JPH10149549A - Multibeam optical disk device and recording/reproducing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multibeam optical disk device making possible parallel recording/reproducing of the data even for a recording disk for a single light beam and capable of increasing a recording/reproducing speed of an optical disk. SOLUTION: This device is provided with plural optical heads 221, 222 emitting light beams recording/reproducing, and is constituted so as to record/ reproduce by plural light beams. Further, the device is provided with plural control mechanisms (coarse actuator 220, etc.) with different control precision, and is constituted so as to improve moving precision of respective light beams by controlling these optical heads. By such a constitution, the matter that plural light beams are, controlled independently and precisely becomes possible, and the matter that plural light beams are used, and furthermore, the recording/ reproducing is performed on the general recording disk at high speed becomes possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk apparatus for recording or reproducing data on a recording disk in parallel with a plurality of laser beams at the same time.

[0002]

2. Description of the Related Art There has been known a method of recording and reproducing data in parallel using a plurality of light beams in order to increase the recording or reproducing speed of an optical disk. For this type of device, Digest Paper, 7th IEE, Symposium, Mass Storage System, (198
5 years) Paragraphs 17 to 21 (Digest of papers, 7th l EEE
Symposium on Mass Strage Systems (1985) PP17-21)
And so on.

[0003]

In the above prior art, a recording disk dedicated to parallel recording was required. On the other hand, the effect of increasing the speed was little for the recording and reproduction of a general-purpose recording disk that performs recording and reproduction with a single light beam.

[0004] A problem in reproducing the general-purpose recording disk with a plurality of light beams will be described below with reference to FIGS. 3 and 4.
FIG. 3 is a diagram showing a positional relationship between a recording track on the general-purpose recording disk and light spots of a recording / reproducing light beam. Data is recorded on the spiral, and the spiral corresponding to the position rotation of the recording disk is called one track.

There are two light beams, recording and reproducing light beams 1 and 2, for recording and reproducing adjacent tracks. FIG. 4 is a diagram showing a temporal position change of the light beam during reproduction. From time t1, beam 1 starts reproducing data from the head of the nth track, and beam 2 starts reproducing data from the head of the (n + 1) th track. At time t2, beam 1 reaches the end of the n-th track. Since track n + 1 has already been reproduced by beam 2, beam 1 must jump to track n + 2. However, since jumping between tracks requires a time of about several hundred μs, the n + 2, n + 3
The beginning of the track cannot be played. Similarly, at the time of recording, a part that is not recorded remains. Here, two light beams are used for the description, but even if three or more light beams are used, a portion where recording and reproduction are not performed at the time of a jump between tracks similarly occurs.

An object of the present invention is to enable parallel recording and reproduction of data even on a recording disk for a single light beam, and to increase the recording and reproduction speed of an optical disk. An object of the present invention is to provide a multi-beam optical disk device.

A second object of the present invention is to provide an optical disk apparatus capable of improving the reproducing speed even on a recording disk having many errors during reproduction.

[0008]

In order to achieve the above object, the present invention provides a means for moving a plurality of light spots of recording light or reproduction light between recording tracks independently of other light spots, By this means, another data point is moved during recording / reproduction of data by one light point.

Since one recording / reproducing light point can move between tracks independently of the other recording / reproducing light point, one light spot jumps one track after the light point jump destination track. The recording / reproduction of the light spot on the track is continued, and the recording / reproduction of the jumped light point is performed in the area where the recording / reproduction of the jumped light spot is left by the light point during the recording / reproduction. In the example of FIG. 4, the reproduction of the beam 2 is continued during the jump of the beam 1, and the area where the beam 1 is reproduced and left is reproduced by the beam 2.

[0010] This makes it possible to eliminate a recording / reproducing area due to a beam jump, so that parallel recording / reproducing with a plurality of light spots can be executed, and high-speed recording / reproducing becomes possible.

Further, when there is an error in the reproduced data by one light spot, the other light spot is jumped to the error occurrence position on the recording disk and retried, so that the reproducing operation can be continued. The reproduction speed can be improved even for a recording disk having many errors.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1, 2, 5, 6, 7, 8, 9 and 9.
This will be described with reference to FIG. 10, FIG. 11 and FIG. FIG. 2 shows a schematic configuration of a mechanism and the like, in which a recording disk 101 and a spindle motor 102 are shown.
And an actuator 103 and an optical head 104.

