Optical disk signal reproducing apparatus and crosstalk...

Details Optical disk signal reproducing apparatus and crosstalk... Optical disk signal reproducing apparatus and crosstalk...

1999-09-28

2002-08-27

Hindi, Nabil (Department: 2653)

Dynamic information storage or retrieval

Control of storage or retrieval operation by a control...

Control of information signal processing channel

C369S053330, C369S124030, C369S124140

Reexamination Certificate

active

06442114

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical disk signal reproducing apparatus which is designed to irradiate a plurality of laser beams onto an optical disk medium having a high packing recording density and to reproduce information signals recorded therein, and in particular to a crosstalk canceling system for use in the signal reproducing apparatus for canceling crosstalk components of main track information from its adjacent track information by adjusting time differences of individual detected multibeam signals so that the respective reproduced signals have substantially no time lag to accurately cancel the crosstalk components.
2. Description of the Prior Art
In recent years, with the tendency of forming the optical disk medium into high density, a more advanced technique has been required for the optical disk signal reproducing apparatus. Hereinafter, referring to the drawings, explanation is given on an example of the conventional optical disk signal reproducing apparatus.
FIG. 7
shows a block diagram of a conventional optical disk signal reproducing apparatus.
In
FIG. 7
, laser beams L
1
, L
2
, and L
3
are irradiated respectively on the tracks T
1
, T
2
, and T
3
formed onto an optical disk medium or carrier
100
, and their reflective light beams are projected respectively onto the light receiving elements such as photo detectors
111
,
121
, and
131
, and are outputted as the electric detection signals S
1
, S
2
, and S
3
, respectively. Assuming that the track from which the play-back information signal is desired to be actually received is T
2
, the laser beams L
1
, L
3
(sub-beams) are for canceling the crosstalk components leaking in from the adjacent tracks T
1
, T
3
while the laser beam L
2
(main beam) scans the track T
2
. In other words, in obtaining the information reproducing signal SX, gain adjusters
113
,
133
and addition/subtraction operational amplifier
130
perform the following operation:
SX=S
2
−
k
×(
S
1
+
S
3
)
and the crosstalk components are electrically canceled.
However, owing to the following reasons, the three laser beams to be irradiated on the optical disk medium are required to be positioned at some distance in the tangential direction of the tracks to a certain extent, so that it is not possible to cancel the crosstalk only by the simple addition/subtraction as described above. That is to say, ideally, the laser beams L
1
, L
2
, and L
3
are required to be irradiated in adjacent relations in the track radial direction. By so practicing, because the track pitch is narrow, the respective laser beams overlap one another, with the inevitable consequence that the beams to be projected onto the light receiving elements mutually overlap, making it impossible to receive the respective beams independently by the light receiving elements
111
,
112
, and
113
. Therefore, it becomes necessary to dispose the respective beams at a certain distance in the tangential direction. At this time, however, there arise time differences in the output signals of the light receiving elements
111
,
121
, and
131
in proportion to the separation distance in the tangential direction. Accordingly, the time differences that occurs here are corrected by the first and second variable delay elements
112
,
122
.
In fact, however, it is difficult to control strictly the mutual distances between the laser beams L
1
, L
2
, and L
3
. Even if they could be successfully controlled, it is necessary to set the delay amounts to be generated by the above variable delay elements
112
,
122
to be always in optimum condition, because the time differences fluctuate depending on the rotational linear velocity of the optical disk medium. The multiplier
141
detects the correlation between the output signal of the light receiving element
131
and the output signal of the second variable delay element
122
, and the multiplier
142
detects the correlation between the output signal of the second variable delay element
122
and the output signal of the first variable delay element
112
, respectively. A maximization control means
136
carries out the delay adjustments of the above delay elements
112
,
122
, respectively, which is disclosed, for example, in the Japanese Patent KOKAI Publication HEI7-176052.
According to the above constitution, however, there are such problems that it is difficult to realize such delay elements
112
,
122
that should satisfy both the delay amount and the delay precision, and in the case of using the multipliers
141
,
142
, the delay amounts cannot be adjusted exactly to the optimum amount because of low detection sensitivity of the correlation signals.
These subjective problems are analyzed as follows. First, the distance between the laser beams L
1
and L
2
in the track tangential direction or the distance between the laser beams L
2
and L
3
in the track tangential direction is required to be about 10 &mgr;m, in consideration of the configurations and dispositions of the light receiving elements
111
,
121
, and
131
. On the other hand, the recording density of the information recorded in the optical disk medium is, taking an example of a DVD (digital video disk) having PWM recording using, for example, 8-16 modulation codes, about 0.1 &mgr;m per 1 channel bit. Accordingly, even the distance of 10 &mgr;m corresponds to a difference of as much as 100 bits.
As to the distinctive means for delaying, there has first been considered a constitution to make a series connection of a plurality of analog delay elements in a ladder form. However, the analog delay element itself has a group delay characteristic, and according to a trial calculation, the delay amount in which the group delay does not matter is at most about 10 bits, and it is almost impossible to realize a delay of 100 bits.
As a method for obtaining a delay amount, there can be considered, for example, a sample and hold type filter such as a switched capacitor filter or a digital filter. By using such a filter, it is possible to maintain group delay without respect to the delay amount. However, as the delay precision is determined by the sampling clock in the sample and hold type filter, if it is desired to elevate precision, the sampling clock frequency must be increased, in proportion to which the number of sample holding elements and the operation speed must be increased. In case of using a channel clock for the sampling clock, as 1 clock corresponds to 1 bit delay, if it is desired to make a delay of 100 bits, there are required 100 pieces of sample holding elements, and yet the respective sample holding elements are required to operate at 27 MHz in case of the DVD reproduction at standard speed.
However, in the case of using a channel clock, the delay amount is variable only at the rate of one bit by one bit, and there should be problems in the point of the delay precision. According to an experiment, in order to make idealistic cancellation of crosstalk, matching of the delay of at least ¼ precision is necessary. When it is desired to realize this state by sample holding, the sampling frequency is required to be 108 MHz or four times the channel clock, and the necessary sample holding elements are increased by a factor of 4 (i.e., 400 elements).
SUMMARY OF THE INVENTION
The present invention has been developed to solve these problems and has an object to provide an optical disk signal reproducing apparatus wherein the optimum delay amount can be set very precisely with a simple construction of a lower cost, having a sufficient resolution accuracy, by realizing necessary delay adjustments by using rough delay means of a digital control system constituted by a shift register and fine delay means of an analog control system constituted by group delay filters, and further by executing search of the optimum delay amount by using jitter detecting means.
In order to achieve the object mentioned above, the present invention provides an optical disk signal reproducing app

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