JPH0348727Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a focus servo device for a video disc player (a servo device for moving the objective lens of a video disc player in a direction perpendicular to the disc).

In a video disc player, a tracking servo loop is opened to move an objective lens in the diametrical direction of the disc for random access to select a predetermined frame. In this case, the known 4
When performing focus servo using a split detector, the tracking servo is closed and the focus servo is on during normal play, so
The difference between the sum outputs of the two detectors is controlled to be equal, and the amplifier for driving the objective lens of the focus servo device does not become saturated, but when scanning based on a scan command such as a random access command, , the tracking servo loop is opened, and the objective lens is rapidly moved in the radial direction of the disk to perform a jump operation or the like. At this time, since the pick-up crosses between the tracks, the image between each track, that is, the tracking component appears as the intensity of light on the four-divided detactor. This tracking component appears as a difference between the sum outputs of each detector, and it is fine if the sum output differences are equal, but in reality, the difference between the sum outputs becomes large and becomes the output of the focus servo, so the focus The amplifier for driving the objective lens of the servo device becomes saturated. If the objective lens drive amplifier becomes saturated, the operating reference point of the objective lens that is moving up and down will shift significantly, resulting in a large so-called objective lens offset, which has the disadvantage of making it impossible to read the frame number required for random access. It was hot.

The present invention was developed in view of the above-mentioned drawbacks, and when scanning is performed based on a random access command, even if the input to the focus servo amplifier increases compared to the input during normal playback operation. It is an object of the present invention to provide a focus servo device in which the output of a focus servo amplifier does not become saturated.

An embodiment of the present invention will be described below with reference to a block diagram of the drawings.

In the figure, 1 is a buffer amplifier, 2 is a variable resistor for closed loop gain adjustment that divides the output of the buffer amplifier 1, 3 is an objective lens driving amplifier that receives the output of the variable resistor 2 as input, and 4
is an objective lens driving coil, which is driven by the objective lens driving amplifier 3. buffer amplifier 1,
The variable resistor 2, the objective lens driving amplifier 3, and the coil 4 constitute a focus servo device.

On the other hand, the output of the objective lens driving amplifier 3 is taken out through a coupling capacitor 5, and a diode 6
The waveform is detected by a smoothing circuit consisting of a capacitor 7, and then applied to the gate of a field effect transistor 8 whose drain and source are respectively connected between the input terminal of the objective lens driving amplifier 3 and the ground. There is.

Note that a power source is connected to the gate of the field effect transistor 8.
B is connected to apply a predetermined bias voltage to the gate of the field effect transistor 8.

Therefore, when the input of the objective lens driving amplifier 3 increases above a predetermined value, the output of the objective lens driving amplifier 3 is rectified and smoothed to increase the gate voltage of the field effect transistor 8. Therefore, the resistance between the drain and source of the field effect transistor 8 is reduced. Therefore, as mentioned above, even if the input signal to the objective lens driving amplifier 3 during scanning based on a random access command is several times to several tens of times higher than the input signal during normal image reproduction operation, the objective lens driving amplifier 3 is Amplifier 3 does not saturate due to automatic gain control.

As explained above, according to the present invention, a field effect transistor is connected to the input of the focus servo amplifier constituting the focus servo device, and when the output voltage of the focus servo amplifier increases, the output voltage By applying a rectified and smoothed signal to the gate of the field effect transistor, the input voltage of the focus servo amplifier is lowered to avoid saturation, so the focus servo device becomes stable and the offset of the objective lens becomes smaller. , even if scanning is performed based on a random access command, it is possible to reliably read a necessary frame number.

[Brief explanation of the drawing]

The figure is a block diagram of one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Buffer amplifier, 2... Variable resistor, 3... Objective lens driving amplifier, 6... Diode, 7... Capacitor, 8... Field effect transistor.

Claims (1)

[Claims for Utility Model Registration] In a focus servo device for moving an objective lens of an optical disk player in a direction perpendicular to the disk, a drain connected between the input of a focus servo amplifier constituting the focus servo device and the ground;・
A field effect transistor having a source connected thereto, and a series circuit consisting of a rectifying diode and a smoothing capacitor that detect the output of the focus servo amplifier and generate a DC voltage, and a predetermined voltage is connected to the gate of the field effect transistor. A focus servo device characterized in that a DC voltage of the series circuit is applied together with a bias voltage.