Method and circuit for generating a tracking error signal using differential phase detection

[0001] This application claims the benefit under 35 U.S.C. § 119 of Taiwan Application No. 092119638 filed Jul. 18, 2003. The disclosure of the prior application is hereby incorporated by reference herein in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention generally relates to the processing of a tracking error signal in disk server system, and more particularly, to a method for generating a tracking error signal using a differential phase detection circuit.

[0004] 2. Description of the Prior Art

[0005] Generally speaking, a pickup head is used in an ordinary optical pickup system to read data recorded on the disk tracks. Normally, there are many tracks on a disk and the pickup head moves between different tracks to read the data, the action of which is called cross-track motion. After the cross-track motion, pickup head needs and re-lock the track in order to read the data stably. When the pickup head cannot properly lock the track and read the data along the track, a tracking error is thus produced and the data cannot be read correctly.

[0006] FIG. 1 shows block diagram for generating DVD tracking error signal in a traditional disk system. Through the lens 104, the pickup head 102 focuses on a track on the surface of disk 100. And a quadrant photodetector 106 inducts signal A, signal B, signal C, and signal D. The oblique signals (signal A and signal C or signal B and signal D) go through I/V amplifier 108, 110 and are added to generate signal (A+C) and signal (B+D). Signal (A+C) goes through equalizer 112 and, after being equalized and amplified, goes through a comparator 116 for digitization. Similarly, signal (B+D) goes through equalizer 114 and comparator 118 to become digital signal.

[0007] There is almost no phase difference between the signal (A+C) and signal (B+D) when the pickup head 102 locks track correctly. Little variation among signal A, signal B, signal C and signal D inducted by the quadrant photodetector could be processed by a circuit to obtain a pickup head location. For instance: after signal (A+C) and signal (B+D) are received by phase detector 120, if the signal (A+C) is a phase lead signal, an UP clock is generated; otherwise, if signal (A+C) is phase delay signal, a DOWN clock is generated, as shown in FIG. 2. The up and down clock then go through a low pass filter 122 and 124 respectively and finally generate tracking error signal. In other words, when the crossing track of the pickup head 102 locks on the new track, no tracking error signal is generated because there is no phase difference between the signal (A+C) and signal (B+D). However, when the pickup head 102 has not completely locked, there is a phase difference between the signal (A+C) and signal (B+D), and thus a tracking error signal is generated. Optical systems adjust the pickup head location by the deviation of this tracking error signal to correctly lock the track.

[0008] According to Seong-Yun Jeong, Jung-Bae Kim and Jin-Yong Kim's report, “Analysis of DPD Signal Offset Caused by Optical Asymmetry” on SPIE Vol. 3109, the tracking error signal could be processed as the equation below, which means signals inducted by quadrant photodetector could be expressed by the equations below:
A=a cos(ωt−Φ_A))
B=b cos(ωt−Φ_B)
C=c cos(ωt−Φ_C)
D=d cos(ωt−Φ_D)
and the tracking error signal generated is

[0009] $\mathrm{Tracking}\mathrm{Error}=\mathrm{Phase}\left(A+C\right)-\mathrm{Phase}\left(B+D\right)=\frac{{\mathrm{\Delta \Phi }}_{\mathrm{CD}}+\frac{c}{a}{\mathrm{\Delta \Phi }}_{\mathrm{AB}}}{1+\frac{c}{a}}+\frac{\left(1-\frac{c}{a}\right)\left({\Phi }_A-{\Phi }_C\right)}{\left(1+\frac{c}{a}\right)}$

[0010] Wherein A, B, C and D are amplitude of signals inducted by 4 cells in a quadrant photodetector, a, b, c, and d, and Φ_A, Φ_B, Φ_C, and Φ_D, are, respectively, phase of their corresponding signals.