FIG. 5 shows the internal configuration of the optical head 04 and the electronic circuit configuration. The internal configuration of the optical head 104 is roughly divided into a focusing optical system, a first beam optical system, and a second beam optical system. Is done.

The stop-down optical system includes an objective lens 110 and fixed mirrors 112 and 113. The first beam optics is
Outputs and detects one light beam (hereinafter, referred to as beam 1), and outputs the first galvanometer mirror 114 and the first beam splitter 1
15, a first wavelength filter 116, a second beam splitter 117
, A first coupling lens 118, a first laser diode 119, a first reproduced signal detector 120, a third beam splitter 121, a first position signal detector 122, and a focus signal detector 123.
It consists of. The second beam optical system outputs and detects a second light beam (hereinafter, referred to as beam 2), and outputs a second galvanic mirror 124, a fourth beam splitter 125, a second wavelength filter 126, and a fifth beam. It comprises a splitter 127, a second coupling lens 128, a second laser diode 129, a second reproduced signal detector 130, and a second position signal detector 131. The first laser diode 119 emits light of 0.78 μm. The first wavelength filter 116 transmits 0.78 μm light,
0.83 μm light is not transmitted. The second laser diode 129
Transmits 0.83 μm light. The second wavelength filter 126 is 0.83.
μm light is transmitted, and 0.78 μm light is not transmitted. When the beam 1 from the first beam optical system is focused on the recording disk, the beam 2 from the second beam optical system is also focused on the recording disk. Therefore, in the first and second beam optical systems, lights of 0.78 μm and 0.83 μm are dominant, respectively.

The electronic circuit is divided into a servo thread circuit and a recording / reproducing system circuit. The servo circuit is a position signal detector 12
The fine position control circuits 144 and 148 for controlling the fine position of the light spot on the recording disk and the jump control between the tracks by driving the galvanometer mirrors 114 and 124 by the position signals from the position 2 and 131 and the coarse actuator 103 are driven. A coarse position control circuit 145 for controlling the movement of the optical head 104, and a focus control circuit 141 for driving the objective lens 110 by a signal from the focus signal detector 123 to focus the optical beam on the recording disk. , A seek control circuit 150 for controlling the start time of the seek and jump.

On the other hand, the recording / reproducing system circuit modulates the digital data for recording and drives the laser diodes 119 and 129, and the write modulation circuits 142 and 146, and the signals from the reproduction signal detectors 120 and 130 into digital data. Readout demodulation circuit 14 for conversion
3, 147, a sector detection circuit 149 for checking the ID (identification) of the reproduced sector, an error detection / correction circuit 152 for adding an error detection code to the data to be recorded, and performing error detection and correction of the reproduced data, and recording / reproducing. A buffer memory 155 for temporarily storing data, a host connection circuit 154 for transferring data and instructions to and from a host computer (not shown), a buffer memory 155 and a host connection circuit
154, write modulation circuits 142, 146 and read demodulation circuits 143, 14
It comprises a buffer memory control circuit 153 for controlling the transfer of data between the devices 7 and a main control circuit 151 for interpreting commands from the host computer and controlling the entire apparatus.

FIG. 6 is a diagram showing the configuration of the buffer memory 155, and includes a first buffer area (hereinafter, referred to as BE) 161 and a first buffer area 161.
The second BE162, the third BE163, and the fourth BE164 are divided into four equal parts for use. Each BE has a capacity to store data for one track, and operates as a ring buffer for the host.

FIG. 7 is a diagram showing a specific configuration of one track, which is divided into n sectors SCT0 to SCTn-1. One sector consists of ID part, DATA part, ID part and D
It is divided into GAP between ATA parts and GAP (ISG) between sectors. At the head of the ID section and the DATA section, there is a SYNC for synchronizing when demodulating the reproduction signal. Following the SYNC, the ID section is composed of IDAM indicating the ID section, TRK # indicating the track number, SCT # indicating the sector number, and CRC for detecting an error during reproduction. DATA part follows SYNC, then DAT
DTA indicating that it is part A, and D where data is recorded
It is composed of an ATA and an ECC for performing data error detection and correction during reproduction.

