JPH0798938A - Optical reproducing device - Google Patents

Abstract

PURPOSE:To reduce the phase lag of a reproduced signal due to the frequency characteristic of an optical head by providing a phase advancing means which advances a phase at the high-frequency area of one output of a pair of photodetectors, and an addition means which adds the output on the outer output of the photodetector. CONSTITUTION:Reflected light from a rotating optical disk 30 is received earlier timewisely than the photodetectors B, C by the photodetectors A, D. The output of the photodetectors A, D, after being current-voltage converted by an adder 2, are added, and that of the photodetectors B, C, after being current-voltage converted, are added by an adder 3. The output 21 of the adder 3 in high frequency-compensated by phase lead, etc., then, a signal 22 is outputted from the phase advancing means 4. The output S0 of an adder 5 is used as an RF reproduction signal. The lowering of amplitude and the phase lag in a high frequency area can be corrected by the phase advancing means 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for recording or reproducing on an optical recording medium, and more particularly to an optical reproducing apparatus for reproducing recorded information without phase delay.

[0002]

2. Description of the Related Art When reproducing information recorded on an optical recording medium such as an optical disk, a reproduction frequency characteristic is band-limited due to an optical characteristic of an optical pickup, so that reproduction of an optical disk recorded at high density is not possible. Intersymbol interference is particularly likely to occur. Therefore, as a method of improving the waveform characteristics of the reproduced signal, a method of increasing the gain of the high frequency component by a cosine equalization circuit has been proposed. A waveform equalization device using a neural network has also been proposed. By the way, in an exchangeable optical recording medium such as a compact disc for music, the angle formed by the optical axis of the optical head and the recording surface of the recording medium deviates from 90 ° due to exchange of the medium or the like, resulting in poor orthogonality. In this case, coma aberration occurs,
As a result, the jitter of the reproduction signal may increase, and the error rate of the reproduction signal may increase.

The above-mentioned waveform equalizer compensates for characteristic deterioration due to coma aberration when an optical recording medium such as an optical disk is tilted, and a dispersion of recording marks (hereinafter also referred to as pits) when manufacturing the optical recording medium. The main purpose is to correct the waveform distortion of the high frequency signal when the frequency characteristics are deteriorated due to variations in the characteristics of the optical head or the like, and it is intended to correct the waveform distortion without specifying the cause of the waveform distortion. . In addition, before binarizing an analog signal in a signal detection system, these equalize an RF reproduction signal (a total signal of reflected light amount by a finite aperture) output from an optical head with a certain frequency characteristic, It improves the jitter characteristic, and
The waveform characteristics are macroscopically corrected regardless of the cause of the waveform distortion of the F reproduction signal.

Hereinafter, a conventional optical reproducing apparatus will be described with reference to FIGS. 3, 4 and 5 by taking reproduction of an optical disk as an example. FIG. 3 is a diagram showing a general optical head in an optical disk device. In FIG. 3, the optical disk 30 fixed on the turntable is rotated at a predetermined rotation speed or a predetermined linear speed. When reproducing the optical disc 30, the laser light generated by the laser light source 11 of the optical head 20 is condensed by the objective lens 14 to form a light spot on the recording surface of the optical disc 30.

The signal train (pit train) on the optical disk 30 is formed in a concentric or spiral shape to form an information track. Further, since the optical disk 30 is replaceable, and there is surface wobbling or eccentricity with respect to the optical axis of the optical head 20, the laser light forms a light spot on the recording surface of the optical disk 30 and is on the center line of the information track. Focus control and tracking control are performed so as to trace accurately. The length of the pit in the pit string is modulated by the information signal so as to be an integral multiple of the reference length. In FIG. 3, the laser light emitted from the laser light source 11 is collimated by the collimator lens 12,
It is guided to the objective lens 14 via the beam splitter 13.

The laser light is condensed by the objective lens 14 to form a light spot on the recording surface of the optical disc 30 in which information pits are formed. The reflected light from the optical disk 30 is guided to the condenser lens 15 and the cylindrical lens 16 via the objective lens 14 and the beam splitter 13. This condenser lens 15 and the cylindrical lens 1
At 6, the reflected light is collected, received by the photodetector 1 and photoelectrically converted. FIG. 4 is a diagram showing a main part of a conventional optical reproducing apparatus. The photodetector 1 is a four-division photodetector as shown in FIG. 4, and by calculating the four outputs, an RF reproduction signal, a tracking error signal for tracking control, and a focus control for focus control. The error signal is detected.