[0011] FIG. 3 shows that this sort of differential phase detection will be very sensitive to signal amplitude. FIG. 3 is to illustrate the influence on phase of the signal variation of a quadrant photodetector. For example, when the signal A on quadrant photodetector weakens, it causes bigger variation of the phase of signal (A+C). Thus, the variation of the signal becomes a miscarriage of judgment in relation to the phase variation and this affects the offset of tracking error signal and further affects the locking track point.

[0012] FIG. 1 shows the traditional phase detection circuit and the variation of the signal that causes misjudgment of phase and error for locking track point. FIG. 4 shows an improved phase detection circuit for generating a tracking error signal. The quadrant photodetector inducting signal A, signal B, signal C and signal D are digitized separately by different circuits. Separate cells induct phase difference that's caused by track crossing. This can avoid phase misjudgment from the signal variation and furthermore, accurately induct the lock track point.

[0013] However, in FIG. 4, the phase variation of signals inducted separately by quadrant photodetector is smaller and the signals make the phase signals generated by circuits that generate tracking error signal more sensitive to the phase delay and the phase lead which exist in the circuit, which can also create misjudgment of tracking error signal.

[0014] In view of prior art, there are many disadvantages in traditional tracking error circuit. The present invention provides a circuit for generating a tracking error signal using a differential phase detector to solve the problems of traditional circuit. The purpose of the present invention is to use a circuit to improve the phase difference which is caused by a circuit and further reduce lock point offset caused by non-physically produced signal phase difference and to increase accuracy of track locking and stability of tracking.

[0015] In accordance with the foregoing purpose, the present invention provides a circuit for generating a tracking error signal using a differential phase detection which comprises a quadrant photodetector for receiving an optical signal and inducting splitting signal A, splitting signal B, splitting signal C and splitting signal D. Splitting signal A and splitting signal C being added by an adder to generate a group signal (A+C). Splitting signal B and splitting signal D being added by another adder to generate a group signal (B+D). A plurality of equalizers for receiving, equalizing and amplifying the splitting signal A, splitting signal B, splitting signal C, splitting signal D, group signal (A+C), and group signal (B+D) (if a signal itself is clear and powerful enough, we can disable the corresponding equalizer). A plurality of phase detectors for receiving the output from the plurality of equalizers (or receiving directly these signal when they are clear enough) and comparing the phase difference of splitting signal A and group signal (A+C), group signal (A+C) and splitting signal B, splitting signal C and group signal (B+D), and group signal (B+D) and splitting signal D, and outputting the adjustment signals respectively. A circuit for eliminating the same phase difference in the adjustment signals by adding and subtracting (in case of high frequency noise, add low pass filter for filtering); finally, mixing adjustment signals to obtain a tracking error signal. Wherein, a plurality of comparators could be further comprised, which are connected between a plurality of equalizers and a plurality of phase detectors for transferring signals to digital signals. And splitting signal A, splitting signal B and group signal (A+C) go through a physically-equaled circuit to phase detector, and splitting signal C, splitting signal D and group signal (B+D) go through another physically-equaled circuit to phase detector. Therefore, splitting signal A, splitting signal B and group signal (A+C) circuits produce the same phase difference, and splitting signal C, splitting signal D and group signal (B+D) circuits produce the same phase difference, too. Wherein the physically-equaled circuit means that all components, all wires and geometric distribution in space and even insulating materials for insulation have no artificial difference. Which means except impurity and defect that cannot be completely removed in the real world, all parts in a circuit that could be artificially controlled are the same.

[0016] The present invention also provides a method for generating a tracking error signal using a differential phase detector circuit. Firstly, reading a plurality of splitting signals, wherein splitting signal A, splitting signal B, splitting signal C, and splitting signal D generated by the quadrant photodetector being read by a pickup head, group signal (A+C) and group signal (B+D) formed by mixing splitting signal A and splitting signal C, and splitting signal B and splitting signal D. And connecting said plurality of signals to a plurality of phase detectors. A plurality of phase detectors then generates a plurality of up clock signals and down clock signals. And then processing the plurality of up clock signals to obtain an up signal and processing the plurality of down clock signals to obtain a down signal for eliminating phase difference produced from going through the circuit. Finally, comparing the up signal and the down signal to obtain and output tracking error signal. Of course, it could be further filtered to eliminate noise or transferred to digital signal before further processing if needed. Of course, the present invention can also be applied to photodetectors other than quadrant photodetectors, such as sextant photodetectors or octant photodetectors. Variations of the method provided by the present invention comprise those in which signals generated by photodetectors are divided into two parts (two group signals) and all signals comprised by each group signal are added and the same process is carried out.

