JP2008108325A - Information reproducing unit, and information reproduction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an information reproducing unit and an information reproduction method for accurately reproducing information on an optical disk recorded with high density by canceling crosstalk precisely. <P>SOLUTION: Before crosstalk cancellation processing, phase differences between a reproduction signal at a read target track, and a reproduction signal at n tracks before and a reproduction signal at n tracks after are calculated each by a cross-correlation function, and each phase is matched based on the calculated phase differences. After the phase matching, crosstalk cancellation processing is performed for subtracting the reproduction signal at n tracks before and the reproduction signal at n tracks after from the reproduction signal of the read target rack. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an information reproducing apparatus and an information reproducing method.

  With the advance of digital technology and the improvement of data compression technology, optical discs such as DVD (Digital Versatile Disc) are drawing attention as media for recording information such as music, movies, photos and computer software. . At the same time, optical disk devices and the like have become widespread as information reproducing devices for reproducing information recorded on optical disks.

  In recent years, there is an increasing need to process large volumes of information such as multimedia, image files, and file servers, and a large capacity of such an optical disk and an information reproducing apparatus corresponding to the large volume are desired. For example, in an information reproducing apparatus compatible with a BD (Blu-ray Disc) that is a large-capacity optical disc, the wavelength of the laser light is 390 nm to 420 nm and the numerical aperture of the objective lens is 0.70 to 0.90. Using the head, the light spot diameter of the laser light is narrowed down to about 0.48 μm. With this configuration, even a mark having a diameter of 0.138 μm or more and 0.160 μm or less on the optical disc can be identified and reproduced, so that high-density optical recording on the optical disc can be realized.

  Here, in the case of an information reproducing apparatus compatible with a larger-capacity optical disc, it is desirable to realize an optical system having a further shorter wavelength and a higher numerical aperture, but this is difficult to realize at present. Therefore, in an information reproducing apparatus equipped with an optical system having the same performance, as one of techniques for realizing a large capacity of an optical disk, there is a crosstalk cancellation technique for reducing the crosstalk component of adjacent tracks. This technique makes it possible to reduce the track pitch of the optical disc and to realize a large capacity of reproducible information.

  As this crosstalk cancellation technique, for example, in Japanese Patent No. 2601174 (Patent Document 1), an accurate delay time can be set depending on variations in optical heads and differences in the radius of the disk. An optical disk signal reproducing apparatus is described in which crosstalk cancellation operates accurately and can realize a significantly higher density than the conventional track density. In the invention described in Patent Document 1, three beams on the disk board surface are used, and crosstalk cancellation is performed by subtracting the read signal of the adjacent track from the read signal of the center track. In the invention described in Patent Document 1, the signals are synchronized by taking the cross correlation of the three beams.

In Japanese Patent No. 3256611 (Patent Document 2), when reading a disk on which recording marks synchronized with the disk radial direction are recorded, read signals for the past two cycles of the disk are stored in a memory. In addition, there is described a disk reproducing apparatus that performs crosstalk cancellation by subtracting a read signal of an adjacent track from a read signal of a central track.
Japanese Patent No. 2601174 Japanese Patent No. 3225611

  However, in the invention described in Patent Document 1, since the maximum amount of light from an LD (Laser Diode) is required during high-speed recording, the amounts of light of the three beams cannot be made the same. Therefore, it is necessary to reduce the light amount of the sub beam on the adjacent track, and a large noise is mixed in the read signal of the adjacent track. Therefore, the effect of canceling the crosstalk cannot be fully utilized. Further, since the sub beam is off-axis of the laser beam, the optical aberration is large and a good light spot shape cannot be obtained. Therefore, it is impossible to cancel crosstalk with high accuracy.

  Furthermore, a PD (Photo Diode) used in a differential push-pull method for obtaining a general track error signal does not have a frequency characteristic up to the RF band, and an expensive PD is required for use in crosstalk cancellation. This is necessary, and the manufacturing cost of the information reproducing apparatus increases.

  In the invention described in Patent Document 2, since the recording mark of each track needs to be recorded synchronously in the disk radial direction, synchronous recording is required in the disk radial direction, and asynchronously in the generally used disk radial direction. It cannot be used for recorded recording marks.

  The present invention has been made in view of the above circumstances, and is an information reproducing apparatus that accurately reproduces information on an optical disc recorded at high density by performing crosstalk cancellation with high accuracy. And it aims at providing the information reproduction method.

  The information reproducing apparatus of the present invention employs the following configuration in order to achieve the above object.

  The information reproducing apparatus of the present invention includes information having reading means for reading a reproduction signal of an RF signal from a reading target track, and storage means for storing a reproduction signal having a length of at least two tracks read by the reading means. In the reproduction apparatus, the phase difference between the reproduction signal of the reading target track and the reproduction signal of n (n is an integer) track before the reading target track, which is stored in the storage unit, is determined as the reproduction signal of the reading target track. First phase difference calculating means for calculating using a cross-correlation function with a reproduction signal n (n is an integer) track before the reading target track, and a reproduction signal of the reading target track stored in the storage means And the reproduction signal after n tracks from the read target track, the phase difference between the reproduction signal of the read target track and the read signal from the read target track. A second phase difference calculating means for calculating based on a cross-correlation function with the reproduction signal of the first reproduction signal, a reproduction signal for the read target track, and a reproduction signal for the previous n tracks based on the output of the first phase difference calculating means Second phase matching is performed between the reproduction signal of the read target track and the reproduction signal after the n tracks based on the output of the first phase matching means for performing phase matching and the output of the second phase difference calculating means. A phase matching unit and a crosstalk cancellation unit that subtracts the reproduction signal before n tracks and the reproduction signal after n tracks from the reproduction signal of the reading target track may be employed. According to such a configuration, it is possible to accurately reproduce the information of the optical disc stored at high density by performing crosstalk cancellation with high accuracy.

  In addition, the first and second phase difference calculating means are respectively inputted with a first input signal and a second input signal, and one or more delay means for delaying the first input signal for a predetermined time. Multiplying means for calculating a product of the first input signal and the second input signal; multiplying means for calculating a product of the output from the delay means and the second input signal; A plurality of low-pass filters that respectively extract the low-frequency components of the outputs of the plurality of multiplying units and an addition / subtraction unit that adds and subtracts the outputs of the plurality of low-pass filters may be employed. According to this configuration, the phase difference between the reproduction signal of the reading target track and the reproduction signal before the n tracks and the reproduction signal after the n tracks is calculated more accurately.