Next, the recording operation in this embodiment will be described with reference to FIGS. FIG. 1 is a diagram schematically showing the movement of a light beam during data recording and the flow of data from a data buffer. FIG. 8 is a flowchart showing an algorithm for moving the light beam. When receiving the command from the host computer (171), the main control circuit 151 analyzes the command (172). If the instruction is not a data read or write, the process is executed (73). If the instruction is a data write, the main control circuit 151
The seek control circuit 150 operates the coarse actuator 103 through the coarse position control circuit 145 to seek the beam 1 and the beam 2 (174). For explanation,
Assume that the instruction is a write to track 7 from track n to track (n + 6) shown in FIG. At time t10, beam 1 starts to seek the (n−1) th track from the kth track, and beam 2 starts to seek the nth track from the (k + 1) th track. The seek control circuit 150 continues the seek until the beam 1 reaches the (n-1) th track (175). When beam 1 reaches the (n−l) th track,
The seek control circuit 150 confirms that the beam 2 has reached the n-th track (176). If the beam 2 has not reached the n-th track, the seek control circuit 150 operates the second galvanometer mirror 124 via the fine position control circuit 148 to cause the beam 2 to jump to the n-th track (17).
7). At time t11 during the seek operation, reception of recording data from the host computer is started, and the buffer memory is started.
155 are stored sequentially from the first BE 161.

When the beam reaches the (n-1) th track and the beam 2 reaches the nth track, the sector detection circuit 149
Starts searching for the recording start sector using the output signals of the read demodulation circuits 143 and 147 (178). When the recording start sector is detected at time t12, data recording is started (179). The data in the first BE 161 of the buffer memory 155 is transferred to the write modulation circuit 142 by the buffer memory control circuit 153, and is recorded on the n-th track by the beam 1 through the laser diode 119. In parallel, the data in the second BE162
The data is transferred to the write modulation circuit 146, and is recorded on the (n + 1) th track by the beam 2 through the laser diode 129.

At time t13, the beam 1 finishes recording data for one track-period (181), and the beam 2 reaches the (n + 1) th track on which data recording has already been completed (18).
3) Because of this, the seek control circuit 150
Operate the first galvanometer mirror 11l4 through 4 and beam 1
Is jumped to the (n + 3) th track two tracks ahead (185). At the same time, at time t13, the buffer memory control circuit 153 switches the source of the data to be transferred to the write modulation circuit 146 from the second BE 162 to the third BE 163 182, and the beam 2 continues to record on the (n + 2) th track. When the transfer from the host to the fourth BE 164 is completed, the data is stored again from the first BE 161. At this point, the first BE
Recording of the data stored in the 161 and the second BE 162 on the recording disk 101 has been completed.

At time t14, when the beam 1 completes the jump to the (n + 3) th track and detects the head of an arbitrary sector on the (n + 3) track (186), the beam 1 starts from the sector where the head is detected. Recording of data is started (187).
At this time, the buffer memory control circuit 153 starts transfer from the area corresponding to the recording start sector of the beam 1 of the fourth BE 164 to the write modulation circuit 143.

At time t15, when the beam 2 completes recording on the (n + 2) th track (181), the buffer memory control circuit 153 starts data transfer from the beginning of the fourth BE 164 to the write modulation circuit 143 (182). , Beam 2 continues to be the (n + 3)
The data is recorded in a portion that the beam 1 of the track has recorded during the jump.

At time t16, when the beam 2 reaches the area where the recording of the beam 1 has been completed (183), the seek control circuit 150 operates the second galvanometer mirror 124 through the fine position control circuit 148, and moves the beam 2 to two tracks. Jump to the previous (n + 5) th track (185). At time t16, the beam 2 changes to the (n
When the jump to the (+5) track is completed and the head of an arbitrary sector on the (n + 5) th track is detected (186), the beam 2 starts recording data from the sector where the head is detected (187). At this time, similarly to the time t14, the buffer memory control circuit 153 starts the transfer from the area corresponding to the recording start sector of the beam 1 of the fourth BE 164 to the write modulation circuit 142. At time t18, the beam 1 reaches the area where the recording of the beam 2 is completed (183).
7) Since the track is outside the recording area (184), no jump is performed, and the write operation of beam 1 ends (188). At time t19, when the beam 2 completes the recording of the (n + 6) th track, the recording of all seven tracks from the nth track to the (n + 6) th track is completed (189), and the write operation of the beam 2 ends. (190), the processing of the write instruction ends. still,
If the number of tracks to be recorded is an even number, the preceding beam reaches the outside of the recording area first (180), and the write operation of the preceding beam ends (191), and the subsequent beam is recorded. When the write operation reaches the completion area (192), the write operation of the succeeding beam ends (193), and the processing of the write instruction ends.

