). The zero cross detection circuit 34a outputs a zero cross signal TZC to the MPU 6 when a zero cross point of the track error signal (TES) level is detected. The off-track detection circuit 34b outputs an off-track signal TOS to the MPU 6 when the level of the track error signal (TES) exceeds a predetermined upper limit value +V₀ or a predetermined lower limit value -V₀. The phase compensation circuit 37 advances the phase of high frequency components of the track error signal (TES) by adding a differential of the track error signal (TES) and a component proportional to the track error signal (TES). The concrete constructions of the above AGC circuit 32, the zero cross detection circuit 34a, the off-track detection circuit 34b, and the phase compensation circuit 37 are each the same as the construction of the similar components shown in Fig. 10 for the focus servo control portion 4. The servo switch 38 closes the track servo loop when receiving an active servo-ON signal TSV from the MPU 6, and opens the track servo loop when receiving a non-active servo-ON signal from the MPU 6. The inversion amplifier 39a inverts the output of the servo switch 38. The power amplifier (PA) 39 amplifies the output of the inversion amplifier 39a to generate a track actuator driving current TVD. The track actuator driving current TVD is supplied to the track actuator 26. Figures 12A and 12B show the construction of the RF signal amplitude detecting portion 5. Fig. 12A shows a construction of an RF sum signal generation circuit. In the construction of Fig. 12A, the aforementioned four outputs (reflection signal) SVa, SVb, SVc, and SVd of the four light sensing detectors of the light sensing device 28, are each applied to one terminal of a corresponding one of capacitors 501a, 501b, 501c, and 501d, where the other terminals of each of the capacitors 501a, 501b, 501c, and 501d are connected to each other. Thus, a sum of the AC components of the above four inputs SVa, SVb, SVc, and SVd is obtained, and is applied to an amplifier 502. Thus, RF (sum) signals RF1 and RF2 (the inverted signal of RF1) are obtained as the outputs of the amplifier 502. An example of the waveform of the above RF (sum) signal RF1 is shown in Figure 13 for each of the cases when a (track) portion of an optical disc where data is written in each sector portion of the track is scanned during the offset adjusting operation, and when a (track) portion of an optical disc where no data is written in each sector portion of the track is scanned during the offset adjusting operation. As explained later, an ID code is preformed before each section of each track when each optical disc is manufactured before data is written. In the construction of Fig. 12B, reference numeral 51 denotes a differential circuit, 52 denotes an envelope generation circuit, 53 denotes a sector mark detection circuit, 54 denotes a shift register, and 55 denotes a sample hold circuit. The above-mentioned RF signals RF1 and RF2 are each input into a differential circuit 51. The differential circuit 51 comprises: a pair of differential circuits each of which comprises a capacitor (C1) and a resistor (R1), a pair of transistors TR1 and TR2, a pair of capacitors 515 and 516, and an amplifier 517. The characteristic of the differentiation of the input RF1, is determined by a capacitor (C1) 511 and a resistor (R1) 513, and the characteristic of the differentiation of the input RF2, is the same as the characteristic of the differentiation of the input RF1. The amplifier 517, together with transistors TRI and TR2 and the capacitors 515 and 516, constitutes a differential amplifier. Thus, the differentiated RF signals DRF1 and DRF2 are obtained as outputs of the amplifier 517. The characteristic of the above differentiation is such that a frequency component which is generated by a modulation caused by a pit located on a track of said optical disc, is extracted from the RF signals RF1 and RF2, and is determined based on the dimensions of the pit, the track, and the beam spot size, and the velocity of scanning of the beam on the track. The above differentiated RF signals DRF1 and DRF2 are input into the envelope generation circuit 52. The envelope generation circuit 52 comprises capacitors 521 and 525, resistors 522 and 524, and a diode 523, and constitutes a detector circuit. The envelope EDRF of the above differentiated RF signals DRF1 and DRF2 are obtained as the output of the envelopes generation circuit 52. An example of the waveform of the above envelope signal EDRF is shown in Figure 13 for each of the cases when a (track) portion of an optical disc where data is written in each sector portion of the track is scanned during the offset adjusting operation, and when a (track) portion of an optical disc where no data is written in each sector portion of the track is scanned during the offset adjusting operation. The sector mark detection circuit 53 in Fig. 12B detects a sector mark in the above-mentioned preformed ID code portion at the beginning of each sector portion of each track, and the construction of the sector mark detection circuit is well-known. The content of the ID code, which is in accordance with the ISO standard, is shown in Fig. 14. The ID code portion includes a sector mark, a VFO code, and an ID number. The VFO code is a synchronization bit consisting of a predetermined repeated