In FIG. 4, the photodetector 1 is a photodetector B, A, divided by two orthogonal dividing lines x, y.
The photodetector 1 is arranged so that the dividing line x corresponds to the center line of the information track of the optical disc 30 and the dividing line y corresponds to the radial direction of the optical disc 30. ing. The arrow shown in FIG. 4 indicates the track direction of the optical disc and the moving direction of the pit row.

The outputs of the photodetectors A and D are added after being converted from current to voltage by the adder 2, respectively, and the outputs of the photodetectors B and C are converted from current to voltage by the adder 3, respectively. Will be added after. The delay circuit 7 delays the output S11 of the adder 2 and outputs a signal S31. This signal S31 is added to the output S21 of the adder 3 by the adder 8. The output S4 of the adder 8 is used as an RF reproduction signal. The delay amount in the delay circuit 7 is set in advance so that the phases of the signal S31 and the signal S21 substantially match.

FIG. 5 is a diagram showing an example of signals of respective parts in FIG. In FIG. 5, signals S1 and S2 are the same as the signal S1.
1, which corresponds to S21, the optical head 2
This is a virtual output signal on the assumption that the frequency band of 0 is sufficiently wide and there is no phase delay during reproduction. In an actual optical head, photoelectric conversion is performed by the photodetector 1 and a current is output, but the high frequency characteristics deteriorate due to stray capacitance of the photodetector 1 and the like. As shown in FIG. 5, for example, in the case of a reproduction signal when a relatively short pit is reproduced, that is, a signal of a relatively high frequency, the original signal S1 is delayed in phase by d as shown by a signal S11, and its amplitude is also reduced. It has been done. Also,
The original signal S2 has a phase d as shown by the signal S21.
However, the delay amplitude is also reduced.

Assuming that the signal S3 is obtained by adding the signals S1 and S2 without phase delay, the signal S3 is a single photodetector having an ideal frequency characteristic at the center of the photodetector 1. It becomes the same phase as the output when is placed. In this case, the signal S1 leads the signal S3 by α, and the signal S2 lags the signal S3 by α. Here, the phase α is proportional to the frequency according to the shape of the pit and the relative speed between the pit and the optical head. However, the signals S11 and S21 actually obtained are delayed in phase from the signals S1 and S2 by d and their amplitudes are also smaller. The signal S1
1 is delayed by 2α in the delay circuit 7, and the phase of its output S31 and the phase of the signal S21 are made equal.

The signals S21 and S31 are added by the adder 8 and a signal S4 having an amplitude doubled is output. This signal S4 has a phase (α + d) with respect to the signal S3.
It will be delayed only. The group delay amount by the delay circuit 7 is constant, and the phase delay amount of the signal is proportional to its frequency. As described above, if at least one pair of photodetectors obtained by dividing by the dividing line are arranged in the direction of the information track of the optical disk, and if the RF reproduction signal is obtained from the pair of photodetectors, the high frequency band is high. In this case, the reproduction signal has waveform distortion because its amplitude is reduced and its phase is delayed due to deterioration of the high-frequency characteristics of the photodetector. In such a case, there is a problem that the signal reproduced from the optical disk contains jitter, and if the jitter increases, the error rate of the reproduced signal from the optical disk increases.

For example, in the case of a compact disc for music,
It is known that if the phase in the high band is disturbed by about 20 ° with respect to the phase in the low band, there is a considerable adverse effect on the jitter. In playback of a read-only optical disc, the phase delay of the playback signal due to the optical head or the like can be approximately predicted,
Since the information pits are adjusted in position so as to compensate for the phase delay at the time of recording on the master, the above problems can be alleviated to some extent. However, if the characteristics of the reproducing system such as the optical head deviate greatly from the predicted characteristics, the above compensation becomes impossible. This is a problem that arises, for example, when a high-density optical disc and a general optical disc are used in the same optical disc device for reproduction.

On the other hand, in the case of a phase change type optical disc such as a recordable type optical disc having a management information area in which information tracks are discretely provided and other recording areas, the management information area is previously stored in the management information area. The formed clock pit (clock mark) is reproduced, and a reference clock signal phase-locked (PLL) with this synchronization signal is obtained. Since the optical disc data is read and written in synchronization with the reference clock signal, the phase delay of the reproduction signal still poses a problem. In this case, since the recording on the optical disc is performed with almost no time delay, if the clock mark cannot be accurately read without the phase delay, the clock mark previously formed in the management information area and the information recorded in the recording area are recorded. There is a problem that the phase of the pits does not match, the signal reproduced from the optical disk contains jitter, and the error rate of the reproduced signal increases.