The above is the operation at the time of recording. When the operation at the time of reproduction also has no error in the reproduced data, the data transfer direction is reversed, but the operations of beam 1 and beam 2 are the same. The description of the operation at this time is omitted. For explanation,
Although recording is started from the beginning of the track, recording and reproduction from the beginning of an arbitrary sector may be performed.

Next, the operation when an error is detected in the reproduced data will be described with reference to FIGS. FIG. 9 is a flowchart showing an algorithm for moving a light beam when a reproduction data error is detected. FIG. 10 is a diagram showing the movement of the light beam and the flow of data to the data buffer when a reproduction data error is detected. At time t21, data reproduction is started, data from the n-th track is reproduced by the beam 1 and transferred to the first BE 161 and data from the (n + 1) -th track is reproduced by the beam 2 and transferred to the second BE 162. .

At time t22, when the erroneous detection / correction circuit 152 detects an error in the reproduced data, data error processing is started. If the erroneous detection and correction circuit 152 determines that error correction is possible (201), it corrects the error in the reproduced data (202), continues the read operation (203), and returns to normal processing. If error correction is not possible (201), rereading is performed. First, it is determined which of the beams reproduced by the beam has an error (204). Since the data detected at time t22 at which the error was detected was reproduced by the preceding beam 1 (204), the beam 1
(207). Trailing beam 2
Continues the read operation until the beam 1 reaches the area where the reproduction has been completed (208). When the beam 1 reaches the reproduced area, the read operation is temporarily stopped without jumping (209).

At time t23, when the beam 2 reaches the sector in which the reproduced data has an error (210), the beam 2 re-reads the sector (211), and the error must be detected by re-reading. If (212), the process returns to normal processing. An error is also detected by re-reading (212), and if error correction is possible (213), an error in the reproduced data is corrected (202).
The read operation is continued (203), and the process returns to the normal processing. If error correction is not possible (213), data error processing is cascaded. However, if an uncorrectable error is detected even after re-reading for the predetermined number of retries (214), the read operation is stopped (215) and the host connection circuit is stopped.
From 154, the host is notified of the abnormal termination, and the read command processing is terminated. At time t23, an uncorrectable error is detected in the re-read data (213), and if the number of retries has not been exceeded (214), the read operation of beam 2 is continued (205), and beam 1 (n + 3) Jump back to the track (206). At this time, the data of the (n + 4) th track and the (n + 5) th track have already been reproduced by the beam 1 and stored in the first BE 161 and the second BE 162.
Rewritten by the data reproduced by the beam 2.

At time t24, the beam 1 reaches the (n + 3) th track and waits until detecting a sector having an error in the reproduced data (210). At time t25, beam 1 re-reads the sector and transfers data to the corresponding location of first BE 161. If there is no error in the reproduced data (212), the process returns to normal processing. Since the data reproduction of the (n + 4) th track has already been completed, 183, at time t26, the beam 1 jumps to the (n + 6) th track to continue the read operation.

FIGS. 11 and 12 show another example of the structure of the optical head. FIG. 11 is an arrangement diagram of the recording disk 101 and the optical head. In this configuration example, the first optical head 221 and the second optical head
222 is moved by a common coarse actuator 220. Figure
Reference numeral 12 denotes the internal configuration of the first optical head 221 and the second optical head 222 and the configuration of an electronic circuit. The first light head 221 includes a first objective lens 231, a first galvanometer mirror 232, a first beam splitter 233, a first coupling lens 234, a first laser diode 235, and a second beam splitter. 236, a first reproduction signal detector 237, a third beam splitter 238, a first position signal detector 239, and a first focus signal detector 240. The second optical head 222 includes a second objective lens 241 and a second
Galvanometer mirror 242, fourth beam splitter 243, second
Coupling lens 244 and second laser diode 245
, A fifth beam splitter 246 and a second reproduced signal detector 247
, A sixth beam splitter 248, and a second position signal detector 249.
And the second focus signal detector 250.

