Optical disc apparatus

Abstract: In an optical disc apparatus for recording/reproducing desired information, e.g. a "write once, read many times" optical disc system, it is desired to securely detect speed information and address data recorded in a pregroove, even when the position of incidence of a light beam (L1) with respect to a light receiving element (24) has been changed. To this end, output signals (SA+SD, SB+SC) of light receiving surfaces of an optical pickup (6) adjacent to each other in the received radial direction of an optical disc (2) have their relative signal levels corrected so that the signal levels of RF components mixed into these output signals are equalized to each other. A difference signal thereof is then generated to recover the speed information and address data.

Description: This invention relates to optical disc apparatus, and more particularly, but not exclusively, to optical disc apparatus of the write once read many times type. It is known to provide so-called write once, read many times optical discs so that desired information may be recorded and read. In at least some write once, read many times optical disc devices, a light beam is caused to illuminate an organic pigment type thin film to form pits whereby desired information may be recorded on a once only basis. By forming pits in this manner, reproduction by an ordinary compact disc player of this type of optical disc is possible. Accordingly, this type of optical disc may be conveniently used in cases where only a small quantity of discs capable of being reproduced by a compact disc player need be produced without having to prepare a stamper for each disc. In this type of optical disc, a pregroove is formed at the pit forming position and time information (ATIP: absolute time in pregroove) is recorded in the pregroove. Thereby, in an optical disc device manipulating such a disc, desired information may be recorded/reproduced on the basis of such time information. In addition, in this type of optical disc, a carrier signal at a frequency of 22.05 kHz is frequency-modulated by address data and the pregroove is wobbled (moved radially) by the resulting frequency-modulated signal. In the optical disc device, the reflected light from the optical disc is received at a light receiving element of which the light receiving surface is divided into two portions in the radial direction of the optical disc. By detecting the wobbling of the pregroove based on such received light, a spindle motor may be rotated to hold the reproduced carrier signal at the frequency of 22.05 kHz and thereby rotate the optical disc at a predetermined rotational speed. Furthermore, the optical disc device is adapted to detect the time information by demodulating the output signal of the light receiving element to yield address data. In the output signal of the light receiving element, time information consisting of address data and speed information for driving the spindle motor may be securely detected before the forming of the pits which carry the recorded write once read many times information. However, after forming the pits, leakage of a signal component of the recorded information (hereinafter referred to as the RF component) into the output signal cannot be avoided due to the fact that the incident light upon the light receiving element is modulated by the pits. In addition, the frequency band of the RF component and that of speed and time information partially overlap as shown in Figure 4 of the accompanying drawings. Conventionally, in the optical disc device, a difference signal is generated from the output signals of the light receiving surfaces of the light receiving element, the light receiving surface of which is divided into the two portions to detect meandering of the groove. When the signal levels of the RF components are equal to each other in the two output signals, it is possible for them to cancel each other's RF component in the difference signal. In this case, speed information and time information may be securely detected. However, the output signal levels of the light receiving surfaces may be different from one another in the optical disc device. In this case, the RF component cannot be completely removed due to the fact that the signal level of the RF component is different between the output signals, and therefore speed information and time information cannot be detected. One possibility for solving this problem, would be that in which a variable resistance, for example, is adjusted to adjust the two output signal levels. However, the incident position of the light beam with respect to the light receiving element in an optical disc device may change due to temperature change and/or aged deterioration. Accordingly, in this case, though speed information and time information may be securely detected directly after such adjustment, it will eventually become impossible to detect speed information and time information. Further, when the signal level of the RF component is changed between the output signals due to tracking errors during reproduction, removing of the RF component is likewise impossible. US Patent No. US-A-4 942 565 discloses an optical disc apparatus of the type using wobbling of the track to store control information for the operation of the apparatus, as reflected by the preamble of claim 1. European Patent Application Publication No. EP-A-0 347 858 (which corresponds to US-A-5 185 732) discloses an optical disc device in which track width is used to store information. EP-A-0 430 541 discloses a system for removing offsets in a tracking error signal. The present invention provides an optical disc apparatus for detecting meandering of a pregroove of an optical disc, said pregroove meandering on the basis of control information for use in recording/reproducing a desired information signal on/from said optical disc, said control information comprising address data and speed data for controlling