pattern, provided for pulling-in a timing signal, and the VFO code is the same for all sectors and tracks. The VFO code, and the ID numbers are repeated three times in each ID code portion. As shown in Fig. 14, the sector mark detection circuit 53 outputs a mark pulse when the sector mark is detected. The mark pulse is delayed in the shift register 54, and the output of the shift register 54 is applied to the sample and hold circuit 55 as a sampling pulse. The timing of the sampling pulse is determined so that a value of an envelope of an RF signal corresponding to the above VFO is sampled at each sector for the purpose of the present invention, i. e., for determining the optimum focus offset value. An example of the timings of the mark pulse and the sampling pulse is shown in Fig. 14. In the sample and hold circuit 55, the envelope of the RF signal is sampled at the timing of the above sampling pulse. The output SEDRF of the sample and hold circuit 55 is shown in Fig. 13 for each of the cases when a (track) portion of an optical disc where data is written in each sector portion of the track is scanned during the offset adjusting operation, and when a (track) portion of an optical disc where no data is written in each sector portion of the track is scanned during the offset adjusting operation. Since the above sampling is carried out regarding the same code signal VFO, a stable sampling in each sector using a rather low speed (available at a low cost) analog to digital converter is possible, and further, a stable and simple operation for determining the optimum offset value is possible. The above sampled value SEDRF is converted into a digital form in an analog to digital converter (not shown), and is then sent to the MPU 6. Figure 15 shows a flow of the operation by the MPU 6 in Fig. 9 to obtain the optimum focus offset value. In the step 601, the MPU 6 makes the track servo system ON, i. e., the MPU 6 sends an active track servo-ON signal to the servo switch 38 to close the track servo loop. In the step 602, the MPU 6 makes the initial value of the focus offset zero. In the step 603, an falling edge of the index signal is detected. The index signal is output at one predetermined angle in each rotation of the optical disc, and the index signal is well-known. When the index signal is detected, the MPU 6 sets the focus offset value in the aforementioned register 461 shown in Fig. 10, and thus the focus offset value is added to the output of the focus servo control portion 4 of Fig. 10. In the step 604, a variable "A" is set as A=N, where N is defined by the equation, N = (the number of sectors in one track) - (the number of sectors necessary for kicking back) - (the number of sectors necessary for detecting a falling edge of the index signal). For example, when the number of sectors in one track is 17, the number of sectors necessary for kicking back is one, and the number of sectors necessary for detecting a falling edge of the index signal is two, N=14. In the step 605, it is determined whether or not the sampling pulse is "1". If the sampling pulse is "1" in the step 605, it is next determined whether or not the sampling pulse is "0" in the step 606. Thus, if it is determined that the sampling pulse is "0" in the step 606, this means that the falling edge of the sampling pulse is detected, and therefore, the MPU 6 reads the sampled value SEDRF of a digital amplitude of the RF signal RF1 from the output of the above-mentioned analog to digital converter in the step 607, and stores the value in the work area (memory) 6b shown in Fig. 9. Next, in the step 608, the above-mentioned variable A is decremented to A-1, and then, in the step 609, it is determined whether or not the variable A is equal to zero. If it is determined that the variable A is not equal to zero, the operation goes to the step 605 to continue the operation of collecting the sampled values. Or if it is determined that the variable A is equal to zero, the operation goes to the step 610 to go back to the starting position in the step 610 (a track back operation is necessary when the track is spirally formed on the recording medium), and then, in the step 611, the focus offset value is incremented to collect the sampled values for the incremented value of the focus offset on the same track. The above track back operation is necessary when the track is formed spirally on the optical disc. When the track is formed concentrically on the optical disc, the track back operation in the step 610 is not necessary. After the step 611, it is determined whether or not the focus offset value exceeds the predetermined maximum value in the step 612. If it is determined that the focus offset value does not exceed the predetermined maximum value, the operation goes to the step 603 to collect the sampling values of SEDRF for the incremented value of the focus offset. If it is determined that the focus offset value exceeds the predetermined maximum value, the operation goes to the step 613 to obtain the focus offset value which gives the maximum SEDRF value using the SEDRF data stored in the memory 6b. Then, in the step 614, the MPU 6 makes the track servo system OFF, i. e., the MPU 6 makes the track servo-ON signal inactive so that the servo switch 38 opens the track servo loop. The timing and the signal levels regarding the above operation are shown in Figure 16.