  Further, the first and second phase difference calculating means are respectively inputted with a first input signal and a second input signal, and one or more delay means for delaying the first input signal for a predetermined time; Multiplying means for calculating a product of the first input signal and the second input signal; multiplying means for calculating a product of the output from the delay means and the second input signal; A plurality of low-pass filters that respectively extract low-frequency components output from the multiplication means, and a maximum value detection means that detects the maximum value of the outputs of the plurality of low-pass filters. According to this configuration, the phase difference between the reproduction signal of the reading target track and the reproduction signal before the n tracks and the reproduction signal after the n tracks is calculated more accurately.

  Further, a configuration may be adopted in which a first FIR filter that has a waveform equivalent to the reproduction signal before n tracks and a second FIR filter that has a waveform equivalent to the reproduction signal after n tracks. According to this configuration, since the frequency characteristic read by the main beam from the waveform equivalent is converted into the frequency characteristic of the crosstalk component, the phase difference is calculated, so that the crosstalk cancellation is performed with high accuracy. Therefore, it is possible to accurately reproduce the information of the optical disk stored at high density.

  In the information reproducing apparatus of the present invention, the first FIR filter and the second FIR filter are adaptive filters that change a waveform equivalent coefficient in accordance with an input signal. The reproduced signal before n tracks is input to the first phase difference calculating means, and the reproduced signal after n tracks whose waveform is equivalent by the second FIR filter is calculated as the second phase difference. It can be set as the structure input into a means. According to such a configuration, a more accurate phase difference can be calculated.

  In the information reproducing apparatus of the present invention, the waveform equivalent coefficient of the first FIR filter is input to the first phase difference calculating unit, and the second phase difference calculating unit includes the second FIR filter. A configuration may be adopted in which a waveform equivalent coefficient is input. According to such a configuration, since the phase difference is calculated from the waveform equivalent coefficient, the phase difference calculating means can be simplified.

  Further, in the information reproducing apparatus of the present invention, the first phase difference calculating means includes a tap where a center tap position of the first FIR filter and a waveform equivalent coefficient of the first FIR filter are maximum. A phase difference between the reproduction signal of the previous read target track and the reproduction signal n tracks before the read target track is calculated based on the difference with the position, and the second phase difference calculating means includes the second FIR filter Based on the difference between the tap position at the center of the first FIR filter and the tap position at which the waveform equivalent coefficient of the second FIR filter has the maximum value, the reproduction signal of the previous read target track and the reproduction signal after n tracks from the read target track The phase difference may be calculated. According to this configuration, it is possible to calculate an accurate phase difference while simplifying the phase difference calculation unit.

  Further, the information reproducing apparatus of the present invention may include a synchronous clock generating unit that reproduces the reproduction signal of the reading target track from the output signal of the crosstalk canceling unit. According to such a configuration, it is possible to reproduce information stably.

  The information reproducing apparatus of the present invention stores an information recording means for recording information on a medium, a reproduction signal having a length of at least two tracks read by the reading means at the time of information reproduction, and a buffer at the time of information recording. It may be configured to have information storage means for storing recording data for preventing underrun. According to such a configuration, it becomes possible to perform crosstalk cancellation with high accuracy without increasing the memory capacity in the recording / reproducing apparatus, and information on the optical disc stored at high density can be accurately reproduced.

  The information reproducing method of the present invention includes information having reading means for reading a reproduction signal of an RF signal from a reading target track, and storage means for storing a reproduction signal having a length of at least two tracks read by the reading means. In the information reproducing method in the reproducing apparatus, the phase difference between the reproduction signal of the read target track and the reproduction signal of n (n is an integer) track before the read target track, which is stored in the storage unit, is calculated as the read target track. A first phase difference calculation procedure that is calculated based on a cross-correlation function between the reproduced signal and the reproduced signal n (n is an integer) track before the read target track, and the read target track stored in the storage means The phase difference between the reproduction signal of the reading target track and the reproduction signal after n tracks from the reading target track is expressed as the reproduction signal of the reading target track and the reading target track. A second phase difference calculation procedure calculated based on a cross-correlation function with a reproduction signal after n tracks from the track, and an output of the first phase difference calculation procedure, and the reproduction signal of the read target track and the n Based on the output of the first phase matching procedure for matching the phase difference with the reproduction signal before the track and the second phase difference calculation procedure, the reproduction signal of the read target track and the reproduction signal after the n track are A second phase matching procedure for matching a phase difference, and a crosstalk cancellation procedure for subtracting the reproduction signal before n tracks and the reproduction signal after n tracks from the reproduction signal of the reading target track; can do. According to such a method, it is possible to accurately reproduce the information on the optical disc recorded with high density by performing crosstalk cancellation with high accuracy.

  The present invention provides an information reproducing apparatus and information reproducing method for accurately reproducing information on an optical disc recorded with high density by performing crosstalk cancellation with high accuracy.

The present invention calculates the phase difference between the reproduction signal of the reading target track and the reproduction signal before n tracks and the reproduction signal after n tracks before the crosstalk cancellation processing, and based on the calculated phase difference. Each phase matching is performed. Then, after the phase matching, a crosstalk canceling process for subtracting the reproduction signal before n tracks and the reproduction signal after n tracks from the reproduction signal of the reading target track is performed.
(First embodiment)
A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is an example of a schematic configuration diagram of an information reproducing apparatus 20 in the first embodiment of the present invention.

  The information reproducing device 20 reproduces the optical disc 15 that is a medium on which information is recorded, and transmits the reproduced information to, for example, a host device 90 connected to the information reproducing device 20 by an appropriate method.

  The information reproducing device 20 includes a spindle motor 21, a seek motor 22, an optical pickup device 23, a laser control circuit 24, a drive control circuit 26, and a reproduction signal processing circuit 28. The information reproducing device 20 includes a buffer memory 34, a buffer manager 37, an interface 38 with the host device 90, a flash memory 39, a CPU 40, and a RAM 41.

  The spindle motor 21 drives the optical disk 15 to rotate. The seek motor 22 drives the optical pickup device 23 in the radial direction of the optical disc 15. The optical pickup device 23 is a laser light source and a light receiving unit for recording and reproducing data, and reads information recorded on the optical disk 15. The laser control circuit 24 controls the laser light source of the optical pickup device 23.