a disc motor, said optical disc apparatus comprising: (i) a light source for emitting a light beam; (ii) an optical system for concentrating said light beam onto an information recording surface of said optical disc and for receiving reflected light of said light beam; (iii) a light receiving element having a light receiving surface divided into at least first and second light receiving surfaces in a radial direction of said optical disc, said light receiving element receiving said reflected light at said optical system and outputting first and second output signals respectively from said first and second light receiving surfaces; (iv) difference signal generation means for generating a difference signal from between said first and second output signals; and (v) detection means for detecting said address data and said speed data from said difference signal; characterised in that said difference signal generation means is responsive to said first and second output signals for correcting relative signal levels of said first and second output signals so as to substantially equalize signal levels of said information signal mixed into said first and second output signals prior to the generation of said difference signal. According to a preferred embodiment of the invention described hereinbelow, for each of the output signals of light receiving surfaces adjacent to each other in the radial direction of the optical disc, the signal level of the RF component mixed into each output signal is detected. The relative signal levels of the output signals are then corrected so that the signal levels of RF components therein are equalized to each other and then a difference signal thereof is obtained. Thus, mixing of RF components into the wobble signal may be reduced, and even when the position of incidence of the light beam with respect to the light receiving element has been changed, speed information and address data recorded in the pregroove may still be securely detected. The preferred embodiment thus provides an optical disc apparatus in which speed information and time information recorded in the pregroove may be securely detected even in such cases where the incident position of the light beam with respect to the light receiving element changes. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram showing an optical disc apparatus; Figure 2 is a block diagram showing a wobble signal detection circuit; Figure 3 is a diagram of signal waveform explanatory of the operation of the apparatus and circuit of Figures 1 and 2; and Figure 4 shows curves indicating spectral characteristics of recorded/reproduced signals. Figure 1 shows an optical disc apparatus 1 which records desired information on a write once, read many times optical disc 2. The optical disc apparatus 1 drives a spindle motor 4 to rotate the optical disc 2 at a predetermined rotational speed and, in this state, drives an optical pickup 6 to record/reproduce the desired information. In the optical pickup 6, a laser diode 10 is driven on the basis of a driving signal output from a driver and a light beam L1 emitted from the laser diode 10 is converted into a collimated beam at a lens 12. The light beam L1 is then guided to an objective lens 16 through a beam splitter 14 and it is concentrated on the optical disc 2 by the objective lens 16. Thus, the light beam L1 of the laser diode 10 is concentrated onto the optical disc 2 to record/reproduce desired information. Meanwhile, a portion of the light beam L1 is reflected at the beam splitter 14 to be incident upon an output light receiving element (not shown). Thus, the optical disc device 1 may control the quantity of the light beam L1 on the basis of the output signal of the output light receiving element. Reflected light L2 from the optical disc 2 is received by the optical disc pickup 6 at the objective lens 16 and is directed to the beam splitter 14 to be reflected. The reflected light L2 then passes to a cylindrical lens 22 which concentrates the light onto a light receiving element 24. The light receiving element 24 has a light receiving surface A to D that is divided into two portions in the radial direction of the optical disc 2 as received through the optical system of the optical pickup 6. The light receiving surface A to D is further divided into two in the direction perpendicular to this direction. Output signals SA to SD respectively of the light receiving surfaces A to D are output to a matrix circuit 28 through amplifiers 26A to 26D. The matrix circuit 28 generates a focus error signal FE by obtaining addition signals SA+SC and SB+SD between diagonally arranged light receiving surfaces and by obtaining a difference signal between the addition signals SA+SC and SB+SD with a subtracter 30. The focus error signal FE is delivered to a phase compensation circuit 31 to drive a focus actuator 32 so that the signal level of the focus error signal FE is brought to a "0" level, thereby effecting focus control. The matrix circuit 28 also obtains addition signals SA+SD and SB+SC of the output signals SA to SD with respect to the light receiving surfaces A, D and B, C which are adjacent to each other in the radial direction of the optical disc 2. The addition signals SA+SD and SB+SC are delivered to a subtracter 33. The subtracter 33 then generates a tracking error signal TE by obtaining a difference signal between the addition signals SA+SD and SB+SC and delivers the tracking error signal TE to a phase compensation circuit 34. A tracking actuator 36 is driven so as to bring the signal level of the tracking error signal TE to a "0" level, thereby effecting tracking control. The optical disc device 1 also delivers the addition signals SA+SD and SB+SC to a wobble signal detection circuit 40, where a difference signal between the addition signals SA+SD and SB+SC is generated by suppressing the RF signal components to generate a wobble signal SW of which the signal level varies