Claims: A focus servo control system with offset error compensation for maintaining an optimum focusing position in an optical disc system during reading and writing operations, comprising: an optical beam impinging means (20, 21a, 22, 24a, 25) for impinging an optical beam on a track of an optical disc; a reflection signal obtaining means (22, 24a, 25, 21b, 28) for obtaining a reflection signal (SVa, SVb, SVc, SVd) having information of an intensity of an optical beam reflected by an optical disc (1); a focus position changing means (27) for changing said focusing position of an optical system; means for obtaining a focus error signal from the reflected beam; a servo control means (4) for controlling said focus position changing means (27) according to said reflection signal, said servo control means being responsive to said focus error signal; and an offset adjusting means (46) for adjusting an offset (VF) which is used to compensate an error arising from a construction of said focus servo control system; characterized in that it further comprises a filtering means (51) for extracting a predetermined frequency component (DRF1) from said reflection signal (SVa, SVb, SVc, SVd), which component is generated in a reflecting signal by a modulation caused by a pit located on a track of said optical disc; and an optimum offset value obtaining means for obtaining an optimum value of said offset (VF), said optimum offset value obtaining means incorporating means for detecting the maximum intensity of the output of said filtering means during a scanning of the focus position, said maximum intensity determining the position of maximum focus, said offset value being determined on the basis of said maximum focus position. A system according to claim 1, wherein said filtering means comprises a differentiating means (51). A system according to claim 1, wherein said optimum offset value obtaining means comprises an enveloping means (52) for obtaining the output of said filtering means (51). A system according to claim 3, wherein said optimum offset value obtaining means further comprises a sampling means (53, 54, 55) for sampling the output of said enveloping means (52). A system according to claim 4, wherein said sampling means (53, 54, 55) carries out said sampling at a portion on a track where a common predetermined pattern (VFO) is written, for all offset values used in said scanning operation. A system according to claim 1, further comprising a track servo control means (3) for maintaining a position of the surface of said optical disc where said optical beam is impinging onto, in an optimum position on a track. A system according to claim 6, wherein said filtering means comprises a differentiating means (51). A system according to claim 6, wherein said optimum offset value obtaining means comprises an enveloping means (52) for obtaining the output of said filtering means (51). A system according to claim 8, wherein said optimum offset value obtaining means further comprises a sampling means (53, 54, 55) for sampling the output of said enveloping means (52). A system according to claim 9, wherein said sampling means (53, 54, 55) carries out said sampling at a portion on a track where a common predetermined pattern (VFO) is written, for all offset values used in said scanning operation.

Inventors: ARAI SHIGERU, IWAMOTO TOSHITAKA, MINAMI AKIRA, SAITOH HIDENORI, SASAKI MASATERU, YANAGI SHIGENORI, ARAI, SHIGERU, IWAMOTO, TOSHITAKA, MINAMI, AKIRA, SAITOH, HIDENORI, SASAKI, MASATERU, YANAGI, SHIGENORI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: G11B7
IPC codes: G02B7, G11B7