JPS62252537A - Focus error detecting system for optical disk device - Google Patents

Abstract

PURPOSE:To obtain a stable focus error signal without an off-setting change by holding the error at the time of reading and separating the band with a conventional error output and executing the synthesizing. CONSTITUTION:By weighting and synthesizing a focus error signal S2 used conventionally and a sample value S1 at a frequency band by an amplifying means 18 with a bank limit, a focus error signal S3 to satisfy both two problems of making a gain constant and removing the influence of an off-setting can be obtained. The focus error signal S3 obtained in such a way is composed of a sample value S1 at the area where the influences of the change of the off-setting are many and the conventional focus error signal S2 at the high area where the influences due to the change of the off-setting are few. Thus, the influence of the off-setting to a focus control is significantly decreased and a sufficient control band can be obtained.

Description

[Detailed Description of the Invention] [Summary] The present invention solves the problem that the offset amount differs when reading/writing information in a focus servo error detection circuit of an optical disk device by holding the error during reading. Then, by band-separating and synthesizing the conventional error output, it is attempted to obtain a stable focus error signal free from offset fluctuations.

[Industrial application field]

The present invention relates to a focus error detection circuit for an optical disc device.

[Conventional technology]

In a writable optical disc, light of low power is irradiated onto the disc medium during reading, and light of high power is irradiated to the disc medium during writing. Therefore, when obtaining the focus error signal due to the reflected light from the medium, the output value and offset amount from the detector will differ, so stable servo control is difficult.
To do this, it is necessary to ensure that these values remain constant throughout the read/write of information.

FIG. 5 shows the configuration of a conventional focus error signal detection circuit in a recordable optical disc device. Conventionally, measures have been taken to keep the gain constant, but no methods have been taken to keep the offset constant.

As shown in the figure, the focus error signal is obtained by receiving the amount of reflected light 5 from the medium by a pair of photodetectors 1, and by taking the difference between the outputs by a differential amplifier 2.

In reality, optical techniques such as a masking method and a cylindrical method are required before receiving light, but these are not related to the present invention and will therefore be omitted.

Read-only (such as a CD (compact disc))
This is all that is needed for a read-only (read-only) device, but for a recordable device, in order to increase the power of the emitted light when writing information, the output of the dynamic amplifier 2 is increased as shown in Figure 6(a). Information light.

At the time [Curve A], it is thicker (,
Therefore, detection sensitivity increases.

This detection sensitivity is approximately proportional to the amount of reflected light from the medium, 5°.
Since the amount of reflected light 5 is proportional to the sum of the respective outputs of the pair of photodetectors 1, by passing it through the divider 3 with the sum signal output from the adder 4 as the denominator,
As shown by curves A and B in FIG. 6(b), error signals with constant sensitivity are obtained.

However, in addition to the gain (sensitivity), the error detection circuit has the problem of an offset that prevents the output of the detector from becoming zero at the just focus point (point P in FIGS. 6(a) and 6(b)). If this offset is always constant, it can be canceled by adding a DC voltage, but normally it is caused by stray light that does not irradiate the medium and enters the photodetector 1, fluctuations in the power distribution due to read/write, or the divider itself. This is caused by complex factors such as offsets, so as shown in Figure 6 (C), it varies depending on the time of reading and writing, and therefore, as shown in Figure 6 + d), the division that adjusts the gain is necessary. Normally, the output of device 3 also does not match.

[Problem that the invention seeks to solve]

As mentioned above, conventional focus error detection circuits cannot cancel offsets during information read/write, and must carefully align optical components and photodetector positions so that offsets do not occur, or It was also necessary to use an expensive divider.

Therefore, it is an object of the present invention to provide a focus error detection method that is not affected by the above offset.

[Means for solving problems]

Most of the writable optical disc devices currently in use are preformatted and pregroove media. Since the present invention is also based on a preformatted and pregroove disc as the optical disc medium, this medium will be described.

A pregroove is a positioning groove carved spirally or concentrically on a disc, and information is recorded on or between these grooves.

The pre-format is a pre-groove section in which information such as a trunk number, sector number, etc. is recorded at dozens of locations on the circumference, depending on the presence or absence of pits, and is also called a pre-pit section or ID section. The preformat section of the slave 4 lever is always in the read state, and the ID indicates that the preformat section is currently being read from the information reproducing circuit.
A signal is emitted.

In order to keep the offset constant throughout read/write, which is the purpose of the present invention, by utilizing the fact that the preformat section is always in the read state, and by holding the focus error signal in the preformat section, it is possible to The signal will not be used, and the offset will not change.However, currently the maximum sampling frequency is around 1KHz, so unless the number of preformat parts per cycle is extremely increased, the sample value alone is sufficient control. Bandwidth cannot be secured.

Therefore, in the present invention, as shown in FIG. 1, the conventionally used focus error signal S2 and the sample value S8 are weighted and synthesized by the frequency band by the band-limited amplification means 18, thereby increasing the gain. A focus error signal S= that satisfies both of the two problems of constantization and removal of the influence of offset can be obtained.

The conventionally used focus error signal St
detects an error in the signals P, , P using the sum signal E obtained by adding the signals P, , Pt output from the pair of photodetectors 1 upon receiving the reflected light 5 in the adder 4. It is obtained by dividing the difference signal obtained by processing in the means 12 by the variable gain amplification means 13.

On the other hand, the sample value SI is obtained by sampling the difference signal while the sample/hold means 17 is reading the preformat section in accordance with the signal I indicating that the preformat section is being read; Other periods can be obtained by holding this.

