JPH02273326A - Optical recording and reproducing device - Google Patents

Abstract

PURPOSE:To prevent malfunction in the lead-in of a focusing servo by starting an AGC action at the point of time when a focusing sum signal exceeds a reference voltage level. CONSTITUTION:Laser light reflected by the surface of a medium is splitted into two by a two-splitting photodetector 1. The divided signal is amplified by a differential amplifier 2 and a summing amplifier 3 and inputted in an AGC circuit 4. At the same time, output from the summing amplifier 3 is compared with Vref1 by a comparator 5. When it is smaller than the Vref1, a switch 41 is turned off and the reference voltage is impressed on the AGC 4, which works as an amplifier with fixed gain. When it exceeds the Vref1, the output of the comparator 5 is switched to set an FF 6. The switch 41 is switched to a focusing sum signal side with the output of the FF 6. When the switch is switched, the AGC 4 outputs a signal which is a focusing error signal/focusing sum signal.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical recording/reproducing device having an optical head that focuses and reflects laser light onto a medium surface and converts the reflected light into an electric signal, particularly a focus pull-in detection device. Regarding.

[Conventional technology]

Conventionally, there is a method as shown in FIG. 2 as a focus servo pull-in technique. In FIG. 2A, reflected light from the medium surface is converted into an electrical signal by a two-split photodetector 21, the difference in the photodetector output is amplified by a differential amplifier 22, and the sum is converted to an electric signal by a summing amplifier 2.
Amplify with 3. The focus error signal and focus sum signal amplified by the differential amplifier and the summing amplifier enter the AGC circuit 4.

The AGC circuit 4 outputs a signal that becomes a focus error signal/focus sum signal. FIG. 2B shows the AGC output waveform. When retracting the focus, the objective lens is brought closer to the medium from a distance, and at that time, the focal point is brought to a focus on the surface of the cover glass of the medium, and then the focal point is brought to a focal point on the recording surface of the medium. The reflectance on the recording surface of normal media is 30% when unrecorded.
, the reflectance of the cover glass 3 surface is about a few percent compared to about 15% during recording, but due to the operation of the AGC circuit 4, the AGO output amplitude ratio at the focal point of the cover glass surface with respect to the medium recording surface is 30~30 when not recording. 35%, and about 60 to 70% during recording. Therefore, the focus is pulled in by detecting the level of 80% of the peak of the AGC output amplitude at the focal point on the recording surface of the medium when no recording is being performed.

[Problem to be solved by the invention]

This conventional method has the drawback that the AGC operation is always active, so there is no difference in the amplitude of the AGC output curve between the cover glass and the recording surface of the medium, and the focus servo may be pulled in at the cover glass surface. .

[Means to solve the problem]

In order to solve the above problems, the present invention provides a two-split photodetector that divides reflected light from a medium surface into two and converts it into an electrical signal, and a differential amplifier that obtains an error signal from the two-split photodetector. , a summing amplifier for obtaining the sum signal from the two-split photodetector, a comparator for detecting the focus sum signal level, a flip-flop set by the comparator output, and a reference voltage and the focus sum signal level by the output of the flip-flop. It has a switch that switches between signals, and an AGC circuit that controls the gain based on the signal from the switch.

〔Example〕

An actual travel example of the present invention will be described below. FIG. 1A is a block diagram of the present invention.

The signal divided by the two-split photodetector is sent to a differential amplifier 2.
It is connected to the summing amplifier 3. The outputs of the differential amplifier 2 and the summing amplifier 3 are connected to an AGC circuit 4. At the same time, the output of the summing amplifier 3 is connected to a comparator 5, to which the reference voltage V r e f 1 is connected. The comparator output goes into flip-flop 6, and the flip-flop output is connected to switch 41. The switch 41 switches between the output of the summing amplifier 3 and the reference voltage.

The laser beam reflected by the medium surface is divided into two parts by a two-part photodetector 1 and converted into electrical signals. The divided signal is sent to a differential amplifier 2. It is amplified by the summing amplifier 3 and enters the AGC circuit 4. At the same time, the output of summing amplifier 3 enters comparator 5, Vrefl,! :Compared. If the summing amplifier output is less than Vrefl, the switch 41 is off and the reference voltage enters the AGC circuit 4, which acts as a constant gain amplifier. The summing amplifier output level is V
When refl is exceeded, the output of the comparator 5 is switched and the flip-flop 6 is set.

The output of the flip-flop switches the switch 41 to the focus sum signal side. When the switch is turned,
The AGC circuit 4 outputs a signal that becomes a focus error signal/focus sum signal.

Next, the output waveforms of each component are shown in FIG. 1B, which shows the output changes when the objective lens is brought closer to the medium from a distance during focus retraction.

The first peak of the focus sum signal is the primary peak of the cover glass surface, which is the first peak of the medium recording surface. Here, the reflectance of the recording surface of the medium is about 30% when unrecorded and about 15% when recorded, whereas the reflectance of the cover glass surface is about several percent.

Therefore, by setting Vrefl to 40% of the focus sum signal peak level on the recording surface of the medium when not recording,
The recording surface and the cover glass surface are separated. ±80% St of the peak of AGC output amplitude at the focal point of the medium recording surface,
S3 and zero cross S2 are detected, and S3 changes from Low to High.
After that, the focus servo is pulled in at the point where S2 changes from Low to High.

〔Effect of the invention〕

As explained above, the present invention starts the AGC operation when the focus sum signal exceeds a certain reference voltage level, so that the difference in the peak level of the AGO output between the cover glass surface and the recording surface of the medium increases and ensures that the recording surface It has the effect of distinguishing and drawing focus.

[Brief explanation of drawings]

FIG. 1A is a configuration diagram of the present invention, FIG. 1B is an output waveform diagram of each part, FIG. 2A is a configuration diagram of the prior art, and FIG. 2B is an AGC output waveform of the prior art. ■...Two-split photodetector, 2...Differential amplifier,...Summing amplifier, 4...AGC2 5...Comparator, 6. ...Flip Flo Ruff.

Claims (1)

[Claims] Dividing the reflected light from the medium surface into two and converting it into an electrical signal 2
a split photodetector, a differential amplifier for obtaining an error signal from the two-split photodetector, a summing amplifier for obtaining a sum signal from the two-split photodetector, and a focus sum signal and a predetermined voltage. An optical system comprising: a voltage comparison means for comparing the two signals; a selection means for selecting the reference voltage signal and the focus sum signal based on the voltage comparison result; and an AGC circuit for controlling the gain using the selected signal. type recording and reproducing device.