  The drive control circuit 26 is a circuit that controls the driving of the spindle motor 21, the seek motor 22, and the optical pickup device 23. The spindle motor control circuit 26 a that controls the driving of the spindle motor 21 and the seek that controls the driving of the seek motor 22. A motor control circuit 26b and a PU (Pick-Up) control circuit 26c that controls driving of the optical pickup device 23 are provided.

  The reproduction signal processing circuit 28 includes an I / V amplifier (current / voltage conversion circuit) 28a, a servo signal generation circuit 28b, an RF signal generation circuit 28c, a wobble signal generation circuit 28d, and a decoder 28e. The reproduction signal read from the optical disk 15 by the optical pickup device 23 is input to the servo signal generation circuit 28b, the RF signal generation circuit 28c, and the wobble signal generation circuit 28d via the I / V amplifier 28a.

  The servo signal generation circuit 28b generates and outputs a servo signal based on the input signal. The RF signal generation circuit 28c reproduces an RF signal corresponding to data. The wobble signal generation circuit 28 d generates a wobble signal used for rotation control of the optical disc 15. The output signal of the RF signal generation circuit 28c and the output signal of the wobble signal generation circuit 28d are input to the decoder 28e. Details of the decoder 28e will be described later. Note that the arrows shown in FIG. 1 indicate the flow of typical signals and information, and do not represent all the connection relationships between the components.

  Hereinafter, an operation when information is reproduced in the information reproducing apparatus 20 will be described.

  When the optical disk 15 is set in the information reproducing apparatus 20 and the CPU 40 receives an information reproducing request from the host apparatus 90 via the interface 38, the information reproducing apparatus 20 starts an information reproducing process. When the information reproduction apparatus 20 receives a reproduction request, the CPU 40 outputs a control signal for controlling the rotation of the spindle motor 21 to the spindle motor control circuit 26b based on the reproduction speed. Based on this, the spindle motor control circuit 26b drives the spindle motor 21 to rotate the optical disc 15.

  When the rotation of the optical disk 15 reaches a predetermined linear velocity, the laser is reproduced and emitted by the laser control circuit 24 in the optical pickup device 23. Then, the optical pickup device 23 performs focus control and track control for the reproduction layer of the optical disc 15. At this time, the servo signal generation circuit 28 b detects the track error signal and the focus error signal based on the output signal from the optical pickup device 23, and outputs it to the drive control circuit 26. In the drive control circuit 26, the PU control circuit 26c drives the tracking actuator and the focusing actuator of the optical pickup device 23 based on the track error signal and the focus error signal to control the optical pickup device 23.

  When the information reproducing process is started in the information reproducing apparatus 20, the CPU 40 transmits the rotational speed data of the spindle motor 21 to the spindle motor control circuit 26a. In addition, the CPU 40 transmits a signal indicating a position where the optical pickup device 23 starts reading information to the seek motor control circuit 26b. The spindle motor control circuit 26a controls the spindle motor 21 based on the rotation number data, and thereby the rotation of the optical disk 15 is controlled. The seek motor control circuit 26b controls the seek motor 22 based on a signal indicating the readout start position, and thereby the optical pickup device 23 is controlled to be positioned at a predetermined information readout start point.

  Further, when the CPU 40 determines that the optical pickup device 23 is located at a predetermined reading start point based on the address information acquired from the optical disc 15 and passed through the reproduction signal processing circuit 28, the optical pickup device 23 uses the optical pickup device 23 to Play back the information recorded in the data area. In the information reproducing apparatus 20, this data is input to the RF signal generation circuit 28c via the I / V amplifier 28a, and is reproduced as an RF signal corresponding to the data by the RF signal generation circuit unit 28c.

  The reproduced RF signal is input to the decoder 28e and demodulated into binary digital data by the decoder 28e. The present invention relates to this decoder 28e.

  The decoder 28e will be described below with reference to the drawings. FIG. 2 is a diagram showing an example of the internal configuration of the decoder 28e of this embodiment.

  The decoder 28e includes a high-pass filter (hereinafter referred to as HPF) 60, an equivalent circuit 61, a crosstalk cancellation circuit 68, and a decoder 66. The processing in the decoder 28e will be described below.

  The RF signal input to the decoder 28e has low-frequency noise removed by the HPF 60, and the equivalent circuit 61 emphasizes high-frequency components whose components have been attenuated due to a decrease in the optical system MTF (Modulation Transfer Function), thereby causing intersymbol interference. Is reduced. Here, the equivalent circuit 61 may have a function of a low-pass filter (hereinafter referred to as LPF) that cuts a high-frequency component so as not to cause aliasing noise during A / D conversion.

  The signal whose intersymbol interference is reduced by the equivalent circuit 61 is input to the crosstalk cancellation circuit 68. This input signal is reduced in crosstalk from adjacent tracks in the crosstalk cancellation circuit 68 and binarized by the decoder 66. Details of processing in the crosstalk cancellation circuit 68 will be described later. Note that the decoder 66 may be constituted by, for example, a slicer or a Viterbi decoder.

  Here, crosstalk from adjacent tracks will be described with reference to FIG. Crosstalk from an adjacent track occurs during reproduction of an information recording medium such as an optical disk. FIG. 3 is a diagram illustrating crosstalk from adjacent tracks. In FIG. 3, only the track necessary for explanation is shown in an enlarged manner.

  The information reproducing apparatus 20 reads information recorded on a track of an information recording medium such as the optical disk 15 using a reading spot which is a light spot of laser light. Here, a track from which information is read by a reading spot is called a reading target track. At this time, as shown in FIG. 3, for example, when the reading spot is larger than the track interval (track pitch), the reproduction signal of the reading target track read by the reading spot is added to the reproduction signal of the previous track or the next track. Playback signal is mixed.

  This mixed signal is crosstalk from the adjacent track. This crosstalk deteriorates the signal quality of the reproduced signal of the read target track. Therefore, in the information reproducing apparatus 20 of the present embodiment, a crosstalk cancellation process is performed to reduce crosstalk.

  The crosstalk cancellation circuit 68 and the crosstalk cancellation process will be described below with reference to the drawings. FIG. 4 is a diagram illustrating the internal configuration of the crosstalk cancel circuit 68 in the information reproducing apparatus 20 of the first embodiment.