[For production]

The focus error signal S, obtained as described above, has a sample value St in the low range, which is more affected by offset fluctuations.
As a result, the high range, which is less affected by offset fluctuations, is constituted by the conventional focus error signal St, so the influence of offset on focus control is significantly reduced, and a sufficient control band can be obtained.

〔Example〕

An embodiment of the present invention is shown in FIG. 2. In the figure, 6 is an ID detection circuit that detects that the preformat section is being read, 7 is a sample/hold circuit, 8 is an adder, and 2
1 is a bypass filter, 22 is a low-pass filter, and other parts that are the same as those of the conventional example shown in FIG. 5 are given the same numbers.

In this embodiment, the ID section detection circuit 6 outputs a signal 'I indicating that the preformat section is currently being read. When this signal I is input, the sample/hold circuit 7 samples the difference signal at that time, and holds the sampled difference signal during that time. The outputs SI and 1 of the sample/hold circuit 7 and the output of the divider 3, that is, the conventional focus Morar signal S2, are respectively filtered by a low-pass filter 22. By sending the signals to the adder 8 via the bypass filter 21, the focus error signal S in this embodiment is obtained by weighting and adding them according to the frequency band.

FIGS. 3(a) and 3(b) show focus error signals S obtained in this manner. , In principle, the above weighting can be done by increasing the specific weight of SI in the low frequency range.If the low frequency side of S is canted using a bypass filter as described above, the result as shown in Fig. 3(a) is obtained. Flat characteristics can be obtained over the entire control band.

However, the low frequency side of the S8 does not necessarily need to be cut as seen in the same figure price), and in any case, the fluctuation in offset amount due to read/write is included only in the S2, so the conventional 88 Compared to a focus error signal based only on the focus error signal, offset fluctuations in the low range are significantly compressed.

Of course, the offset fluctuation can be more compressed by doing as shown in Figure 3(a), but even if the sensitivity of the added result S differs depending on the frequency, as shown in Figure 3(a), it is still lower than the control band. No problem.

Each part of this embodiment shown in FIG. 2 corresponds to FIG. 1 as follows. That is, the differential amplifier 2 includes an error detection circuit 12, a sample/hold circuit 7, a sample/hold means 17, an adder 8. High bass filter 21. In addition, the low-pass filter 22 constitutes the band-limited amplification means 1B.
Configure.

FIG. 4 shows another embodiment of the present invention, in which the input of the sample and hold circuit 7 is inputted from each of the pair of photodetectors 1 to the corresponding peak hold circuits 9 and 9°, compared to the embodiment of FIG. The signal is sent to the differential amplifier 2 via the differential amplifier 2, whereby the difference signal between the two peak values is detected and obtained. This embodiment differs from the previous embodiment only in this point.

Note that the difference signals input to the divider 3 are the respective detection outputs P+,
Since a Pt difference signal is required, in this embodiment, a differential amplifier 2'' is provided in addition to the differential amplifier 2, and the output difference signal D° is input to the divider 3.

Even in the preformat section, since the amount of reflected light decreases due to diffraction in the part with information pits, the sample hold circuit 7
The output becomes a focus error signal for a portion of the preformat section where there are no information pits. Therefore, according to this embodiment, the influence of the so-called crosstalk problem, in which light is diffracted due to information pits and pregrooves and becomes noise, is also reduced on the focus error signal S3.

In the other embodiment shown in FIG. 4, two types of differential amplifiers D and D' are used, but D'' is eliminated and the input of the divider 3 is taken from the other differential amplifier D. However, in this case, it is necessary to be careful in selecting the time constants of the peak hold circuits 9, 9'.

[Effects of the Invention] As described above, according to the present invention, it is possible to reduce the effects of offset fluctuations and crosstalk that are mixed into the focus error signal due to read/write.

[Brief explanation of drawings]

Fig. 1 is a detailed explanatory diagram of the present invention, Fig. 2 is an explanatory diagram of the configuration of an embodiment of the present invention, Fig. 3 is an explanatory diagram of the sensitivity of the focus error signal in the above embodiment, and Fig. 4 is an explanatory diagram of the present invention. FIG. 5 is an explanatory diagram of a conventional focus error detection circuit, and FIG. 6 is an explanatory diagram of the characteristics and offset of a focus error signal, which are problems of the conventional technique. In the figure, 1 is a pair of photodetectors, 2 is a differential amplifier, 3 is a divider, 4 and 8 are adders, and 7 is a sample/amplifier.
12 is an error detection means, 13 is a variable gain amplification means, 17 is a sample/hold means, 18 is a band-limited amplification means, 2
1 is a bypass filter, 22 is a low-pass filter, P
+, Pt are signals indicating the amount of detected light, D, D' are difference signals, E is a sum signal, S, , S. are the sample value and the difference signal at constant gain, respectively, and Ss is the focus error signal obtained by the present invention. 4 adder Basic configuration explanatory diagram of the present invention Fig. l

Claims (2)

[Claims] (1) A pre-formatted pre-groove disk is used as a recording medium, the amount of reflected light (5) from the disk is detected by a pair of photodetectors (1), and the detection output (
In an optical disk device that detects a focus error based on a difference signal (D) between P_1 and P_2), the sample/hold method samples the difference signal (D) while reading the preformat section and holds it during other periods. means (17) and the difference signal (D) to the detection output (P_
1, P_2), and the output (S) of the sample/hold means (17) and the output of the variable gain amplification means (13). (S_2) is weighted by the frequency band and the output (S_3) of the band-limited adder (18) is
) is used as a focus error signal. (2) Peak hold means (9) for holding the peak value of each output of the pair of photodetectors (1).
, 9'), and the difference between the outputs of the peak hold means (9, 9') is used as a difference signal (D) to be input to the sample/hold means (17). A focus error detection method for an optical disc device according to scope 1.