[0017] To conclude the foregoing, in a traditional tracking error circuit frame, the variation of a signal can easily influence the phase difference and make tracking offset and cause locking track point to be offset. In an improved tracking error circuit frame, the phase leading and the phase delay by the circuit still influence the tracking signal. As a result, the present invention provides a tracking error signal using a differential phase detector frame and uses the same phase difference signals to inter-eliminating phase difference for improving phase variation caused by the circuit. Furthermore, it reduces the non-physically produced phase lock point to be offset and increases the accuracy of track locking and stability of tracking.

[0025] In the following, some preferred embodiments of the invention would be described in greater detail. Nevertheless, it should be recognized that the present invention could be practiced in a wider range in other embodiments beside those explicitly described, and the scope of the present invention is not limited by these expressed embodiments but specified in the accompanying claims.

[0026] A preferred embodiment of the present invention is a circuit for generating a tracking error signal using a differential phase detection, which comprises, a quadrant photodetector for receiving an optical signal and generating splitting signal A, splitting signal B, splitting signal C and splitting signal D, wherein splitting signal A and splitting signal C being added by an adder to form a group signal (A+C) and splitting signal B and splitting signal D being added by another adder to form another group signal (B+D). A plurality of equalizers for receiving, equalizing and amplifying the splitting signal A, splitting signal B, splitting signal C, splitting signal D, group signal (A+C) and group signal (B+D). A plurality of phase detectors for receiving the output of equalizers and comparing the phase difference between splitting signal A and group signal (A+C), group signal (A+C) and splitting signal B, splitting signal C and group signal (B+D), and group signal (B+D) and splitting signal D, and outputting adjustment signals respectively. A circuit for eliminating the same phase difference in the adjustment signals by adding and subtracting, Low pass filters and a comparator for filtering and comparing adjustment signals and outputting a tracking error signal. Wherein, it further comprises a plurality of comparators, which are connected between a plurality of equalizers and a plurality of phase detectors for transferring the signal to a digital signal. And foregoing splitting signal A, splitting signal B and group signal (A+C) go through physically-equaled circuit to the phase detector and splitting signal C, splitting signal D and group signal (B+D) go through physically-equaled circuit to phase detector.

[0027] The following is the detailed description of the present invention. Referring to FIG. 5, it is the flow chart of the present invention, generating a tracking error signal using differential phase detection. Firstly, as step 610 shows, the splitting signal A, signal B, signal C and signal D, generated by a quadrant photodetector are read by a pickup head. As the following step 620 shows, the previously generated splitting signal A, splitting signal B, splitting signal C and splitting signal D, including group signal (A+C) and group signal (B+D) mixed by a circuit, are received by and sent by a first equalizer, second equalizer, third equalizer, fourth equalizer, fifth equalizer and sixth equalizer.

