JPS6367259B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video disc playback device equipped with a pick-up means that detects information recorded on a rotatable video disc as a change in capacitance, and particularly relates to a video disc playback device equipped with a pick-up means that detects information recorded on a rotatable video disc as a change in capacitance. Deterioration of the SN ratio due to dust etc. that adheres between the
It is an object of the present invention to provide a video disc playback device having a pick-up stylus pressure control function that can effectively prevent the problem without deteriorating the quality of playback signals (signals such as video, audio, tracking, etc.).

Conventionally, in capacitance detection type video disk playback devices, the disk is rotated while the pick-up needle is in contact with the surface of the disk, so minute dust gets between the disk and the pick-up stylus tip, and as a result, the pick-up needle There was a problem in that the output of the reproduced signal picked up at the end decreased and the S/N ratio deteriorated. It has been found that, as a means for preventing the deterioration of the reproduced signal due to the adhesion of dust, it is effective to increase the stylus pressure of the pick-up needle when dust adheres. In more recent research,
When dust begins to adhere, the amplitude of the reproduced output signal begins to decrease, and as the dust increases, the amplitude of the output signal decreases more rapidly. In this case, it has been found that if the pressure of the pick-up needle is slightly increased when the attached dust is small, the dust is removed and it becomes difficult for the dust to accumulate. Furthermore, in order to remove dust, it is effective to control the stylus pressure of the pick-up needle according to the amplitude of the reproduced signal. However, if the stylus force is simply controlled using a control voltage proportional to the amplitude of the reproduced signal, the stylus force will also change due to variations in the amplitude of the reproduced signal. In other words, variations in the level of the reproduced signal occur due to variations in the disc and the pickup needle, but even in this case, the stylus pressure will be different. This will be explained with reference to FIG. In FIG. 1, the horizontal axis is the signal amplitude, and the vertical axis is the control voltage, which is in a proportional relationship.
Now, assuming the signal amplitude is being played at 5V,
If the signal amplitude decreases due to dust and the control voltage decreases by ΔV, then the signal amplitude will be
This is a 20% drop from 5V to 4V. However, for example, if the disk is bad and the signal amplitude is being reproduced at 2V, the signal output will drop by 50% from 2V to 1V with respect to the change in control voltage ΔV. In other words, for a fixed amount of control voltage,
Since the rate of change of the signal amplitude is different, this signal cannot be used for stylus force control.

The present invention solves the above-mentioned problems, and when controlling the stylus force of a video disc using the information of the playback signal output, it is possible to effectively control the stylus force even if there are variations in the playback signal output. The purpose is to provide a device that can do this. The basic concept will be explained below with reference to FIG. 2, and then embodiments of the present invention will be explained with reference to FIGS. 3 to 5. FIG. 2 is a diagram showing an example of the characteristics of the control voltage used in the present invention, in which the horizontal axis shows the signal amplitude on a logarithmic scale, and the vertical axis shows the control voltage on a linear scale. In this case, the change in signal amplitude with respect to the change ΔV in the control voltage is approximately 70% at the same rate of change whether the signal amplitude is around 5V or around 2V. This means that
This shows that if a control voltage proportional to the logarithm of the signal amplitude is generated, the rate of change in the signal amplitude is constant with respect to the amount of change in the control voltage. When controlling the stylus force of a video disk, by using this control voltage, the stylus force is controlled in accordance with the rate at which the signal amplitude decreases, regardless of the output signal level, and desired performance can be obtained.

FIG. 3 is a block diagram showing an embodiment of the present invention. In the figure, video disk 1
is a turntable 2 that is rotationally driven by a motor 3.
Changes in capacitance caused by pits located on the video disc 1 and recorded as recorded information on the video disc 1 are picked up by a pick-up needle 5a at the tip of the pick-up means 5, and guided to a signal detector 8 via a fly lead 7. The signal detector 8 converts the capacitance change into an electrical signal and supplies a reproduced signal to the head amplifier 9. A magnet 4 is attached to the pick-up means 5, and an electromagnet 6 for controlling stylus force is disposed at a fixed portion near the magnet 4, and these constitute a stylus force control means. A coil is wound around the electromagnet 6, and the position of the magnet 4 is controlled in accordance with the current flowing through the coil, so that the stylus pressure of the pickup needle 5a against the video disc 1 is controlled. The output of the head amplifier 9 is led to a terminal 10 and supplied to a signal processing circuit (not shown), and is also supplied to a logarithmic converter 11 which constitutes a control signal generating means with a rectifier 12 and a low-pass filter 13, which will be described later. Added. This logarithmic converter 11 functions to generate a signal proportional to the logarithm of the amplitude of the output of the head amplifier 9, that is, the signal reproduced from the video disk 1. Then, the logarithmically converted signal is rectified by a rectifier 12, and then passed through a low-pass filter 1.
3 and becomes the control voltage. The low-pass filter 13 sets its time constant to a value suitable for needle pressure control. A control voltage obtained from the low-pass filter 13 is applied to the coil of the electromagnet 16 via an amplifier 14 for driving the needle pressure coil. Note that the needle pressure coil driving amplifier 14 may be included in the needle pressure control means.