  In the crosstalk cancel circuit 68 of the present embodiment, the phase matching process between the reproduction signal of the reading target track and the reproduction signal of the track immediately before the reading target track, the reproduction signal of the reading target track, and one of the reading target tracks A highly accurate crosstalk cancellation process is realized by performing a phase matching process with the reproduction signal of the next track. Details of the phase matching process will be described later.

  Here, the track immediately before the reading target track is called a front track, and the track immediately after the reading target track is called a rear track. The reproduction signals read from these three tracks are stored in a memory 92 provided in the crosstalk cancellation circuit 68. The memory 92 stores a signal read in the past, and a reproduction signal for each of the three tracks can be obtained by designating an appropriate address in each memory space. The memory 92 will be described later.

  Further, the crosstalk cancel circuit 68 of the present embodiment performs processing for converting the frequency characteristics of the reproduction signal of the previous track and the frequency characteristic of the reproduction signal of the subsequent track into the frequency characteristics of the reproduction signal of the read target track.

  In the information reproducing apparatus 20, the reproduction signal of the reading target track is read at the center of the reading spot, whereas the reproduction signals of the front track and the rear track are read at the end of the reading spot. Therefore, the reproduction signals of the front track and the rear track are read at positions shifted from the optical axis of the laser beam for reading the reproduction signal of the reading target track. For this reason, the frequency characteristic of the reproduction signal of the read target track is different from the frequency characteristic of the reproduction signal of the front track and the rear track. Therefore, in the present embodiment, the other frequency characteristic is converted so that this frequency characteristic is matched with one of the frequency characteristics.

  The configuration of the crosstalk cancellation circuit 68 will be described below.

  The crosstalk cancellation circuit 68 includes an A / D converter 91, a memory 92, phase difference calculation circuits 93a and 93b, interpolators 94a, 94b and 94c, a PLL circuit 95, equivalent circuits 96a, 96b and 96c, a subtractor 97, The phase matching circuits 98a and 98b are configured.

  The crosstalk cancellation circuit 68 receives an RF signal from which low-frequency noise has been removed by the HPF 60 and intersymbol interference has been reduced by the equivalent circuit 61. This RF signal is sampled into a binarized digital signal by the A / D converter 91. The sampling clock may be a free-running clock that is not synchronized with the rotation of the optical disk 15. The sampled data is stored at the first address of the memory 92. The memory 92 will be described below.

  The memory 92 is composed of, for example, a shift register, and the stored data is incremented by one address every clock in the direction of the arrow in the figure. The memory 92 is not limited to the shift register, and a normal memory may be used. In this case, the designation of each read address may be incremented by 1 every clock.

  Here, the capacity of the memory 92 must be a capacity capable of obtaining reproduction signals of at least three tracks, that is, the front track, the read target track, and the rear track. Therefore, the memory 92 must have a capacity of at least two cycles of the optical disc 15 as shown in FIG. FIG. 5 is an example of a schematic diagram showing the contents of the memory 92.

  The capacity required for the memory 92 will be described with a specific example. For example, assuming that the outermost peripheral radius of the optical disk 15 is 56.5 mm, the shortest mark length is 0.149 μm, the shortest mark is 2T, and one clock length is 0.149 / 2 = 0.0745 μm, the maximum is 56.5 per round. * 2π [mm] /0.0745 [μm] = 4762684 tracks / round. Therefore, if sampling is performed at 8 bits, the capacity required for the memory 92 is 4762684 * 2 * 8 bits, which is approximately 9.5 Mbytes.

  For example, a RAM (Random Access Memory) provided inside or outside a decoder LSI (Large Scale Integration) is used as the memory 92. Since the memory 92 is used only during the reproducing operation of the information reproducing apparatus 20, it is preferably used also as a buffer RAM for preventing buffer underrun during recording. Here, FIG. 6 shows an example of an RF signal stored in the memory 92 as digital data.

  Next, an outline of processing in the crosstalk cancellation circuit 68 will be described with reference to FIG. FIG. 7 is a flowchart for explaining the outline of the processing of the crosstalk cancellation circuit 68.

  In the crosstalk cancellation circuit 68, the memory 92 stores a reproduction signal input from the A / D converter 91 and having a length of two tracks or more of the optical disk 15 (S71). Here, the reproduction signal of the reading target track, the reproduction signal of the previous track, and the reproduction signal of the subsequent track are recorded in the memory 92.

  Next, in the crosstalk cancellation circuit 68, the phase difference calculation circuit 93a calculates the phase difference A between the reproduction signal of the reading target track and the reproduction signal of the previous track (S72). Details of the phase difference calculation processing in S72 will be described later. In the crosstalk cancellation circuit 68, similarly to S72, the phase difference calculation circuit 93b calculates the phase difference B between the reproduction signal of the read target track and the reproduction signal of the subsequent track (S73).

  Next, in the crosstalk cancellation circuit 68, the phase matching circuit 98a matches the phase difference between the reproduction signal of the reading target track and the reproduction signal of the previous track based on the phase difference A (S74). Details of this phase matching processing will be described later. Similarly to S74, the crosstalk cancellation circuit 68 matches the phase difference between the reproduction signal of the read target track and the reproduction signal of the subsequent track based on the phase difference B by the phase matching circuit 98b (S75).

  Next, in the crosstalk cancellation circuit 68, the equivalent circuit 96a converts the frequency characteristic of the reproduction signal of the reading target track so as to match the frequency characteristic of the reproduction signal of the previous track (S76). Next, the crosstalk cancel circuit 68 converts the frequency characteristic of the reproduction signal of the read target track so as to match the frequency characteristic of the reproduction signal of the subsequent track by the equivalent circuit 96c as in S76 (S77). In this way, the crosstalk cancellation circuit 68 of the present embodiment generates a replica of the crosstalk component that matches the phase of the reproduction signal of the read target track.

  Next, in the crosstalk cancel circuit 68, the subtracter 97 subtracts the replica of the crosstalk component generated from the subsequent track from the reproduction signal of the read target track (S78). Further, the subtracter 97 subtracts the replica of the crosstalk component generated from the previous track from the reproduction signal of the reading target track (S79).

  As described above, the crosstalk cancel circuit 68 of the present invention subtracts the replica of the crosstalk component, which is in phase with the reproduction signal of the read target track, from the read signal of the read target track. It can be performed.