[0028] In the following, as step 630 shows, a phase detector receives splitting signal A and group signal (A+C) outputted by the first equalizer and the fifth equalizer, compares phase difference of splitting signal A and that of group (A+C), and outputs an Up clock signal and a Down clock signal; then a second phase detector receives group signal (A+C) outputted by the fifth equalizer and splitting signal B outputted by the second equalizer, compares the phase difference of group signal (A+C) and that of splitting signal B, and outputs an Up clock signal and a Down clock signal. The third phase detector receives splitting signal C outputted by the third equalizer and group signal (B+D) outputted by the sixth equalizer, compares the phase difference of group signal (B+D) and that of splitting signal C, and outputs an Up clock signal and a Down clock signal. The fourth phase detector receives group signal (B+D) outputted by the sixth equalizer and splitting signal D outputted by the fourth equalizer, compares the phase difference of group signal (B+D) and that of splitting signal D, and outputs an Up clock signal and a Down clock signal. And then, as step 640 shows, a circuit is used to deal with a plurality of lead Up clock signals or delay Down clock signals to eliminate phase differences generated by the circuit, which means an UP signal is achieved by subtracting the result of adding Up clock signals outputted by the third phase detector and the fourth phase detector from the result of adding Up clock signals outputted by the first phase detector and the second detector and making adjustment. Then, two UP signals that are mutually subtracted and compared are outputted to a first low pass filter for filtering the high frequency noise. Similarly, a DOWN signal can also be achieved by subtracting the result of adding Down clock signals outputted by the third phase detector and the fourth phase detector from the result of adding Down clock signals outputted by the first phase detector and the second detector and making adjustment. Then, two DOWN signals that are mutually subtracted and compared are outputted to a second low pass filter. Finally, the output of first low pass filter and output of second low pass filter are added for generating a tracking error signal. Wherein, a plurality of comparators could be further used in transforming a signal outputted from the equalizer to a digital signal before the following procedures; the digitizing process can also take place after forming a tracking error signal.

[0029] As the foregoing said, the characteristics of the present invention such as step 630, splitting signal A, splitting signal B and group signal (A+C) going through a set of physically-equaled circuits to a set of phase detectors, and splitting signal C, splitting signal D and group signal (B+D) going through another set of physically-equaled circuits to another set of phase detectors. So splitting signal A, splitting signal B and group signal (A+C) have the same phase difference produced by a circuit; and splitting signal C, splitting signal D and group signal (B+D) have the same phase difference produced by a circuit. Since the phase difference produced on a circuit is different from the phase between the UP signal and DOWN signal outputted from a phase detector, thus the present invention has a circuit for comparing and processing (subtracting one from another, for example) the phase difference of an UP signal and DOWN signal in the same phase detector set, as described in step 640. In other words, the characteristic of the present invention is based on making received split signals go through physically-equaled circuits to a plurality of phase detectors, and in the processing phase. And to have a circuit between the phase detector and the filter to process the phase, and to subtract one signal from another that has the same phase difference to completely eliminate the influence of phase difference produced by a real circuit. Of course, here it only eliminates the phase difference by inter-subtracting. The adjustment of the whole circuit still depends on the real needs when dealing with the UP clock signal and Down signal and performing an inter-action with the UP clock signal and Down signal.

[0030] FIG. 6 shows the circuit of another preferred embodiment of the invention. Wherein, the pickup head 502, through lens 504 tracking, reads the optical signal on the surface of the disk 500. The quadrant photodetector 506 has four cells: cell A, cell B, cell C and cell D. Cells of the quadrant photodetector 506 receive and induct the optical signal transmitted by the lens 504, and then output splitting signal A, splitting signal B, splitting signal C and splitting signal D. Splitting signal A and splitting signal C are mixed to form group signal (A+C) by a circuit and splitting signal B and splitting signal D are mixed to form group signal (B+D) by another circuit.

[0031] Reference to FIG. 6 continued. In the following, splitting signal A, splitting signal B, splitting signal C, splitting signal D, group signal (A+C) and group signal (B+D) go through a plurality of corresponded equalizers 508 for being equalized and amplified and for filtering the high frequency noise. The signal is transferred to a digital signal by the comparator 510. Which means analog signals received by the quadrant photodetector 506 must go through the comparator 510 to be transferred to and outputted as digital signal.