correspondingly to wobbling of the pregroove. A band-pass filter circuit (BPF) 42 detects the carrier signal component at the frequency of 22.05 kHz from the wobble signal SW and delivers the detected result to a spindle servo circuit 44. Thus, the spindle motor 4 is driven so as to maintain the frequency of the carrier signal component at a frequency of 22.05 kHz, and thereby keep the rotational speed of the optical disc 2 at a certain value. Meanwhile, an address decoder 46 detects frequency-modulated address data by demodulating the wobble signal SW and delivers the detected result to a central processing unit (CPU) 48. Thus, desired information may be recorded/reproduced on the basis of time information represented by such address data. The wobble signal detection circuit 40 is shown in more detail in Figure 2. The signal level of the addition signal SA+SD is corrected so that the RF components (consisting in this embodiment of an addition signal SA+SB+SC+SD obtained by adding the output signals of the light receiving surfaces A to D) mixed into the two addition signals SA+SD and SB+SC are equalized to each other. A difference signal is then generated between the addition signals SA+SD and SB+SC so that the RF components are not mixed into the wobble signal SW. As shown in Figure 3, at the wobble signal detection circuit 40, addition signals SA+SD and SB+SC (Figure 3, (A) and (B)) are input through coupling capacitors 50 and 52 and, of these, the addition signal SA+SD is output to a variable amplifier 54. The addition signals SA+SD and SB+SC represent the quantity of the reflected light beam L1; this varies within a predetermined range according to pits and meandering of the groove in the radial direction of the optical disc 2. For this reason, the signal levels vary periodically at a high frequency corresponding to the pits in a manner biased by a predetermined level and the overall signal level fluctuates corresponding to the meandering of the groove. Thus, the input signals S1 and S2 which have passed through the coupling capacitors 50 and 52 vary about a "0" level (Figure 3, (C) and (D)). The input signal S1 of these is amplified by a predetermined gain at the variable amplifier 54. The wobble signal detection circuit 40 delivers the output signal S1 of the variable amplifier 54 to a clamping circuit 58 through a coupling capacitor 56 and delivers the input signal S2 to a clamping circuit 60. The output signals of the clamping circuits 58 and 60 passes to peak hold circuits 62 and 64 to thereby detect envelope signals S3 and S4 (Figure 3, (E) and (F)) of the input signals S1 and S2. By clamping the input signals S1 and S2 at the clamping circuits 58 and 60 and then detecting the envelope signals S3 and S4 at the peak hold circuits 62 and 64, the wobble signal detection circuit 40 is adapted to detect the envelope signals S3 and S4 by effectively using the dynamic range of the peak hold circuits 62 and 64. The wobble signal detection circuit 40 then generates a difference signal S5 between the envelope signals S3 and S4 at the subtracter 66 and feeds this difference signal S5 back to the variable amplifier 54 via an amplifier 68. Thus, at the variable amplifier 54, gain is adjusted/controlled so that the signal levels of RF components varying according to the pits are equalized to each other between the input signals S1 and S2, thereby correcting the signal level of the input signal S1. The wobble signal detection circuit 40 obtains a difference signal SW (Figure 3, (G)) between the output signal S1 of the variable amplifier 54 and the input signal S2 at a subtracter 70. By outputting the difference signal SW through the band-pass filter circuit 42, a wobble signal SW is generated. In this manner, after correcting the signal level of the input signal S1 so that the signal levels of RF components varying according to pits are equalized to each other between the input signals S1 and S2, the difference signal SW is generated. Thereby, even when the position of incidence of the reflected light beam L2 with respect to the light receiving element 24 has been changed, for example due to aging, it is possible to reduce erroneous mixing of the RF component into the wobble signal SW. Accordingly, the speed information may be securely detected at the spindle servo circuit 44, the spindle motor may be securely driven, and address data may be securely detected at the address decoder 46. It should be noted that, while, in the above described embodiment, a description has been given with respect to a case where peak hold is effected after clamping the input signals S1 and S2 at the clamping circuits 58, 60, the present invention is not limited to this and the clamping circuit may be omitted as required. Further, while, in the above described embodiment, a description has been given with respect to a case where the signal level is corrected at the variable amplifier, the present invention is not limited to this and the signal level may be corrected using a multiplier. Further it is also possible to correct the signal level with respect to both the input signals S1 and S2. As has been described, an optical disc device may be obtained, in which: for the output signals of the light receiving surfaces adjacent to each other in the radial direction of the optical disc, the signal level of the output signal is corrected so that the signal levels are equalized of RF components mixed into the output signals; a difference signal between these output signals is then generated so that mixing of RF components into the wobble signal may be reduced; thereby, for example, even when the position of incidence of the light beam with respect to the light receiving element has been changed, speed information and time information recorded in the pregroove may be securely detected. In summary, Figure 1 illustrates apparatus 1 in which time information or speed information is detected by detecting meandering of a