A detrack detecting system in an optical recording/reproducing apparatus

Abstract: A detrack detecting system for detecting deviation of the beam spot from the track in an optical recording/­ reproducing apparatus in which recording and reproducing of data are performed by irradiating optical beams onto an optical disc. Three beam spots are irradiated at positions on the optical disc which are shifted from each other by a predetermined distance laterally of the track. The reflected lights are respectively detected by three photodetectors whose photosensitive surfaces are divided into two parts and tracking error signals are detected by a push-pull system. A tracking error signal obtained from the centre beam spot of the three beam spots and tracking error signals obtained from other two beam spots are respectively added to obtain the first and second composite tracking error signals. Detracking state is detected based on the polarities of the first and second composite tracking error signals.

Description: The present invention relates to a detrack detecting system for detecting deviation of the beam spot from the track in an optical recording/reproducing apparatus in which recording and reproduction of data are performed by irradiating an optical beam onto an optical disc. There has been put into practice use an optical disc in which the recording medium of its recording surface is such that its reflectance changes in response to light, thereby enabling information to be recorded and reproduced. EP-A-0095766 is an example of an apparatus corresponding to the pre-characterising part of claim 1. In a writable optical disc, for instance, as shown in figure 1 of the accompanying drawings, a spiral track is divided into n sectors S C1 , S c2 , S c3 , ..., S cn and address areas Ad₁, AD₂, AD₃, ... in which track address data is recorded, are formed in the leading portion of each sector. A laser beam modulated by recording data is irradiated onto the data recording areas of the sectors S c1 , S c2 , S c3, ..., so that the temperature of that portion rapidly rises and recording dots are formed onto the disc surface. By irradiating the disc with a laser beam which is of lower intensity than that used for recording, the recorded data can be read out without destroying the recording dots. The laser beam which is irradiated onto the optical disc is controlled in a manner such that, in the recording and erasing modes, the laser power which is supplied is a few times larger than that in the reading mode. Therefore, when a strong vibration is applied to the erasing mode and the tracking servo is thereby made inoperative, a serious accident may result from the laser beam irradiating the wrong area of the disc, ie, data which has already been written may be destroyed or data may be written to the wrong track on the optical disc. Therefore, as shown in figure 2, there is considered a method whereby the amplitude of a tracking error signal which is obtained from the reflected light of the beam spot which was irradiated onto the optical disc is compared with reference levels Et₁ and Et₂. When the tracking error signal outside the reference levels is output, it is regarded that the optical recording/ reproducing apparatus is in a detracking state and this state is detected in the recording mode, the writing mode is cancelled at this time point so that erroneous recording or erroneous erasure of data is prevented. However, in the detrack detecting system as described above, since the level of tracking error signal changes depending on the kind of optical disc or the reflectance of the optical disc, there arises a problem that even if the tracking error signal level is compared with the reference levels, it is difficult to detect accurately that a detrack condition exists. It is an object of the present invention to provide a detrack detecting system in an optical recording/reproducing apparatus which can accurately detect the detracking state even in the case where the reflectances of the optical discs are different. According to the present invention there is provided a detrack detecting system in an optical recording/reproducing apparatus in which recording and reproducing of data are performed by irradiating beam spots onto an optical disc, comprising: means for irradiating the disc with a first beam spot which follows the centre of a data recording track; means for irradiating the disc with second and third beam spots at positions which are shifted laterally of the track from the first beam spot by a predetermined distance to the inner circumferential side and the outer circumferential side of the track respectively; and first, second and third photodetectors for detecting light reflected from the first, second and third beam spots, respectively; characterised by means for deriving first, second and third tracking error signals from the first, second and third photodetectors respectively; means for adding the first and second tracking error signals and for adding the first and third tracking error signals to form first and second composite tracking error signals, respectively; and means for detecting the detracking state, in which the first beam spot is deviated from the track, based on the polarities of the first and second composite tracking error signals. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figure 1 is an explanatory diagram of a writable optical disc; Figure 2 is an explanatory diagram of tracking error signals; Figure 3 is a block diagram showing an embodiment of the present invention; Figure 4 is an explanatory diagram of beam irradiating positions; Figure 5 is an explanatory diagram of phase differences among three tracking error signals; Figure 6 is an explanatory diagram of composite tracking error signals; Figure 7 is a waveform diagram for explaining detrack signals; Figure 8 is an explanatory diagram of beam irradiating positions in another embodiment of the present invention; Figure 9 is an explanatory diagram of phase differences among these tracking error signals in another embodiment; and Figure 10 is an explanatory diagram of composite tracking error signals in another embodiment. Figure 3 shows a schematic block diagram of a detrack detecting system of an optical recording/reproducing apparatus of the present invention. In figure 3, reference numerals 1a, 1b and 1c respectively denote detectors to which the reflected light of the first, second and third beam spots which were irradiated onto an optical disc are input. Each of the detectors 1a-1c is divided into two sensitive areas (eg, a₁, a₂ in the case of detector 1a) which produce independent outputs according to the detected intensity of the reflect light. Reference numerals 2a, 2b and 2c indicate comparing circuits which calculate the difference between the signals which are output from photosensitive surfaces a₁-a₂, b₁-b₂ and c₁-c₂ of the detectors 1a, 1b and 1c respectively; 3a, 3b, 3c, 3d and 3e represent adding circuits; and 4a, 4b and 4c denote coefficient circuits. As will be explained hereinlater, reference numerals 5a, 5b and 6 denote 0-level comparators and a logic circuit for discriminating the polarities of composite tracking error signals and detecting the detracking state. Figure 4 shows the positions of the beam spots irradiated onto the optical disc. The three beam spots are irradiated in a manner such that the first beam spot S A is located on a track T of the optical disc and the second and third beam spots S B and S C are located so as to be shifted laterally of the track from the first beam spot S A by a 1/4 track pitch, to the inner circumferential side and the outer circumferential side radially of the optical disc, respectively. Therefore, for example, when the three beam spots S A , S B and S C are traversed in the radial direction of the optical disc, the phases of the tracking error signals which are obtained at this time are set such that tracking error signals e b and e c which are obtained from the second and third detectors 1b and 1c have a phase difference of 90° for a tracking error signal e a which is output from the first detector 1a as shown in fig 5. Accordingly, when the tracking error signals e b and e c which are obtained from the second the third detectors 1b and 1c are supplied to the adding circuit 3a and are added, the tracking error signal component is set to 0. However, a DC component E D which is output due to the skew of the optical disc and the optical offset is detected. The DC component E D is multiplied with a coefficient K D by the coefficient circuit 4a and is input to the second adding circuit 3b. In the adding circuit 3b, the DC component included in the tracking error signal e a obtained from the first detector 1a is cancelled, thereby forming a tracking error signal Et without an offset. The tracking error signal Et and the signals which are obtained by multiplying coefficients K b and K c to the tracking error signals e b and e c are respectively added by the adding circuits 3c and 3d. Thus, as shown in the vector diagram of fig 6, composite tracking error signals (Et + e b ·K b and (Et + e c ·K c ) are formed. The detracking state can be detected from these composite tracking error signals by suitable processing. For example, as shown in fig 7, the composite tracking error signals (Et + e b ·K b ) and (Et + e c ·K c ) are respectively input to the comparators 5a and 5b. Waveforms S₁ and S₂ which are inverted at zero-cross points are generated. When those waveforms are supplied to the AND gate 6 whose one input terminal is used as a negative logic input, an S DT signal is output from the AND gate 6. Since the output S DT is set to the H level at positions before and after (a range of ±α) of an on-track point T o of the tracking error signal, when the output S DT of the AND gate 6 is set to the L level, such a state can be taken to be a detracking state. The on-track range, namely, the range (±α) which is regarded as a state in which the tracking servo is being correctly executed can be determined by the values of the coefficient values K b and K c which are multiplied to the second and third tracking error signals e b and e c in fig 3. When K b = K c = 1, if the beam spot were deviated from the centre point of the track by one ±45°, the servo is taken to be in a detracking state. As the values of K b and K c are large, the discriminating range of the on-track is widened. In fig 3, as shown by the broken line, by supplying the outputs S₁ and S₂ of the comparators 5a and 5b to an OR gate 7 one of whose input terminals is used as a negative logic input, it is possible to obtain a signal S DT ′ in which a range of ±β (β=180° - α) from the track point T o can be taken is indicating the on-tracking state. On the other hand, by adding the tracking error signals e b and e c which are obtained