JPH041929B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for reproducing recorded information by focusing a light beam onto an optically readable information recording carrier, and particularly relates to a method for reproducing recorded information by following and scanning an information track. This paper relates to a tracking error signal generation method necessary for this purpose.

[Prior art and its problems]

In general, information such as audio, video, and various data is recorded on a record carrier by changing its optical form, for example, with or without pits, and the recorded information is read by irradiating this with a light beam at a minute spot. Optical information reproducing (recording) devices are known. Examples of such devices include optical disks known as compact disks and video disks. In optical information reproducing devices such as optical disks, one of the conditions for accurately reading information on an information recording carrier is that the light beam accurately scans the center of the information track (pit row). It is necessary to perform so-called automatic track scanning control (hereinafter referred to as tracking servo) to control the irradiation position of the light beam. This tracking servo requires a tracking error signal corresponding to the deviation (tracking error) between the tracking center and the irradiation position of the light beam. As a method of obtaining this tracking error signal, a photodetector having a light receiving surface divided into two in the tangential direction of the track detects the reflected light from the information recording carrier, and the differential signal of the output signal is used as the tracking error signal. Method (Push
-Pull method) is known.

However, in the method of using a differential signal as a tracking error signal, the influence of the tilt of the record carrier or the deviation of the drive unit due to the tracking servo drive method may be superimposed on the original tracking error. This will be explained below using FIGS. 1 and 2. FIG. 1 shows a case where the influence of the tilt of a record carrier such as a disk is superimposed on the tracking error. In this case, the optical system is such that, as shown in FIG. The information recording carrier 5 is irradiated. The light reflected from the information recording carrier 5 passes through the objective lens 4 again, is reflected by the beam splitter 3, and is guided to the photodetector 6 and detected as shown in FIG. 1b. Photodetector 6
The light-receiving surface of the information recording carrier 5 is divided into two parts at least in the tangential direction of the information track of the information recording carrier 5, and a tracking error signal is obtained by obtaining a differential signal of the photocurrent of each divided part 6a and 6b. There is. However, if the record carrier 5 such as a disk is tilted as shown in FIG. It shifts from the center. Therefore, the difference due to the shift is superimposed on the tracking error signal obtained from the photodetector 6. FIG. 2 shows a case where the influence of the deviation of the drive section is superimposed on the tracking error. The optical system is similar to that shown in FIG. 1, but is provided with a drive section 7 for driving the objective lens as a tracking servo drive system. However, if the objective lens 4 is deviated in the tracking direction (direction perpendicular to the tangential direction of the track) by this drive unit 7, the light receiving surfaces 6a, 6 are shifted as shown by broken lines in FIG. 2b.
The light beam illuminated on b is shifted from the center,
The difference due to this shift is superimposed on the tracking error signal.

Conventionally, there is a method disclosed in Japanese Patent Laid-Open No. 164448/1983 as a method for reducing the effect of the above-mentioned shift of a light beam on the light receiving surface of a photodetector. This means that the gain of one of the signals S ₁ and _{S 2} obtained from the light receiving surfaces 6 _a and 6 b of the photodetector 6 shown in FIG.
By using the differential signal S′ _B after variable control according to SA as the tracking error signal, the differential signal of S ₁ and S ₂
This reduces the shift component contained in S _B. Therefore, S′ _B becomes S′ _B = S ₁ −β・S ₂ ……(1). Here, β is a function β=f(S _A ) of _SA .

Now, if the focused light beam is accurately irradiating the track center line and the light beam on the light receiving surface is shifted as described above, each light receiving surface 6a,
The signals S ₁ and S ₂ obtained from 6b are as shown in FIGS. 3a and 3b. Each peak level of S ₁ and S ₂ is S _1p ,
S _2p and the bottom levels when modulated at high frequency by the pit are S _1L and S _2L , then the low frequency components necessary for the tracking servo of S _A and S' _B are: S _A = S _1p − S _2p ……(2) S′ _B =S _1p +S _1L /2−βS _2p +S _2L /2 ……(3) Here, assuming that the amount of shift of the optical beam is small compared to the beam diameter, the sum signal in the pit-free area is
When S _0p is 1, S _0p = S _1p + S _2p = 1 (4), and when the sum signal at the pit is S _0L , S _0L = S _1L + S _2L (5). Next, if the rate at which the light beam moves from the light receiving surface 6a to 6b is α, then the above equations (2) and (3) are as follows: S _A = (1/2 + α) - (1/2 - α) = 2α... (6) S′ _B ≒1/2 [{(1/2+α) + (1/2+α)S _0L } −β{(1/2-α)+(1/2-α)S _0L }] = 1 /2 {(1/2+α)-β(1/2-α)} ・(1+S _0L ) ...(7). To remove the shift amount of the light beam, S′ _B
= 0, so from equation (7), β becomes β = 1/2 + α/1/2 - α ... (8). Although JP-A-57-164448 does not specifically disclose anything about the conversion from S _A to β, according to this method, S ₂ in equation (1)
A gain variable circuit is required, and a divider circuit in equation (8) is also required. In this way, Tokukai Sho
The technology disclosed in Publication No. 57-164448 requires an arithmetic circuit for an analog multiplier such as an analog multiplexer, which increases the number of elements such as ICs.
Compared to other tracking error signal generation methods, this method requires more and more complicated circuits to be added, which offsets the advantage of the simple optical system of the Push-Pull method.