  Next, the processing in the crosstalk cancellation circuit 68 will be described in further detail along the signal paths of the reproduction signals of the front track, the read target track, and the rear track.

  First, the signal path of the reproduction signal of the previous track and the processing of the crosstalk cancellation circuit 68 will be described.

  The crosstalk cancel circuit 68 designates a certain address in the memory 92 and reads data at a position corresponding to the previous track, that is, a reproduction signal of the previous track. The read reproduction signal is input to the interpolator 94a. In the interpolator 94a, the reproduced signal is interpolated in accordance with the timing of the reproduced clock with an accuracy of 1 clock or less. That is, in the reproduction signal, sample values which are interpolation data are embedded and interpolated between two sampling points in synchronization with the reproduction signal clock of the read target track and at intervals of one clock or less of the reproduction signal.

  The reproduction signal of the previous track passes through the interpolator 94a and is then input to the phase difference calculation circuit 93a. The phase difference calculation circuit 93a calculates the phase difference between the reproduction signal of the reading target track and the reproduction signal of the previous track. Here, the phase difference calculation process in S72 described in FIG. 7 will be described.

  FIG. 8 is a diagram illustrating an example of a circuit configuration of the phase difference calculation circuit 93a.

  The phase difference calculation circuit 93a includes delay units 70a to 70e having a fixed delay amount, multipliers 71a to 71f, LPFs 77a to 77f, an adder / subtractor 73, and a subtractor 74. The phase difference calculation circuit 93a calculates the product of each signal input from the input 1 and passed through the delay devices 70a to 70e and the input 2 by the multipliers 71a to 71f. With the above operation, the cross-correlation between the input 1 and the input 2 can be obtained. In this embodiment, it is assumed that the reproduction signal of the reading target track is input to the input 1 and the reproduction signal of the previous track is input to the input 2. Thereafter, each signal passes through the LPFs 77a to 77f and is added / subtracted by the adder / subtractor 73.

  Here, the calculation formula of the cross correlation is shown in (Formula 1).

この数１において、ｘが入力１、ｙが入力２、Ｒが相互相関値である。光ディスク１５において、ディスク盤面上での位置を考えると、読取目標トラックの再生信号と隣接トラック（すなわち前トラック又は後トラック）の再生信号との位置（位相）がラジアル方向で整合している場合には、相互相関はｋ＝０の原点を中心として対称となる。よって、位相差算出回路９３ａの出力が０となるということは、入力１と入力２の位相が整合している、即ち再生信号の読み取り位置がラジアル方向に整合していることを示す。

In this equation 1, x is input 1, y is input 2, and R is a cross-correlation value. Considering the position on the disk surface of the optical disk 15, the position (phase) of the reproduction signal of the reading target track and the reproduction signal of the adjacent track (that is, the front track or the rear track) is aligned in the radial direction. The cross-correlation is symmetric about the origin of k = 0. Therefore, the output of the phase difference calculating circuit 93a being 0 indicates that the phases of the input 1 and the input 2 are matched, that is, the read position of the reproduction signal is matched in the radial direction.

  Here, in the phase difference calculation circuit 93a, since the input 1 is passed through the delay units 70a to 70e, the phase delayed by the delay units 70a to 70e is actually delayed when the output of the adder / subtracter is 0. In this state, input 1 and input 2 are synchronized. Therefore, the subtracter 74 performs a process of reducing the constant C corresponding to the delay by the delay units 70a to 70e in order to compensate for this phase delay. As a result, the output of the phase difference calculation circuit 93a is set to 0 while the phases of the input 1 and the input 2 are matched. That is, the output of the phase difference calculation circuit 93a is a phase difference signal between the input 1 and the input 2.

  The phase difference calculation circuit 93a may have a configuration as shown in FIG. 9, for example, in addition to the configuration shown in FIG. FIG. 9 is a diagram illustrating another example of the circuit configuration of the phase difference calculation circuit 93a.

  In the example shown in FIG. 9, the adder / subtracter 73 shown in FIG. 8 is replaced with a maximum value detector 75. Here, the maximum value detector 75 selects the value of k that takes the maximum value from the cross-correlation values corresponding to the delay amounts of the delay units 70a to 70e. Here, k is assumed to be 0-5. In the example shown in FIG. 9, the selected value of k or the value obtained by subtracting the constant C is used as the output of the phase difference detector 93a.

  The phase matching circuit 98a performs phase matching based on the output of the phase difference calculation circuit 93a. The phase matching process in S74 described with reference to FIG. 7 will be described below.

  The phase matching circuit 98a uses the interpolator 94a to match the phase difference of the phase difference calculated by the phase difference calculation circuit 93a with respect to the clock width of the reproduction signal of the read target track. Of the phase difference calculated by the phase difference calculation circuit 93a, the phase matching circuit 98a has an address suitable for matching the phase difference in the memory 92 for components larger than the clock width of the reproduction signal of the read target track. And the playback signal of the previous track is read from the specified address.

  According to this, it becomes possible to cancel the crosstalk with the reproduction signal of the previous track phase-matched with the reproduction signal of the reading target track. Therefore, it is possible to perform crosstalk cancellation with the playback signal of the previous track that has been phase-matched even on a normal optical disk in which the playback signal is not recorded in synchronization with the disk radial direction. It can be performed. In the present embodiment, on the principle of using cross-correlation in the phase difference calculation, phase difference information with a better SN ratio can be obtained as the crosstalk increases. Therefore, even when the track interval is narrowed in the optical disc, high-performance crosstalk cancellation processing can be performed, which is advantageous for increasing the density of the optical disc.

  Note that the number of delay units, multipliers, and LPFs in the phase difference calculation circuit 93a is not limited to the numbers shown in FIGS. 8 and 9, and at least one delay unit is used in the same configuration. , Two LPFs are sufficient. However, the greater the number of delay units, multipliers, and LPFs, the more accurate the phase difference calculation using cross-correlation, so that the signal can be reproduced more reliably.

  Since the signal path of the reproduction signal of the rear track and the processing of the crosstalk cancellation circuit 68 are the same as in the case of the front track, description thereof will be omitted. Note that the crosstalk cancellation processing of the rear track is performed by the interpolator 94c, the phase difference calculation circuit 93b, and the phase matching circuit 98b.

  The reproduction signal phase-matched by the phase matching circuit 98a converts the frequency characteristic of the reproduction signal of the reading target track into the frequency characteristic of the reproduction signal of the previous track in the equivalent circuit 96a. The frequency characteristic conversion process in S74 described with reference to FIG. 7 will be described below.

