JPH0685225B2 - Tracking error signal detector - Google Patents

Description

The present invention relates to a method for detecting a tracking error signal in an optical disc or the like that reproduces information by light.

[Background technology and its problems]

An optical disk reproducing apparatus that irradiates light (laser light or the like) on an optical disk, which is a recording medium, and reads the information recorded on the optical disk from the return light, has at least a tracking servo for tracking a spiral recording track. And a device for detecting a tracking error signal for knowing which part of the recording track the position where the light spot is radiated is provided.

FIG. 1 (a) is a principle diagram showing an example of a method (Push Pull method) for detecting such a tracking error signal. Reference numeral 1 is an optical disk board, and 2 is an optical spot irradiating the optical disk board 2 It is a part of the shaft mechanism.

An objective lens 3 and a focus drive coil (not shown) for driving the objective lens 3 in the vertical direction, a tracking drive coil for driving the objective lens 3 in the radial direction, and the like are provided inside the biaxial machine 2. There is.

A polarization beam splitter 4 is attached to the objective lens 3.
Laser light is incident from the light emitting source 5 via the.

The reflected light reflected by the optical disc board 1 goes straight through the polarization beam splitter 4 and is incident on the light receiving element 6.

An optical system for detecting the RF signal and the focus error signal from the reflected light is omitted.

As shown in FIG. 1 (b), the light-receiving element 6 has a light-receiving surface composed of A and B divided in two in the track direction.
The light diffracted by the pits formed on the optical disk board 1 at A and B and reflected is received. Therefore, the intensity distribution of the light incident on the light receiving surfaces A and B of the light receiving element 6 changes depending on the position of the pit P.

That is, as shown in FIGS. 2 (a), (b), and (c), when one pit P passes through the light spot irradiated on the optical disc board 1, the reflected light incident on the light receiving element 6 is reflected. The light intensity distribution of the spot changes like S ₁ , S ₂ , and S ₃ .

In this case, as shown in FIGS. 2A and 2C, in the detrack state where the recording track of the optical disc board 1 and the center of the objective lens 3 do not coincide with each other, the light intensity of the light receiving surfaces A and B is increased. The distributions are non-uniform on the left and right, and when they match as shown in FIG. 2 (b), they are symmetrical on the left and right.

Therefore, the detection signals output from the light receiving surfaces A and B are
When input to the subtraction circuit 7 shown in the figure, the tracking error signal can be detected from the difference output of A-B.

Such a tracking error signal detection method is generally called a push-pull method or a far-field method and has an advantage of simplifying an optical system.

However, when the tracking servo is performed by such a method, even when the detracking state occurs as shown in FIGS. 2 (a) and 2 (c), the reflected light spots S _{1 to} S ₃ are reflected by the light receiving surface A, B
If the objective lens 3, the polarization beam splitter 4, and the light receiving element 6 are installed in the biaxial mechanism 2 so as to be symmetric with respect to the dividing line, the problem will occur. , Light receiving element 6, or light receiving element 6
In a device in which the polarization beam splitter 4 is fixed and only the objective lens 3 moves in the track direction, the positions of the reflected light spots S _{1 to} S ₃ of the light receiving element 6 in the case of detracking are shown in FIG. , (C) as shown by the alternate long and short dash line, there is a problem that a DC offset occurs in the tracking error signal and an accurate tracking error signal cannot be detected.

That is, as shown in FIG. 3A, when the biaxial mechanism 2 is moved (traversed) in the track direction, a positive / negative tracking error signal component et should be output every time the recording track is passed. 6 reflected light spot S ₁
When S ₃ is shifted, the DC offset signal ed is superimposed on the tracking error signal component et as shown in FIG. 3 (b).

Then, the subtraction circuit 7 does not always output an accurate tracking error signal.

Therefore, normally, in such a tracking error detection method, the polarization beam splitter 4 and the light receiving element 6 are also housed in the biaxial mechanism 2 and are configured to move together with the objective lens 3 by a tracking coil. There is a problem in that the weight of the biaxial mechanism 2 increases and the size increases.