[Purpose of the invention]

An object of the present invention is to provide a tracking error signal generation method that easily and reliably corrects a beam shift component superimposed on a tracking error signal by taking advantage of the simplicity of the optical system using the Push-Pull method. There is a particular thing.

[Summary of the invention]

The present invention detects reflected light from an information recording carrier using a photodetector having a light receiving surface divided into two in the track tangent direction of the information recording carrier, and outputs two output signals obtained from the two divided light receiving surfaces. After generating a differential signal B, which is a signal obtained by detecting the average value of the high frequency components of the differential signal and the two output signals, or a differential signal C, which is a signal obtained by detecting the envelope of the high frequency components of each output signal, the differential signal A is generated.
and differential signal B at a ratio of approximately 1:π or the ratio of differential signal A and differential signal C to approximately 1:1 to generate a difference signal between the two signals as a tracking error signal. The purpose is to obtain a generation method.

〔Effect of the invention〕

According to the present invention, a differential signal S _A of two output signals of a photodetector obtained by the Push-Pull method and a differential signal of a signal obtained by detecting the average value of high frequency components of the two output signals
S _B is set to approximately 1:π, and a difference signal S′ _B is generated,
Alternatively, by setting the differential signal S _A and the differential signal S _C of the signal obtained by envelope detection of the high frequency components of the two output signals at approximately 1:1, and generating the difference signal S' _C as the tracking error signal, Contains no shift components,
Alternatively, a reduced tracking error signal can be generated using only a simple arithmetic circuit, and moreover, stable and reliable tracking servo can be performed.

[Embodiments of the invention]

Hereinafter, shift compensation according to the present invention will be explained as shown in FIGS. 4 and 5.
This will be explained together with the signal generation circuit shown in the figure. In the optical system as shown in FIG. 1, while the focused light beam is currently scanning the center of the track of the information recording medium, the light beam is on the light-receiving surface of the photodetector due to the reasons mentioned above. Assume that the is shifting. First, the first method of the present invention uses a signal generation circuit as _shown in _{FIG .} is multiplied by β _B = 1/π by the compensation rate adjustment circuit 10, and each output signal S ₁ and S ₂ is passed through a high pass filter 11 to extract a high frequency component, which is then averaged. After passing through the detection circuit 12 to generate a differential signal S _B of both signals,
The difference signal S'B between the differential signal S _A and _{the differential signal S B is} used as a tracking error signal. Next, a second method of the present invention uses a signal generation circuit as shown in FIG.
The differential signal S _A of each output signal S ₁ and S ₂ from b is multiplied by β _C =1 by the compensation rate adjustment circuit 10, and each output signal S ₁ and S ₂ is passed through a high-pass filter (High Pass filter).
Filter) 11 to extract the high frequency components, and then pass each of these through the envelope detection circuit 13 to generate a differential signal S _C of both signals, and then a difference signal between the differential signal S _A and the differential signal S _C. S′ _C is the tracking error signal.

