JPH0476825A - Velocity detector - Google Patents

Abstract

PURPOSE:To exactly detect a velocity signal by specifying the cut-off frequency of the low-pass filter of a waveform shaping means and varying a time constant corresponding to the above-mentioned velocity signal when generating a comparing signal from a tracking error signal. CONSTITUTION:The output of a photodetector 7 is subtracted by a differential amplifier 8, and a generated tracking error signal (a) is inputted to a variable band filter 9 and a low-pass filter 10 respectively. The output is compared by a comparator 11, and a track pulse is outputted. A required frequency compo nent is selectively taken out by the filter 9 so as to remove a noise component, and by outputting an offset fluctuation component corresponding to moving velocity, the stable track pulse is obtained. On the other hand, the cut-off fre quency of the low-pass filter 10 is set at the 1/10 of the tracking error signal frequency, and by suppressing an influence to a comparing voltage due to the fundamental frequency component of the error signal, offset deviation included in the error signal is exactly detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a speed detection device, and more particularly to a speed detection device that detects the moving speed of an optical head when accessing a track of an optical disk device.

[Conventional technology]

Conventionally, this type of speed detection device positions a light spot focused by an objective lens at a predetermined position on an optical disk medium when recording or reproducing information. When the light spot crosses the group, the tracking error signal becomes zero between and on the grooves, so the positive/negative polarity is detected to generate a track pulse, and this track pulse is counted to find the distance traveled. The frequency of the track pulse is frequency-voltage converted to determine the moving speed when accessing the track.

[Problem to be solved by the invention]

This conventional speed detection device only detects whether the tracking error signal has positive polarity or negative polarity when generating track pulses, so it is difficult to detect whether the tracking error signal has positive polarity or negative polarity.
), if a lens position shift occurs during acceleration and deceleration of the optical head during track access, an offset will occur in the tracking error signal, and as a result, the track pulse will be detected at a position shifted from the center of the track. As shown in FIG. 4, in the case of an optical disk medium in which holes are formed on the medium surface due to recording, the amplitude of the tracking error signal decreases as the interval between pits becomes narrower.
If data is recorded off-track from the center of the track due to disturbances during recording, an offset will occur in the tracking error signal in the recording section. ) ~ Same figure (C)
As shown in Figure 2, there is a drawback that when lens positional displacement occurs, it becomes difficult to accurately detect track pulses.

[Means to solve the problem]

The invention converts light into electrical signals. a photodetector; a differential amplifier that generates a tracking error signal from the output of the photodetector; a waveform shaping device that detects track crossing from the tracking error signal; In a speed detection device having a frequency-voltage conversion circuit for converting into a signal, the waveform shaping means includes a low-pass filter whose time constant is varied according to the speed signal when generating a comparison signal from the tracking error signal. have.

Alternatively, the cutoff frequency of the low-pass filter may be 1/10 of the frequency of the tracking error signal.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. The light emitted from the semiconductor laser 1 passes through the collimating lens 2
, passes through the beam splitter 3 and the objective lens 5, and enters the optical disk medium 6.

4 is a lens drive mechanism that drives the objective lens 5. The reflected light from the optical disk medium 6 passes through the objective lens 5, is reflected by the beam splitter 3, and is irradiated onto the photodetector 7. The output of the photodetector 7 is divided into a pair of output signals parallel to the track and subtracted by a differential amplifier 8 to generate a tracking error signal a. This tracking error signal a is input to a band variable halter 9 and a low-pass filter 10, respectively. This variable band filter 9 is set so that a frequency component corresponding to the speed signal passes through the tracking error signal a during track access. Furthermore, the low-pass filter 10 is a band variable filter 9.
This is a circuit that generates a comparison voltage of the signal that has passed through the bandpass filter 9, and is set in advance to be one or more orders of magnitude lower than that of the variable band filter 9, that is, 1
The cutoff frequency of the low-pass filter characteristic changes depending on the speed signal. The outputs of the variable band filter 9 and the low-pass filter 10 are compared by a comparator 11, and one track pulse per track is output from the comparator 11. And this track pulse is F/V
A converter 12 performs frequency-voltage conversion to produce a speed signal. 2(a) to 2(C) show the relationship between the speed signal and the cutoff frequency of the low-pass filter 10. The frequency of the tracking error signal during track access is proportional to the moving speed of the optical head, and this signal component includes, in addition to the tracking error signal, a positional deviation component from the optical center of the lens, and a track error signal during data recording. Contains off-center components, etc. Among these, most of the lens position deviation components are components of several hundred Hz or less, and the deviation components from the track center during data recording are several KHz or less, considering factors such as disturbances, and are due to changes in the recording pattern. The fluctuation includes up to around the data recording frequency.

Furthermore, the frequency of the tracking error signal during track access varies over a very wide range from several hundred Hz to several hundred KHz, and appears in the tracking error signal depending on the moving speed of the optical head. Disturbances in the tracking error signal due to data recording are averaged and reduced because it takes a long time to cross one track at low speeds, but as the speed increases, the time to cross a track becomes shorter and
The effects of the interval between recording pits and the deviation of pits from the track center during recording tend to appear as disturbances in the tracking error signal. Therefore, in order to obtain a stable tracking pulse, in order to remove the noise component of the tracking error signal, the variable band filter 9 is used to selectively extract the necessary frequency components of the tracking error signal according to the moving speed of the optical head. If it is possible to output the offset fluctuation component of the tracking error signal (track position deviation component due to disturbance during data recording of lens position deviation component data, change in pit interval, etc.) according to the moving speed, stable tracking pulses can be obtained. It can be seen that this is possible. Here, in order to perform stable track detection, the cutoff frequency of the low-pass filter 10 needs to be 1/10 or less of the frequency of the tracking error signal when the optical head moves. The influence of the components on the comparison voltage can be suppressed.

〔Effect of the invention〕

As explained above, the present invention detects the lens position deviation component included in the tracking error signal when detecting the moving speed of the optical head during track access.

By detecting deviation components of pits from the track center due to disturbances, etc. by varying the cutoff frequency of the filter according to the moving speed, it is possible to accurately capture the offset deviation included in the tracking error signal. , it is possible to generate a stable comparison voltage even when the lens position shifts due to changes in acceleration during track access, and when pits shift from the track center during data recording. By counting , track counting errors can be reduced, which has the effect of enabling high-speed and highly accurate track access operations.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 (
3(a) to 3(C) are signal waveform diagrams during track access in the present embodiment, FIGS. 3(a) to 3(C) are signal waveform diagrams during conventional track access, and FIG. FIG. 3 is a waveform diagram of a tracking error signal. 1... Semiconductor laser 2... Collimator lens,
3... Beam splitter, 4... Lens drive mechanism,
5... Objective lens, 6... Optical disk medium, 7...
・Photodetector, 8...Differential amplifier, 9...Variable band filter, 10...Low pass filter, 11...Comparator, 12...F/V converter, a...Tracking error Signal, b...speed signal.

Claims (1)

[Claims] 1. A photodetector that converts light into an electrical signal, a differential amplifier that generates a tracking error signal from the output of the photodetector, and a waveform shaper that detects track crossing from the tracking error signal. and a frequency-voltage conversion circuit for converting a signal frequency output from the waveform shaping means into a speed signal, when the waveform shaping means generates a comparison signal from the tracking error signal,
A speed detection device comprising a low-pass filter whose time constant is variable according to the speed signal. 2. The speed detection device according to claim 1, wherein the cutoff frequency of the low-pass filter is 1/10 or less of the frequency of the tracking error signal.