[Object of the Invention]

The present invention has been made to solve such a problem, and provides a tracking error detection method capable of removing a DC offset component in a push-pull tracking error detection method in which a light receiving element is a fixed type. It is provided.

[Outline of Invention]

According to the present invention, in a push-pull tracking error detection method in which a light receiving element is fixed, a tracking error signal including a DC offset component output from the light receiving element is pitted by a first and a second sample hold circuit. It is designed to remove the DC offset component contained in the tracking error signal by separating and sampling the signal of the part and the signal of the land part and calculating the held voltage by the differential amplifier circuit. is there.

Therefore, the DC offset component can be eliminated even in the push-pull type tracking error signal detection method in which the light receiving element is fixed.

〔Example〕

FIG. 4 is a block diagram of a tracking error signal detection system according to an embodiment of the present invention, in which 6 is a light receiving element irradiated with the reflected light spot of the optical disk, and 7 is a tracking signal detection device. Is a subtraction circuit for.

Reference numeral 8 indicates an adder circuit that outputs the sum of the detection signals of the light receiving surfaces A and B, and reference numerals 9 and 10 indicate first and second sample and hold circuits for sampling and holding the tracking error signal.
As will be described later, the first sample and hold circuit 9 outputs a tracking error signal when the light beam irradiates the pits, and the second sample and hold circuit 10 outputs the light beam between the pits, that is, in the land portion. It samples the signal when it is illuminated.

11 is a differential amplifier circuit for detecting a difference signal, 12 is an RF signal waveform shaping circuit, 13 is a delay circuit, and 14 and 15 are edge pulse generation circuits that generate pulses at the rising and falling points of the RF signal. Is.

Reference numeral 16 denotes a sample voltage selection circuit, which is composed of a third sample hold circuit 17 and a timing circuit 18. The sorting circuit 16 detects a more accurate tracking error signal and can be omitted.

Next, the circuit operation of FIG. 4 will be described with reference to the waveform chart of FIG.

When the shape of the pit information of the optical disk is formed by long and short pits as shown by PM in FIG. 5, the output of the adder circuit 8 which detects the reflected light is the light amount at the pit portion as shown by the RF signal. Is decreased, and the amount of light is increased in the non-pit part.

When this RF signal is shaped in the waveform shaping circuit 12, it becomes a D ₁ waveform, and when it is delayed (τ) in the delay circuit 13 by the length of the irradiation spot of the light beam, a D ₂ waveform is obtained.

When the rising and falling points of this D ₂ waveform are detected by the edge pulse generation circuits 14 and 15, pulse waveforms P ₁ and P ₂ are obtained from the outputs.

The pulse waveforms P ₁ and P ₂ are supplied as sampling pulses for the first and second sample and hold circuits 9 and 10.

The subtraction circuit 7 is included in the first and second sample hold circuits 9 and 10.
Since the tracking error signal output from the first sample hold circuit 9 is supplied from the first sample hold circuit 9,
As shown in the PM waveform in the figure, a tracking error signal in the state of the spot Sp where the light beam is applied to the pit portion is output, and the second sample and hold circuit 10 outputs the light beam between the pits, that is, the land. The signal at the spot Sm in the portion of is output.

By the way, as described above, in the push-pull method, the tracking error signal cannot be detected when the light beam receives the spot Sm on the land portion. However, when the spot Sm is not on the recording track, that is, in the detrack state in which the optical axes of the objective lens 3 and the light receiving element 6 do not coincide, the light receiving element 6 shifts the spot as described above, and thus the DC offset is generated. The component (ed) should be detected.

On the other hand, a tracking error signal component (et) is obtained from the spot Sp when the light beam is on the pit, and at the same time, a DC offset component (ed) is also included when this spot Sm is not in the center of the recording track. Has been.