[0032] In circuit constitution showed in FIG. 6, group signal (A+C) and group signal (B+D) could be shared with focus error. And in this preferred embodiment, splitting signal A, splitting signal B and group signal (A+C) pass through the physically-equaled circuit and the influence of the phase leading and phase delay produced by a circuit are the same. Similarly, splitting signal C, splitting signal D and group signal (B+D) pass through the physically-equaled circuit and the influence from the phase leading and phase delay produced by the circuit are the same, too. In other words, for the real circuit, splitting signal A and splitting signal B inducted by cell A and cell B of a quadrant photodetector 506 and group signal (A+C), inducted by mixing cell A with cell C pass through the physically-equaled circuit to a phase detector 512 and a phase detector 514 for signal phase comparing. Since these signals pass through the physically-equaled circuit, the phase influence that is caused by circuit must also be the same. Splitting signal A, splitting signal B and group signal (A+C) pass through the circuit and generate a phase difference, which is defined as ΔΦ₁. Splitting signal C, splitting signal D and group signal (B+D) pass through a circuit and generate a phase difference, which is defined as ΔΦ₂. According to the foregoing, the comparative phase outputted by each phase detector could be explained by the equations below:
Phase Detector 512=Phase(A)−Phase(A+C)+ΔΦ₁
Phase Detector 514=Phase(B)−Phase(A+C)+ΔΦ₁
Phase Detector 516=Phase(C)−Phase(B+D)+ΔΦ₂
Phase Detector 518=Phase(D)−Phase(B+D)+ΔΦhd 2

[0033] Signals generated by the phase detector go through a low pass filter 520 and a comparator 522 for filtering. A tracking error signal is obtained by putting output signal generated by each phase detector through low pass filter 520 and filtering process of comparator 522, which could be explained by the equation below.
TE(Tracking Error)=[Phase(A)−Phase(A+C)+ΔΦ₁]−[Phase(B)−Phase(A+C)+ΔΦ₁]+[Phase(C)−Phase(B+D)+ΔΦ₂]−[Phase(D)−Phase(B+D)+ΔΦ₂]

[0034] By extending the equation, ΔΦ₁ and ΔΦ₂ in the equation will be inter-eliminated, and phase (A+C) and phase (B+D) are inter-eliminating, too. The equation will be finally reduced as follows:
TE=[Phase(A)−Phase(B)]+[Phase(C)−Phase(D)]

[0035] By referring to the reduced equation above, we can see ΔΦ₁ and ΔΦ₂, phase difference produced by the circuit, are eliminated. Therefore it could be understood that tracking error signal generated by the present invention can respond to real signal phase, which is not influenced by the phase difference that's caused by a circuit.

[0036] This preferred embodiment and circuit structure showed in FIG. 4 both first separate signal A, signal B, signal C and signal D inducted by a quadrant photodetector and use each separate cell to induct the phase difference which is caused by the crossing track for generating tracking error signal. In traditional tracking error circuit, variations such as phase delay or phase lead will occur on its circuit and will influence phase of phase detector. However, the present invention provides a differential phase and a circuit to eliminate the phase differences caused by the circuit. As a result, this can completely eliminate phase difference on the circuit and enormously reduce the probability of an offset tracking point or tracking error and efficiently improve the accuracy of tracking error.

[0037] Moreover, the generation of tracking error signal in the present invention only needs group signal (A+C) and group signal (B+D), therefore as long as group signal (A+C) and group signal (B+D) could be produced, there is no limit to the amount of wires and phase detectors and the sort of circuit needed to eliminate the phase of the Up clock signal and Down clock signal. The present invention only needs to keep an equal phase difference for every wire (at least making some circuits connected to several phase detectors with which output signals will have direct interaction have the same phase difference), compare this phase difference with Up clock signal and Down clock signal of phase detector, and then process Up clock signal from different phase detectors with a circuit (eliminating phase difference of Up clock signal caused by the circuit). Similarly, a circuit is used to process Down clock signal from different phase detectors (eliminating phase difference of Down clock signal caused by the circuit) to ensure complete elimination of phase difference on the circuit.

[0038] Of course, it is to be understood that the present invention is not limited by these disclosed embodiments. Various modification and similar changes are still possible within the spirit of the present invention. In this way, the scope of the present invention should be defined by the appended claims.