from the second and third detectors 1b and 1c by the adding circuit 3e, a tracking error signal Et (90°) whose phase is shifted by 90° can be obtained. By using this tracking error signal, the direction of movement of the optical head can be detected. Fig 8 shows irradiating positions of beam spots showing another embodiment of the invention. In the embodiment, the second and third beam spots S B and S C are arranged so as to be shifted laterally of the track from the first beam spot S A by 1/3 track pitch, respectively. Therefore, in this case, as shown in the vector diagram of fig 9, three tracking error signals e a (Et), e b and e c having phase differences of 120° are generated. Fig 10 shows phase waveforms of traverse signal of the three tracking error signals and composite tracking error signals. In the case of the embodiment as well, the composite tracking error signals (Et + e b ·K b ) and (Et + e c ·K c ) are input to the comparators 5a and 5b and the detrack detection signal S DT can be obtained from the AND gate 6 by the system as mentioned above. In the case of the embodiment, when the coefficient values K b and K c are set to 1, the on-track range (±α) is set to ±60° and as the coefficient values are small, the discriminating range of the on-track is narrowed. As described above, according to the detrack detecting system of the invention, three beam spots are irradiated onto the optical disc, the polarities of the tracking error signals which are obtained from the three beam spots are discriminated by the logic circuit and the like, and the detracking state is thereby detected. Therefore, the system can be used with optical discs having different reflectances or where the kinds of recording media provided on the recording surfaces of the optical discs differ and the detracking state can still be accurately detected.

Claims: A detrack detecting system in an optical recording/reproducing apparatus in which recording and reproducing of data are performed by irradiating beam spots onto an optical disc, comprising: means for irradiating the disc with a first beam spot (SA) which follows the centre of a data recording track; means for irradiating the disc with second and third beam spots (S B ,S C ) at positions which are shifted laterally of the track from the first beam spot by a predetermined distance to the inner circumferential side and the outer circumferential side of the track respectively; and first, second and third photodetectors (1a,1b,1c) for detecting light reflected from the first, second and third beam spots, respectively; characterised by means (3a,4a,3b,4b,4c) for deriving first, second and third tracking error signals from the first, second and third photodetectors respectively; means (3c,3d) for adding the first and second tracking error signals and for adding the first and third tracking error signals to form first and second composite tracking error signals, respectively; and means (5a,5b,6) for detecting the detracking state, in which the first beam spot is deviated from the track, based on the polarities of the first and second composite tracking error signals. A detrack detecting system according to claim 1, wherein the second and third beam spots are irradiated at positions which are shifted from the first beam spot by a 1/4 track pitch. A detrack detecting system according to claim 2, wherein means (3a,4a,3b) are provided to eliminate a DC component included in the first tracking error signal by adding the second tracking error signal and the third tracking error signal and subtracting a signal obtained by multiplying by a predetermined coefficient the resulting signal from the first tracking error signal. A detrack detecting system according to claim 1, wherein the second and third beam spots are irradiated at positions which are away from the first beam spot by a 1/3 track pitch. A detrack detecting system according to any one of the preceding claims, wherein the deriving means includes means (4a,4b,4c) for multiplying, by respective predetermined coefficients, the second and third tracking error signals and the first tracking error signal. A detrack detecting system according to any one of the preceding claims, the detecting means detects the polarities of the first and second composite tracking error signals by producing respective detection signals by detecting zero-crossing points of the first and second composite tracking error signals respectively logically AND-ing one of the detection signals with the logical inverse of the other. A detrack detecting system according to any one of the preceding claims, wherein, the detecting means detects polarities of the first and second composite tracking error signals by producing respective detection signals by detecting zero-cross points of the first and second composite tracking error signals respectively and logically OR-ing one of the detection signals with the logical inverse of the other. A detrack detecting system according to any one of the preceding claims, wherein, in use, the moving direction of the beam spot is detected by subtracting the second and third tracking error signals. A detrack detecting system according to any one of the preceding claims and including means for inhibiting writing operations to the disc while a detracking condition is detected.

Inventors: WACHI SHIGEAKI C O SONY CORPOR, WACHI, SHIGEAKI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B7