Therefore, the above-mentioned first and second sample hold circuits
When the signals obtained from 9 and 10 are input to the differential amplifier circuit 11 to cancel the DC offset component (ed), the differential amplifier circuit
Only the tracking error signal can be obtained from 11. In this case, it is preferable to adjust the output level of the second sample hold circuit 10 by the level adjusting circuit (11a) in order to completely cancel the DC component.

Since the irradiation spot diameter on the optical disk is usually larger than the length of the shortest length pit,
As shown in the figure, the sampling voltage after the shortest long pit or after the shortest long space may detect the tracking error signal in a state where it is on only a part of the pit as seen in the spot Se.

Therefore, in order to remove the sample voltage detected at such a spot Se, a third sample hold circuit 17 is provided. Then, the timing circuit 18 forms the pulse waveform P ₃ shown in FIG. 5, which is used as the third sample hold circuit.
In addition to 17, it is preferable to reselect the sample voltage output from the differential amplifier circuit 11.

In this case, the timing circuit 18 is configured to output the sampling pulse (P ₃ ) when the pit signal or the space signal is long, for example, 2T or more, and the pulse waveform
Do not capture the voltage sampled by the X mark of P ₁ and P ₂ .

Note that too long pit signals and space signals may be caused by optical disc dropouts or scratches. Therefore, for example, do not output the sampling pulse (P ₃ ) for pit signals of 13T or more or space signals. You may

The means for forming the sampling pulses (P ₁ , P ₂ ) is not limited to the above embodiment, and other circuit configurations can be adopted.

〔The invention's effect〕

As described above, according to the tracking error detection method of the present invention, even in the case of the push-pull tracking error detection method in which the light receiving element or the light receiving element and the polarization beam splitter are fixed outside the biaxial mechanism, Accurate tracking error because the reflected light of the irradiated light beam is divided into the pit part and the land part, and the subtraction output signal is used to obtain the tracking control signal with the DC offset component electrically removed. There is an advantage that a signal can be detected.

As a result, the biaxial mechanism is lightened and the size is reduced, so that the tracking servo function is improved.

[Brief description of drawings]

1 (a) and 1 (b) are schematic diagrams of an optical system showing a tracking error signal detection mechanism by a push-pull method, and FIGS. 2 (a), 2 (b) and 2 (c) are incident on a light receiving element. Explanatory drawing showing the state of the reflected light spot, FIG. 3 (a),
FIG. 4B is a waveform diagram of a tracking error signal when the objective lens is traversed, and FIG. 4 is a block diagram showing an embodiment of a tracking error signal detection system according to the present invention.
FIG. 5 is a main waveform diagram showing the operation of FIG. In the figure, 1 is an optical disc board, 2 is a biaxial mechanism, 3 is an objective lens, 4 is a polarization beam splitter, 5 is a light emitting source, 6 is a light receiving element, 7 is a subtraction circuit, 8 is an addition circuit, and 9 and 10 are 1, 1st and 2nd sample hold circuits, 11 shows the differential amplifier circuit.

Claims (1)

[Claims] 1. A light receiving element having first and second light receiving surfaces for receiving return light from the optical disk, which is arranged with a track of the optical disk interposed therebetween, and a detection output from the first and second light receiving surfaces. A first subtraction circuit that outputs a difference signal, an addition circuit that outputs a sum signal of the detection outputs from the first and second light receiving surfaces, and a sum signal that is output from the addition circuit is irradiated with a light beam. A delay circuit that delays by the length of the spot, a detection unit that detects the rising and falling points of the sum signal output from the delay circuit, and a detection unit that detects the rising point of the sum signal. A first sample and hold circuit that samples the difference signal from the first subtraction circuit based on the output, and the first sample and hold circuit based on the detection output from the detection means that detects the falling point of the sum signal. A second sample and hold circuit for sampling the difference signal from the subtractor circuit and a second subtractor circuit for calculating the difference between the outputs from the first and second sample and hold circuits are provided. A tracking error signal detecting device for detecting a tracking error signal for driving an objective lens.