pregroove with respect to an optical disc 2 on which the pregroove is caused to meander based on time information or speed information, so as to record/reproduce a desired information signal on/from the optical disc 2 on the basis of the detected result SW, comprising: a light source 10 for emitting a light beam L1; optical systems 12, 14, 16, 22 for concentrating the light beam L1 onto the information recording surface and for receiving reflected light L2 of the light beam L1; a light receiving element 24 having a light receiving surface divided in the radial direction of the optical disc 2 into first and second light receiving surfaces A, D and B, C receiving the reflected light L2 received at the optical systems 12, 14, 16, 22 and outputting first and second output signals SA+SD and SB+SC respectively from the first and second light receiving surfaces A,D and B,C; a difference signal generation means 40 for detecting the difference signal SW between the first and second output signals SA+SD and SB+SC; and an information detection means 46 for detecting the speed information or the time information from the difference signal SW; and wherein the difference signal generation means 40 generates the difference signal SW after correcting the signal level of the first and/or second output signal SA+SD and/or SB+SC such that the signal levels of an information signal mixed into the first and second output signals SA+SD and SB+SC are substantially equalized to each other. Further, the difference signal generation means 40 detects envelopes S3 and S4 of first and second output signals SA+SD and SB+SC and corrects the signal level of the first and/or second output signal SA+SD and/or SB+SC on the basis of the envelopes S3 and S4. Thereby, the signal level of the first and/or second output signal SA+SD and/or SB+SC is corrected such that the signal levels of the information signal mixed into the first and second output signals SA+SD and SB+SC are substantially equalized to each other. Furthermore, in the present apparatus, the difference signal generation means 40 clamps the first and second output signals SA+SD and SB+SC at clamping circuits 58, 60 and then peak-holds them at peak-hold circuits 62, 64 to detect the envelopes S3 and S4 of the first and second output signals SA+SD and SB+SC. The difference signal SW is generated after correcting the signal level of the first and/or second output signal SA+SD and/or SB+SC such that the signal levels of the information signal mixed into the first and second output signals SA+SD and SB+SC are substantially equalized to each other. Thereby, the amount of the information signal mixed into the difference signal SW may be reduced even where the incident position of the light beam with respect to the light receiving element 24 has been changed. The envelopes S3 and S4 of the first and second output signals SA+SD and SB+SC are detected and the signal level of the first and/or second output signal SA+SD and/or SB+SC is corrected on the basis of the envelopes S3 and S4. Thereby, the signal level of the first and/or second output signal SA+SD and/or SB+SC may be corrected such that the signal levels of the information signal mixed into the first and second output signals SA+SD and SB+SC are substantially equalized to each other. The first and second output signals SA+SD and SB+SC are clamped at the clamping circuits 58, 60, and are then peak-held at the peak-hold circuits 62, 64. Thus, the envelopes S3 and S4 may be detected of the first and second output signals SA+SD and SB+SC.

Claims: An optical disc apparatus for detecting meandering of a pregroove of an optical disc (2), said pregroove meandering on the basis of control information for use in recording/reproducing a desired information signal on/from said optical disc, said control information comprising address data and speed data for controlling a disc motor (4), said optical disc apparatus comprising: (i) a light source (10) for emitting a light beam (L1); (ii) an optical system (6) for concentrating said light beam onto an information recording surface of said optical disc and for receiving reflected light of said light beam; (iii) a light receiving element having a light receiving surface (24) divided into at least first and second light receiving surfaces (AD, BC) in a radial direction of said optical disc, said light receiving element receiving said reflected light at said optical system and outputting first and second output signals (SA+SD, SB+SC) respectively from said first and second light receiving surfaces; (iv) difference signal generation means (40) for generating a difference signal (SW) from between said first and second output signals; and (v) detection means (44, 46) for detecting said address data and said speed data from said difference signal; characterised in that said difference signal generation means (40) is responsive to said first and second output signals for correcting relative signal levels of said first and second output signals so as to substantially equalize signal levels of said information signal mixed into said first and second output signals prior to the generation of said difference signal (SW). Apparatus according to claim 1, wherein said difference signal generation means (40) is operative to detect envelopes of said first and second output signals and to correct relative signal levels of said first and second output signals on the basis of said envelopes. Apparatus according to claim 2, wherein said difference signal generation means (40) is operative to clamp said first and second output signals at clamping circuits (58, 60) and then to peak hold said first and second output signals at peak hold circuits (62, 64) to detect the envelopes of said first and second output signals.

Inventors: IIMURA SHINICHIRO, IIMURA, SHINICHIRO, Iimura, Shinichiro, c/o Intellectual Property Div.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: G11B7
IPC codes: G11B7, G11B27