The electronic circuit drives the second objective lens 241 according to the signal from the second focus signal detector 250, as shown in FIG. 5, to control the focus of the light beam on the recording disk.
1 is added to the servo system circuit.

The other circuits are the same as in FIG.

As described above, according to the first embodiment, data can be recorded / reproduced in parallel to a general-purpose single spiral recording disk using two optical beams. Recording and playback speed can be almost doubled. In addition, when an error in the reproduced data is detected, re-reading of the sector in which the error has been detected can be executed in parallel with the reproduction of the normal data, so that the reliability of the data can be ensured and the reproducing speed can be improved.

Hereinafter, a second embodiment of the present invention will be described with reference to FIGS.
FIG.

The second embodiment is an example in which the present invention is applied to three light beams. FIG. 11 is an arrangement diagram of the recording disk 101 and the optical head. In this configuration example, the first optical head 301
The second optical head 302 and the third optical head 303 are moved by a common coarse actuator 304. The internal configurations of the optical heads 301, 302, and 303 and the configuration of the electronic circuit are the same as those in FIG. 12 except that a third optical head 303 is added.

FIG. 14 shows a method of moving the light beam. At time t40, beam 1 starts reading the nth track, beam 2 starts reading the (n + 1) th track, and beam 3 starts reading the (n + 1) th track. At time t41,
When beam 1 completes reading the nth track and beam 2 completes reading the (n + 1) th track, beam 1 jumps to the (n + 5) th track and beam 2 jumps to the (n + 4) th track.
Jump to the track. Beam 3 continues to be the (n +
3) Read the track. When the jump between the beam 1 and the beam 2 is completed at time t42, the beam 1 starts reading the (n + 5) th track, and the beam 2 starts reading the (n + 4) th track. At time t43, beam 3 becomes beam 2
When beam 2 reaches the area where beam 1 has been read and beam 2 has reached the area where beam 1 has been read, beam 3 is shifted to the (n +
8) Jump to track, beam 2 jumps to track (n + 7). The beam 1 continues to read the (n + 6) th track.

By repeating the above read and jump,
Read the data in the required area. The same applies to the case of a light.

FIG. 15 is a diagram showing another method of jumping the light beam. At time t50, the three light beams start reading, and at time t51, reading of data for one track is completed, beam 1 reaches the area where beam 2 has been read, and beam 2 reaches beam 3 To the area where the reading has been completed. Immediately, beam 1 jumps to track n + 4. Beam 3 continues to read the (n + 3) th track. The beam 2 jumps to the (n + 5) th track at a time t52, waiting for the end of the jump of the beam 1.

In this method, the timing of the beam jump is shifted, and the beam does not jump to two beams at the same time, so that the peak of the power consumption can be suppressed.

As described above, according to the second embodiment, data can be recorded / reproduced in parallel on a general-purpose single spiral recording disk using three light beams. Speed can be almost tripled.

[0041]

According to the present invention, the remaining recording / reproducing area caused by the jump of a light beam is supplemented by another light beam, so that data can be recorded / reproduced in parallel even on a recording disk for a single light beam. Therefore, the recording / reproducing speed of the optical disk can be increased.

Further, according to the present invention, retry can be performed in parallel while data reproduction is continued upon detection of an error in the reproduced data, so that the reproduction speed of a recording disk having many errors in the reproduced data can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a recording operation according to a first embodiment.

FIG. 2 is a configuration diagram of a first embodiment.

FIG. 3 is a diagram showing an arrangement of a recording disk and a light beam of a conventional apparatus.

FIG. 4 is a diagram illustrating the operation of a conventional device.

FIG. 5 is a configuration diagram of an optical head and an electronic circuit according to the first embodiment.

FIG. 6 is a configuration diagram of a buffer memory of FIG. 5;

FIG. 7 is a configuration diagram of tracks and sectors on a recording disk.