  The equivalent circuit 96a in the present embodiment uses, for example, an FIR (Finite Impulse Response) filter. The FIR filter is a filter that obtains a desired frequency characteristic by inputting data to a delay line, extracting an output from a tap, and multiplying the outputs by a coefficient. This FIR filter obtains desired filter characteristics by changing coefficients and the number of tap stages. In the present embodiment, for example, this coefficient (waveform equivalent coefficient) is set to a fixed value given in advance, and the frequency characteristic is converted by this coefficient. Similarly, the equivalent circuit 96c may be configured by an FIR filter. The equivalent circuit 96a may of course be an adaptive filter whose waveform equivalent coefficient is variable.

  Next, the signal path of the reproduction signal of the reading target track and the processing of the crosstalk cancellation circuit 68 will be described.

  Data stored at a position corresponding to the reproduction signal of the reading target track, that is, the reproduction signal of the reading target track is read from an appropriate address in the memory 92. The read reproduction signal is input to the interpolator 94b, the data between the sample points is complemented in accordance with the phase of the reproduction clock, and the phase is adjusted with an accuracy of one clock or less, and then input to the equivalent circuit 96b. .

  The PLL circuit 95 compares the phase of the reproduction signal clock output from the interpolator 94 b with the reproduction clock generated by the PLL circuit 95. The PLL circuit 95 adjusts address designation to the interpolator 94b or the memory 92 based on the error signal indicating the phase difference, and reproduces the reproduction signal of the read target track. Also, the PLL circuit 95 increments the designated address to the memory by one address per clock in order to advance the reading position. In practice, rotational jitter of the optical disk 15 exists, but is eliminated by phase comparison by the PLL circuit 95 described above, phase adjustment of one clock or less by the interpolator 94b based on the phase comparison, and address adjustment to the memory 92. Here, the PLL circuit 95 may use a slicer or the like.

  Thereafter, the reproduction signal of the read target track is subjected to frequency characteristics suitable for the decoder 66 by the equivalent circuit 96b, for example, PR (Partial Response) characteristics when the decoder 66 is a Viterbi decoder (for example, PR (1, 2, Perform waveform equivalence to 2, 2, 1)). Thereafter, the crosstalk signal is subtracted to complete the crosstalk cancellation process.

  According to the present embodiment, when reading the signal of the adjacent track, the reproduction signal is read out by the light spot using 100% of the LD power, so that there is less noise and high accuracy compared to the three-beam type information reproduction apparatus. Crosstalk cancellation processing can be performed.

  Here, FIG. 10 shows a modified example of the crosstalk cancel circuit 68. In the example shown in FIG. 10, the output of the subtractor 97 is the input of the PLL circuit 95. Therefore, it is possible to reproduce the reproduction signal of the reading target track with a signal having a good S / N ratio after the crosstalk cancellation processing, and it is possible to reproduce information stably.

  FIG. 11 shows another modified example of the crosstalk cancellation circuit 68. In the example shown in FIG. 11, in addition to the example shown in FIG. 10, the signals after waveform equalization by the equivalent circuits 96a and 96c are input to the phase difference calculation circuits 93a and 93b. Therefore, since the phase difference is calculated by the equivalent circuits 96a and 96c using the reproduction signal matched with the frequency characteristic of the crosstalk component, the accuracy of the phase difference calculation based on the cross correlation can be improved.

  Further, according to the present embodiment, not only for three tracks but also for crosstalk cancellation processing in, for example, five tracks and seven tracks, phase difference calculation circuits, phase matching circuits, interpolators, and equivalent circuits for the number of tracks are prepared. Thus, processing can be performed without changing the optical system. In this case, although the circuit scale of the crosstalk cancellation circuit becomes large, it is possible to perform a more accurate crosstalk cancellation process.

  In addition, when three beams are generated by the optical system and the crosstalk canceling process is performed, it is difficult to satisfy the quality of the outer spot with five beams or more due to the influence of aberration. On the other hand, in the present embodiment, the crosstalk cancellation processing with five beams, that is, five tracks can also be performed, and this embodiment is advantageous also in this respect.

  Furthermore, this embodiment may be combined with a differential push-pull method for obtaining a track error signal using three beams. In this case, the information reproducing apparatus 20 may use one beam corresponding to the reading target track for the crosstalk cancellation process and three beams for tracking.

  Further, in the information reproducing apparatus, when there is an optical aberration such as radial tilt, crosstalk between adjacent tracks increases and signal quality deteriorates. On the other hand, according to the present embodiment, the influence of this optical aberration can be reduced by phase matching and waveform equivalence. For this reason, the drive margin in the information reproducing apparatus can be improved. Therefore, not only the capacity of the information recording medium can be increased, but also highly reliable reproduction processing can be performed in the current information recording medium standard.

  Here, the description has been made using the optical disk in the present embodiment, but other than this, for example, a circular information recording medium such as an HDD (hard disk drive) or a card type information recording medium as shown in FIG. This embodiment can be applied. FIG. 12 is an example of a diagram showing a card-type information recording medium.

  Further, FIG. 13 shows the effect of the first embodiment. In FIG. 13, when the crosstalk cancel processing for each track pitch is not performed on a disk having a wavelength of 405 nm and an objective lens NA of 0.65 and a shortest mark length of 200 nm, the crosstalk cancel processing described in Patent Document 1 is performed. In this case, each jitter value when the crosstalk cancellation processing of the present embodiment is performed is shown. Here, for the jitter value related to the crosstalk cancellation process described in Patent Document 1, 1/10 of the light amount of the main beam that reads the read target track is set as the light amount of the sub beam.

  According to FIG. 13, it can be seen that the jitter value is reduced in the crosstalk cancellation processing of the present embodiment compared to the crosstalk cancellation processing described in Patent Document 1. Here, when the jitter value at a track pitch of 0.40 μm when the crosstalk cancellation processing is not performed is used as a reference, the crosstalk cancellation processing described in Patent Document 1 has a track pitch of 0.32 μm, whereas in the present embodiment, In the crosstalk cancellation process, the track pitch is 0.29, and there is an effect of increasing the capacity by 10%. In addition, the present embodiment can be realized at a lower cost than the invention described in Patent Document 1 that requires a special PD.