[0014]

As described above, when a plurality of photodetectors are arranged in the time axis direction and a reproduction signal is to be obtained from the outputs of the plurality of photodetectors, the height of the optical head is increased. Due to the deterioration of the range characteristics, the reproduced signal has waveform distortion,
In particular, there was a problem in that it was accompanied by phase distortion. The phase distortion is mainly due to the delay of the phase in the high frequency region during photoelectric conversion by the photodetector, which means that the recording density of the recording medium is high and the minimum pit length is reduced, or the recording medium is widened to have a wider band. This is a particular problem when the relative speed with the optical head is increased. The optical reproducing apparatus of the present invention has been made in view of the above problems, and an object thereof is to reduce the waveform distortion of a reproduced signal due to the frequency characteristic of the optical head, particularly the phase delay.

[0015]

In the optical reproducing apparatus of the present invention, a photodetector comprising at least one pair of photodetectors, the pair of photodetectors corresponding to the information track direction of the optical recording medium. The output of the optical recording medium is read out without any phase delay by performing high-frequency compensation of one output and adding the output to the other output. That is, in an optical reproducing apparatus in which the reflected light from the optical recording medium is received by at least one pair of photodetectors corresponding to the direction of the information track, the output of one of the pair of photodetectors is high. The information recorded on the optical recording medium is provided with a phase advancing means for advancing the phase in the region and an adding means for adding the output of the phase advancing means and the output of the other of the pair of photodetectors. It is an optical reproducing device for reading out data without phase delay.

[0016]

In the optical reproducing apparatus of the present invention, at least one pair of photodetectors A and B are arranged so as to correspond to the direction of the information track of the optical recording medium. The pair of photodetectors are irradiated with the reflected light from the optical recording medium with a certain time difference. The outputs of the pair of photodetectors are both affected by the deterioration of the high-frequency characteristics of the photodetectors, and the phase is delayed by d at a certain frequency in the high band, but the reflected light is emitted later. The output of the photodetector B is subjected to high frequency compensation by the phase advancing means, and the phase is advanced by 2d at a certain frequency in the high frequency range. The output of the photodetector B is advanced and then added to the output of the photodetector A. The output of the photodetector A is in phase advance with respect to the virtual reproduction signal having no phase delay, and the signal to which the output of the photodetector B is advanced is with respect to the virtual reproduction signal having no phase delay. Since the phases are delayed and the amounts of the lead phase and the feed phase are made equal, the phase of the added signal is substantially in phase with the virtual reproduction signal having no phase delay.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical reproducing apparatus according to an embodiment of the present invention will be described below with reference to FIGS. 1, 2 and 3 by taking an optical disk reproducing apparatus as an example. FIG. 1 is a diagram showing a main part of an optical reproducing apparatus according to the present invention. Components having the same functions and functions as those in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted. The optical disk reproducing apparatus 10 of the present embodiment is characterized by the processing method of the signal output from the optical head, but the optical disk 3 shown in FIG.
Similar to the conventional example, the information reading from 0 is performed using a general optical head 20.

In FIG. 1, the photodetector 1 is a four-division photodetector built in the optical head. As in the conventional example shown in FIG. 4, the division line x is the information track of the optical disk 30. It is arranged so as to correspond to the center line. The arrow shown in the figure indicates the direction of the information track and the moving direction of the pits on the optical disk.
Therefore, the reflected light from the rotating optical disc 30 is received by the photodetectors A and D earlier than the photodetectors B and C in terms of time. The outputs of the photodetectors A and D are current-voltage converted by the adder 2 and then added, and the outputs of the photodetectors B and C are current-voltage converted by the adder 3 and then added.

In the phase advancing means 4, the output S21 of the adder 3 is compensated in the high frequency range by a phase advancing circuit or the like, and the signal S22 is output. This signal S22 is added to the output S11 of the adder 2 in the adder 5. Output S0 of the adder 5
Are used as RF reproduction signals. The phase advance means 4 corrects the decrease in amplitude and the delay in phase in the high frequency range. This situation will be described with reference to FIG.

FIG. 2 is a diagram showing an example of signals of respective parts in FIG. In FIG. 2, signals S1 and S2 are the same as the signal S1.
1, which corresponds to S21, the optical head 2
This is a virtual output signal on the assumption that the frequency band of 0 is sufficiently wide and there is no phase delay during reproduction. However, in the actual reproducing device, the output current of the photodetector 1 is affected by the stray capacitance of the photodetector 1 and the high frequency characteristics are deteriorated. As shown in FIG. 2, for example, at a relatively high frequency corresponding to a relatively short pit of an information track, the original signal S1 is the signal S11.
As shown by, the phase is such that the delay amplitude is reduced by d. Further, the original signal S2 has a phase delayed by d as shown by a signal S21 and the amplitude thereof is reduced.