FIG. 8 is a flowchart of a normal recording / reproducing operation.

FIG. 9 is a flowchart of a data error process.

FIG. 10 is a diagram illustrating the operation of data error processing.

FIG. 11 is a diagram showing another configuration of the optical head.

FIG. 12 is a configuration diagram of an optical head and an electronic circuit of FIG. 11;

FIG. 13 is a configuration diagram of an optical head according to a second embodiment.

FIG. 14 is an operation explanatory diagram of the second embodiment.

FIG. 15 is an operation explanatory diagram of the second embodiment.

[Explanation of symbols]

101: Recording disk, 102: Spindle motor, 103, 220, 30
4 ... Coarse actuator, 104,221,222,301,302,303
... optical heads, 110, 231, 241 ... objective lenses, 112, 113 ...
Fixed mirror, 114, 124, 232, 242 ... Galvano mirror, 11
5, 117, 121, 125, 127, 233, 236, 238, 243, 246, 24
8 ... beam splitter, 116, 126 ... wavelength filter, 118,
128, 234, 244… coupling lens, 119, 129, 235,
245 laser diode, 120, 130, 237, 247 reproduced signal detector, 122, 131, 239, 249 position signal detector, 12
3, 240, 250: Focus signal detector, 141, 251: Focus control circuit, 142, 146: Write modulation circuit, 143, 147: Read demodulation circuit, 144, 148: Fine position control circuit, 145: Coarse position control Circuit, sector detection circuit, 150: seek control circuit, 151: main control circuit, 152: erroneous detection and correction circuit, 153: buffer memory control circuit, 154: host control circuit, 155: buffer memory

Continued on the front page (72) Inventor Seiji Honda 292 Yoshida-cho, Totsuka-ku, Yokohama-shi Inside the Microelectronics Device Development Laboratory, Hitachi, Ltd. Inside the factory (72) Inventor Jin Komatsu 2880 Kofu, Odawara-shi, Kanagawa Prefecture Inside Odawara Plant, Hitachi, Ltd. Yasushi 2880 Kokufutsu, Odawara-shi, Kanagawa Prefecture Inside the Odawara Plant of Hitachi Co., Ltd. 1-280, Central Research Laboratory, Hitachi, Ltd.

Claims (7)

[Claims] 1. An optical disk apparatus having means for converging recording light or reproduction light at a plurality of positions on a recording disk and recording or reproducing data at a plurality of positions on the recording disk at the same time. Means for moving each of the plurality of light spots between recording tracks independently of the other light spots, and moving another light spot during recording or reproduction of data by one light spot. Optical disk device. 2. The optical disc apparatus according to claim 1, wherein said condensing means condenses recording light or reproduction light at a plurality of positions by a set of objective lenses. 3. The optical disk device according to claim 1, wherein said moving means comprises a plurality of moving means having different precisions, and two or more light spots share a coarse precision moving means. 4. The recording track is spirally formed on the recording disk, and data is simultaneously recorded or reproduced on the plurality of continuous tracks by the plurality of light spots. When another light spot during recording or reproduction reaches an already recorded or reproduced area, the light spot is caused to seek to a position earlier than the position of the other light spot. Optical disc device according to 1, 2, or 3 5. An apparatus according to claim 5, further comprising: means for detecting an error in the reproduced data. When an error is detected in the data reproduced from one of the plurality of tracks on the recording disk by one light point, the other light point is detected. 5. The optical disk device according to claim 1, wherein the data is reproduced again from a position where the error is detected. 6. A multi-beam optical disk recording / reproducing apparatus for converging a plurality of recording beams or reproducing beams on a plurality of tracks spirally formed on a recording disk, and simultaneously recording and reproducing data at a plurality of positions on the recording disk. A method for recording / reproducing a multi-beam optical disk, wherein each of the plurality of beams is independently illuminated with another beam so as to be movable on the track. 7. A multi-beam optical disk recording / reproducing method according to claim 6, wherein said plurality of beams are converged on said plurality of adjacent tracks, and one of said plurality of beams is simultaneously recorded or unrecorded. A multi-beam optical disk recording / reproducing method, wherein when another beam being reproduced reaches a position where recording or reproduction has already been performed, the beam is sought to a track ahead of a track position of the other beam.