  Further, according to the present embodiment, it is possible to increase the density in the track direction of the information recording medium. Further, the density in the track line direction can be improved by using a super-resolution reproduction technique that reads information smaller than the reading spot diameter. Therefore, according to the present embodiment, the capacity of the information recording medium can be greatly increased.

(Second embodiment)
A second embodiment of the present invention will be described below with reference to the drawings. In the second embodiment, the decoder 28eA is different from the first embodiment in comparison with the first embodiment. Therefore, in the second embodiment, only points different from the first embodiment will be described, and those having the same functional configuration as the first embodiment are the same as the reference numerals used in the description in the first embodiment. The description is omitted.

  In this embodiment, the phase difference between the reproduction signal of the reading target track and the reproduction signal of the previous track and the reproduction signal of the subsequent track is acquired from the waveform equivalent coefficients of the equivalent circuits 96a and 96c. Hereinafter, the decoder 28eA of the present embodiment will be described with reference to FIGS.

  FIG. 14 is an example of a diagram illustrating an internal configuration of the decoder 28eA of the second embodiment. FIG. 15 is a diagram for explaining the internal configuration of the crosstalk cancellation circuit 68A in the information reproducing apparatus 20A of the second embodiment.

  The decoder 28eA of the present embodiment includes an HPF 60, an equivalent circuit 61, a crosstalk cancellation circuit 68, a decoder 66, an intersymbol interference generator 63, and a subtractor 64. The intersymbol interference generator 63 and the subtractor 64 are subtracted. An error signal for calculating the waveform equivalent coefficient is obtained by the calculator 64.

  A binarized signal that is an output of the decoder 66 is input to the intersymbol interference generator 63. The intersymbol interference generator 63 generates and outputs an ideal read waveform in accordance with PR (Partial Response) characteristics from the binarized signal. The read waveform obtained here is input to the subtractor 64.

  The subtractor 64 compares and subtracts the ideal read waveform from the signal after the crosstalk cancellation processing output from the crosstalk cancellation circuit 68, and outputs an error signal of both. This error signal is input to the equivalent circuit 96a and the equivalent circuit 96c of the crosstalk cancellation circuit 68A. In the equivalent circuits 96a and 96c, using this error signal, a waveform equivalent coefficient is calculated by an adaptive equivalent algorithm such as a least mean square (LMS) algorithm or an inductive least square (RLS) algorithm.

  In the crosstalk cancellation circuit 68A, as shown in FIG. 15, the waveform equivalent coefficients calculated by the equivalent circuits 96a and 96c are input to the phase difference calculation circuits 93a and 93b, respectively. The phase difference calculation circuits 93a and 93b use this error signal to calculate the phase difference.

  In the present embodiment, the adaptive equivalent algorithm used when calculating the waveform equivalent coefficient optimizes the waveform equivalent coefficient as a correlation value between the target signal and the error signal delayed according to each coefficient. Therefore, the waveform equivalent coefficient indicates the cross-correlation between the target signal and the error signal. Here, since the error signal of this embodiment is an error signal for the reproduction signal of the reading target track, the waveform equivalent coefficient calculated by the equivalent circuit 96a and the equivalent circuit 96c includes the reproduction signal of the reading target track and the adjacent track. The cross-correlation information with the reproduced signal is included.

  Therefore, in the present embodiment, it is possible to obtain phase difference information between the reproduction signal of the reading target track and the reproduction signals of the front track and the rear track adjacent to the reading target track from the waveform equivalent coefficient obtained here.

  According to such a configuration, in the present embodiment, it is not necessary to detect the cross-correlation in the phase difference calculation circuits 93a and 93b as described in the first embodiment, and a circuit therefor is unnecessary. Therefore, the circuit can be simplified.

  Here, a method for obtaining the phase difference information from the waveform equivalent coefficient will be described.

  As a method of obtaining the phase difference information from the waveform equivalent coefficient, there is a method of outputting a shift from the maximum value of the waveform equivalent coefficient tap center as a phase difference signal. In this method, for example, Tmax−Tm is output as a phase difference signal, where Tmax is the tap position taking the maximum value of the waveform equivalent coefficient and Tm = the number of taps / 2 which is the center tap position.

  In the present embodiment, this method may be applied to phase difference calculation. In that case, the phase difference calculation circuits 93a and 93b output the deviation from the tap center of the maximum value of the waveform equivalent coefficient received from the equivalent circuits 96a and 96c, respectively, as a phase difference signal.

  Here, FIG. 16 shows an example of a waveform equivalent coefficient. In the example shown in FIG. 16, a 19-tap FIR (Finite Impulse Response) filter is used. In this case, since the center is the 10th tap, the phase difference calculation circuits 93a and 93b output a phase difference signal slightly shifted in the positive direction. Upon receiving this phase difference signal, the phase matching circuits 98a and 98b adjust the read addresses of the signals from the interpolators 94a and 94c and the memory 92, and adjust the phase so that the maximum value of the waveform equivalent coefficient is at the center of the tap. Do.

  In the present embodiment, the phase matching of the reproduction signals from the reading target track, the previous track, and the rear track is performed by the above operation. Therefore, it is possible to perform a highly accurate crosstalk cancellation process.

  As described above, the present invention has been described based on each embodiment, but the present invention is not limited to the requirements shown here, such as the shapes described in the above embodiment and combinations with other elements. With respect to these points, the present invention can be changed within a range that does not detract from the gist of the present invention, and can be appropriately determined according to the application form.

  The present invention can be applied to an information reproducing apparatus and an information reproducing method.

It is a schematic block diagram of the information reproducing | regenerating apparatus 20 in 1st embodiment of this invention. It is a figure which shows an example of the internal structure of the decoder 28e of this embodiment. It is a figure explaining the crosstalk from an adjacent track. It is a figure explaining the internal structure of the crosstalk cancellation circuit 68 in the information reproducing | regenerating apparatus 20 of 1st embodiment. 3 is a schematic diagram showing the contents of a memory 92. FIG. 6 is a diagram illustrating an example of an RF signal stored in a memory 92. FIG. 5 is a flowchart for explaining the outline of processing of a crosstalk cancellation circuit 68. It is a figure which shows an example of the circuit structure of the phase difference calculation circuit 93a. It is a figure which shows another example of the circuit structure of the phase difference calculation circuit 93a. FIG. 10 is a diagram showing a modification of the crosstalk cancellation circuit 68. FIG. 10 is a diagram showing another modification of the crosstalk cancellation circuit 68. It is a figure which shows a card-type information recording medium. It is a figure which shows the effect by 1st embodiment. It is an example of the figure which shows the internal structure of the decoder 28eA of 2nd embodiment. It is a figure explaining the internal structure of the crosstalk cancellation circuit 68A in the information reproduction apparatus 20A of 2nd embodiment. It is a figure which shows the example of a waveform equivalent coefficient.