With this configuration, if dust adheres between the video disc 1 and the pick-up needle 5a, the drop in the playback signal is detected, the stylus pressure is increased, and the attached dust is removed. As a result, terminal 1
A reproduced signal of a constant value can be stably obtained from 0. Here, since the control signal is proportional to the logarithm of the amplitude of the reproduced signal, even if the signal output varies due to variations in the video disk 1 and the pickup needle 5a, the control signal is proportional to the logarithm of the amplitude of the reproduced signal. As mentioned above, the pressure control amounts are the same.

FIG. 4 is a block diagram of a main part of another embodiment of the present invention, mainly showing only the structure of the part that generates the control signal voltage. The difference between FIG. 4 and FIG. 3 is the configuration of the control signal generating means 15, that is, the output signal of the rectifier 12 is applied to the first and second low-pass filters 13a and 13b, and the respective outputs are sent to the differential amplifier 16. In addition, the output of the differential amplifier 16 is used as a signal for controlling the stylus force. In this embodiment, the time constant of the first low-pass filter 13a is set sufficiently large, and the time constant of the second low-pass filter 13b is selected to be relatively small. That is, the first low pass filter 1
The output of 3a is the average reproduction signal output, and the output of the second
Since the output of the low-pass filter 13b is an instantaneous reproduction signal output, if the instantaneous output becomes lower than the average value, the differential amplifier 16 is configured to detect the amount. Therefore, when the output decreases from the average playback output level due to dust etc. adhering to the video disk, a control voltage corresponding to the amount is obtained and the stylus pressure is increased.
In this case as well, the logarithmic converter 11 operates effectively as in the embodiment shown in FIG.

FIG. 5 is a circuit diagram showing an example of the configuration of the logarithmic converter 11 used in the present invention. This is the operational amplifier 20
The collector-emitter path of a transistor 21 whose base is grounded is connected between the input terminal and output terminal of the transistor 21, and the transistor 21 is used as a logarithmic conversion element. Here, if the signal amplitude applied to the input terminal 17 is Vi, the collector current Ic of the transistor 21 is determined by the resistance value R of the input resistor 19, and Ic=Vi/R.

Transistor emitter-base voltage V _BE
is generally given by the following equation.

V _BE ≒kT/qlnIc/Io K: Boltzmann's constant T: Absolute temperature q: Electron charge Io: Reverse saturation current The signal amplitude Vo obtained at the output terminal 18 is equal to V _BE . Therefore, Vc≒alogVi+b However, a and b are constants, and the output amplitude is proportional to the logarithm of the input amplitude.

In addition, although the above-mentioned Example explained the structure which rectified after logarithmically transforming, it cannot be overemphasized that exactly the same effect can be obtained even if it rectifies and then logarithmically transforms.

As described in detail above, the present invention controls the stylus force of the capacitance-sensing pick-up needle using a control signal proportional to the logarithm of the playback signal output, and the stylus force is controlled at a constant rate regardless of variations in the playback output. A predetermined change in stylus pressure is applied to a change in signal amplitude. Therefore, according to the present invention, it is possible to effectively remove dust and the like that enter between the disk and the pick-up stylus tip, thereby providing a stable reproduced signal without deterioration of the S/N ratio.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing exemplary characteristics of signal amplitude versus control voltage for explaining the principle of the present invention,
FIG. 3 is a block diagram of main parts showing one embodiment of the present invention, FIG. 4 is a block diagram of main parts showing another embodiment of the present invention, and FIG. 5 is a structural example of a logarithmic converter used in the present invention. FIG. 1...Video disk, 4...Magnet, 5
. . . Pick-up means, 5a . ,1
3, 13a, 13b...Low pass filter, 14
. . . Needle pressure coil drive amplifier, 15 . . . Control signal generating means, 16 . . . Differential amplifier, 20 .

Claims (1)

[Scope of Claims] 1 Pickup means for detecting information recorded on a video disk as a change in capacitance; stylus pressure control means for controlling the stylus pressure of a pickup stylus of the pickup means; control signal generating means for generating a control signal proportional to the logarithm of the amplitude of the reproduced signal, the control signal of the control signal generating means being supplied to the stylus force control means to control the stylus force of the pick-up needle. A video disc playback device characterized in that it is configured as follows. 2. The video disc playback device according to claim 1, wherein the control signal generating means includes a logarithmic converter, a rectifier, and a low-pass filter. 3. The control signal generation means includes a logarithmic converter, a rectifier, first and second low-pass filters having different time constants, and a differential amplifier that obtains a difference signal between each output signal of the first and second low-pass filters. A video disc playback device according to claim 1, characterized in that the video disc playback device is configured to include the following. 4. A patent claim characterized in that the logarithmic converter includes an operational amplifier and a transistor whose collector-emitter path is connected between the input terminal and output terminal of the operational amplifier and whose base is grounded. A video disc playback device according to item 1, item 2, or item 3.