Assuming that the signals S1 and S2 are added without a phase delay to obtain a signal S3 (shown in FIG. 5),
The signal S1 leads the signal S3 by α, and the signal S2 lags the signal S3 by α. However, the actually obtained signal S11,
The phase of S21 is delayed from the signals S1 and S2 by d and the amplitude thereof is smaller. The signal S21 is advanced in phase by about 2d by the phase advancing means 4, and its output S22
And the phase of the signal S11 are made substantially equal.

The phase advancing means 4 is composed of a primary phase advancing circuit as shown in FIG. FIG. 6 is a diagram showing an example of the phase advancing means. In FIG. 6, the signal S21 is input to the negative phase input terminal of the differential amplifier 41 through the resistor R1 and the capacitor C1 shown in the figure, and the output thereof is passed through the inverting circuit 42 for inverting the polarity. It is output as the signal S22. A feedback resistor R2 is connected from the output terminal of the differential amplifier 41 to the negative phase input terminal, and the phase advance means 4 is connected.
The amplification degree in the low frequency range of 1 is set to 1, but there is no phase change at an extremely low frequency. The cut-off frequency ωc of this first-order phase advance circuit is expressed as ωc = 1 / C1R1, and the amplification is increased and the phase is advanced on the high frequency side near the frequency.

The cutoff frequency when the frequency characteristic of the photodetector 1 is approximated to the characteristic of the first-order low pass filter is ωs.
Then, the cutoff frequency ωc of the phase lead circuit is set to approximately 1/2 of the cutoff frequency ωs or slightly lower than 1/2. In the first-order low-pass filter, the amount of phase delay is approximately proportional to the frequency and the amount of group delay is substantially constant on the low frequency side near the cutoff frequency ωs. In the first-order phase advance circuit, the amount of phase advance is approximately proportional to the frequency and the amount of group delay is substantially constant on the low frequency side near the cutoff frequency ωc.
For example, when the phase delay d in the photodetector 1 is 15 ° at the frequency ωp corresponding to the shortest pit, the cutoff frequency ωs of the photodetector 1 is about 3.73ωp. , Ωc is 0.464ωs (1.732ω)
(corresponding to p), which advances the phase of the signal by 30 ° at the frequency ωs and increases the amplitude by 1.15. The phase advance by the phase advancing means 4 is limited to about 40 °, and if the phase is advanced more than this, the amount of group delay is not constant, which is not suitable.

As described above, the phase of the signal S22 is set to be 2d ahead of the signal S21, and the amplitude thereof is slightly increased. Then, the signals S11 and S22 are added by the adder 5 and the signal S0 is output. The phase of the signal S0 substantially coincides with that of the virtual signal S3, and the information is transmitted from the optical disk 30 without any phase delay. Read. As described above in detail, the reproduction signal from the optical disc 30 has almost no phase delay and little waveform distortion, and, for example, the pit (clock mark) of the reference clock recorded on the optical disc is accurately reproduced. A phase-locked (PLL) reference clock signal is generated, and a reproduction signal with less jitter is obtained from the optical disk in synchronization with the reference clock, and as a result, the error rate of the signal-processed reproduction signal is reduced. These effects are not limited to the case of the diffractive type in which the information pit uses the diffraction of the reflected light, and the same effect is obtained even in the case of the phase change type in which the change of the reflectance is used.

[0025]

According to the optical reproducing apparatus of the present invention, there is almost no phase delay of the reproduced signal due to the frequency characteristic of the optical head, and the waveform distortion of the reproduced signal is satisfactorily reduced. Further, there is an effect that a photodetector having a relatively large light receiving area and high sensitivity can be used.

[Brief description of drawings]

FIG. 1 is a diagram showing a main part of an optical reproducing apparatus of the present invention.

FIG. 2 is a diagram showing an example of signals of respective parts in FIG.

FIG. 3 is a diagram showing a general optical head in an optical disk device.

FIG. 4 is a diagram showing a main part of a conventional optical reproducing device.

FIG. 5 is a diagram showing an example of signals of respective parts in FIG.

FIG. 6 is a diagram showing an example of a phase advancing means.

[Explanation of symbols]

 1 Photodetector (4-division photodetector) 2, 3 Adder 4 Phase advance means 5 Addition means 10 Optical reproducing apparatus of the present invention 20 Optical head 30 Optical recording medium (optical disk)

Claims (1)

[Claims] 1. An optical reproducing device in which reflected light from an optical recording medium is received by at least one pair of photodetectors corresponding to the direction of an information track. Recording is performed on the optical recording medium by providing a phase advancing means for advancing the phase of the output in the high frequency band and an adding means for adding the output of the phase advancing means and the other output of the pair of photodetectors. An optical reproducing device characterized by reading the recorded information without phase delay.