Explanation of symbols

15 optical disc 20 information reproducing device 23 optical pickup device 26 drive control circuit 28 reproduction signal processing circuit 28e, 28eA decoder 68, 68A crosstalk cancel circuit 91 A / D converter 92 memory 93a, 93b phase difference calculating circuit 94a, 94b, 94c Interpolator 95 PLL circuits 96a, 96b, 96c Equivalent circuit 97 Subtractors 98a, 98b Phase matching circuit

Claims (10)

An information reproducing apparatus comprising: a reading unit that reads a reproduction signal of an RF signal from a reading target track; and a storage unit that stores a reproduction signal having a length of at least two tracks read by the reading unit.
The phase difference between the reproduction signal of the reading target track and the reproduction signal of n (n is an integer) track before the reading target track, which is stored in the storage means, is determined as the reproduction signal of the reading target track and the reading target track. First phase difference calculating means for calculating using a cross-correlation function with a reproduction signal before n (n is an integer) from
The phase difference between the reproduction signal of the reading target track and the reproduction signal after n tracks from the reading target track, which is stored in the storage means, is calculated after the nth track from the reproduction signal of the reading target track and the reading target track. Second phase difference calculating means for calculating based on the cross-correlation function with the reproduction signal;
First phase matching means for performing phase matching between the reproduction signal of the reading target track and the reproduction signal before the n tracks based on the output of the first phase difference calculation means;
Second phase matching means for performing phase matching between the reproduction signal of the read target track and the reproduction signal after the n tracks based on the output of the second phase difference calculation means;
An information reproducing apparatus comprising crosstalk canceling means for subtracting the reproduction signal before n tracks and the reproduction signal after n tracks from the reproduction signal of the reading target track. Each of the first and second phase difference calculating means receives a first input signal and a second input signal,
One or more delay means for delaying the first input signal for a predetermined time;
Multiplication means for calculating a product of the first input signal and the second input signal;
Multiplication means for calculating a product of the output from the delay means and the second input signal;
A plurality of low-pass filters that respectively extract low-frequency components of outputs of the plurality of multiplication means;
2. An information reproducing apparatus according to claim 1, further comprising addition / subtraction means for adding / subtracting outputs of the plurality of low-pass filters. A first input signal and a second input signal are respectively input to the first and second phase difference calculating means,
One or more delay means for delaying the first input signal for a predetermined time;
Multiplication means for calculating a product of the first input signal and the second input signal;
Multiplication means for calculating a product of the output from the delay means and the second input signal;
A plurality of low-pass filters that respectively extract low-frequency components of outputs of the plurality of multiplication means;
2. The information reproducing apparatus according to claim 1, further comprising maximum value detecting means for detecting a maximum value of outputs of the plurality of low-pass filters. A first FIR filter that waveform-equivalents the reproduction signal before n tracks;
4. The information reproducing apparatus according to claim 1, further comprising a second FIR filter that has a waveform equivalent to the reproduced signal after n tracks. The reproduced signal before n tracks whose waveform is equivalent by the first FIR filter is input to the first phase difference calculating means,
5. The reproduction signal after n tracks whose waveform is equivalent by the second FIR filter is input to the second phase difference calculation means. 6. Information playback device. The first FIR filter and the second FIR filter are adaptive filters that change a waveform equivalent coefficient according to an input signal, and the first FIR filter has a waveform included in the first phase difference calculation means. The equivalent coefficient is entered,
5. The information reproducing apparatus according to claim 4, wherein a waveform equivalent coefficient of the second FIR filter is input to the second phase difference calculating unit. The first phase difference calculating means is configured to determine a pre-read target based on a difference between a tap position at the center of the first FIR filter and a tap position at which a waveform equivalent coefficient of the first FIR filter has a maximum value. Calculating the phase difference between the reproduction signal of the track and the reproduction signal n tracks before the reading target track;
The second phase difference calculating means is configured to determine a pre-read target based on a difference between a tap position at the center of the second FIR filter and a tap position at which a waveform equivalent coefficient of the second FIR filter has a maximum value. 7. The information reproducing apparatus according to claim 6, wherein a phase difference between a reproduction signal of the track and a reproduction signal after n tracks from the read target track is calculated.   8. The information reproducing apparatus according to claim 1, further comprising a synchronous clock generating unit that reproduces a reproduction signal of the target reading track from an output signal of the crosstalk canceling unit.   An information recording means for recording information on a medium, a reproduction signal having a length of at least two tracks read by the reading means at the time of information reproduction, and recording data for buffer underrun prevention at the time of information recording 9. The information reproducing apparatus according to claim 1, further comprising an information storage unit. In an information reproducing method in an information reproducing apparatus, comprising: reading means for reading a reproduction signal of an RF signal from a reading target track; and storage means for storing a reproduction signal having a length of at least two tracks read by the reading means.
The phase difference between the reproduction signal of the reading target track and the reproduction signal of n (n is an integer) track before the reading target track, which is stored in the storage means, is determined as the reproduction signal of the reading target track and the reading target track. To a first phase difference calculation procedure for calculating based on a cross-correlation function with a reproduction signal before n (n is an integer) track,
The phase difference between the reproduction signal of the reading target track and the reproduction signal after n tracks from the reading target track, which is stored in the storage means, is calculated after the nth track from the reproduction signal of the reading target track and the reading target track. A second phase difference calculation procedure for calculating based on the cross-correlation function with the reproduction signal;
Based on the output of the first phase difference calculation procedure, a first phase matching procedure for matching a phase difference between the reproduction signal of the reading target track and the reproduction signal of n tracks before;
A second phase matching procedure for matching a phase difference between the reproduction signal of the reading target track and the reproduction signal after the n tracks based on the output of the second phase difference calculation procedure;
An information reproduction method comprising: a crosstalk cancellation procedure for subtracting the reproduction signal before n tracks and the reproduction signal after n tracks from the reproduction signal of the read target track.

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