Hereinafter, it will be further explained numerically that the shift compensation of the light beam is performed by the first and second methods of the present invention. Now, when the output signals S ₁ and S ₂ from the two light receiving surfaces 6a and 6b are as shown in FIG. 3, the amount of shift is smaller than the diameter of the light beam on the light receiving surface. Then, first S _{1
The differential} signal S _{A of S 2 and}
)}≒α(1+S _0L )...(9), and the differential signal S _B of the amplitude components of S ₁ and S ₂ according to the first method is S _B = 1/2π {(S _1p − S _1L ) −(S _2p −S _2L
)}≒α/π(1−S _0L )……(10). Further, the differential signal S _C of the envelope detection signal of the amplitude components of S ₁ and S ₂ according to the second method is S _C = 1/2 {(S _1p - S _1L ) - (S _2p - S _2L )
}≒α(1−S _0L )……(12). Therefore, in order to cancel the shift components included in S _B and S _C with S _A , if the compensation rate by the first method is β _B and the compensation rate by the second method is β _C , then S′ _B = S _B −β _B S _A =α/π(1−S _0L )−β _B
α(1+S _0L )=0……(12) S′ _C = S _C −β _C S _A = α(1−S _0L )−β _C α(
1+ _S0L )=0...(13) It is sufficient. That is, from equations (12) and (13), β _B ,
If β _C is set as β _B =1/π·1−S _0L /1+S _0L (14) β _C =1−S _0L /1+S _0L (15), the shift component becomes zero. In this case, S _0L
is S _0L = S _1L + S _2L , and since this value changes due to variations in the degree of modulation among the information recording carriers, it is conceivable to change the compensation rates β _B and β _C depending on each information recording carrier. Since this S _0L is unrelated to the amount of movement α of the light beam, stable tracking servo can be obtained even if β _B or β _C is fixed. In this case, β _B and β _C approach those obtained when S _0L =0 in equations (14) and (15), and β _B =1/π, β _C =1. Therefore, although the values of β _B and β _C differ slightly depending on the specifications of the record carrier and the system specifications related to the tracking servo, the allowable range can be set to ±50%.

The fact that β _B and β _C can be fixed in this way means that S _A
It is possible to obtain a tracking error signal in which the superposition of the beam shift component is corrected without the need for a gain variable circuit, and without the need for an arithmetic circuit for generating β _B and β _C , which is conventionally required.

In addition, in the above embodiment, the tracking error signal is
S′ _B = S _B −β _B・S _A , S′ _C = S _C −β _C・S _A , but this is S″ _B = S _A − S _B /β _B , S″ _C = S _A − It may be S _C /β _C. Furthermore, in carrying out the present invention, the shapes of the optical head and the light receiving surface are not limited to those shown in the drawings, and the application thereof is not limited to optical information reproducing devices, but also optical information recording devices, It can be applied not only to recording and reproducing devices but also to optical detection in systems that require tracking error detection.

[Brief explanation of drawings]

1 to 3 are diagrams for explaining the conventional method, and FIGS. 4 and 5 are diagrams for explaining an embodiment of the method of the present invention. 1...Light source, 2...Collimation lens, 3
...Beam splitter, 4...Objective lens, 5...
... Information recording carrier, 6 ... Photodetector, 7 ... Tracking servo drive mechanism, 10 ... Compensation rate adjustment circuit, 11 ... High pass filter
Filter), 12...average detection circuit, 13...envelope detection circuit.

Claims (1)

[Scope of Claims] 1. The reflected light of the light focused and irradiated onto the information recording carrier is optically detected by a photodetector having a light-receiving surface divided into two at least in the tangential direction of the information track of the information recording carrier, After generating a differential signal A of two output signals obtained from the light receiving surface and a differential signal B of a signal obtained by detecting the average value of high frequency components of the two output signals, the differential signal A and the differential signal B are generated. A tracking error signal generation method characterized in that a difference signal between both signals is generated with a ratio of approximately 1:π and used as a tracking error signal. 2. The method for generating a tracking error signal according to claim 1, wherein the information recording carrier is an optical disk. 3. The tracking error signal generation method according to claim 1, wherein the light source of the focused irradiated light is a semiconductor laser. 4. The reflected light of the light focused and irradiated onto the information recording carrier is detected by a photodetector having a light-receiving surface divided into two at least in the tangential direction of the information track of the information recording carrier, and the light is obtained from the divided light-receiving surface. After generating a differential signal A of two output signals and a differential signal C of a signal obtained by envelope detection of high frequency components of the two output signals, the ratio of the differential signal A and the differential signal C is set to approximately 1: 1. A tracking error signal generation method characterized in that a difference signal between both signals is generated as a tracking error signal. 5. The tracking error signal generation method according to claim 1, wherein the information recording carrier is an optical disk. 6. The tracking error signal generation method according to claim 1, wherein the light source of the focused irradiated light is a semiconductor laser.