JPH0480451B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a focus search circuit used in a pickup device of an optical playback device.

B. Prior Art In an optical playback device, when starting a playback operation, the objective lens of the pick-up device is moved vertically with respect to the disk to detect the focused position (so-called focus search operation). What is required is known, for example, from the magazine "Electronics", July 1983 issue, pages 73-75.

In such a focus search operation, the speed at which the objective lens is moved up and down with respect to the disk becomes a problem. That is, if the speed at which the objective lens is moved is too fast, the objective lens cannot be stopped at the in-focus position, and the focus search operation becomes unstable. On the other hand, if the moving speed is too slow, the focus search operation time becomes long, and in particular, a focused state cannot be obtained in one cycle of focus search operations, and two or three focus search operations are required. In this case, it will take a longer time.

C. Problems to be Solved by the Invention It is an object of the present invention to obtain a suitably fast focus search operation and to ensure that an in-focus state is obtained.

D. Measures to solve the problem Zero cross of focus error signal (focus point)
Previously, a level detection circuit was provided to detect that the focus error signal reached a predetermined level, and the objective lens drive circuit was driven to apply a brake pulse to the objective lens drive circuit in response to the output of this level detection circuit. It is equipped with a brake pulse generation circuit.

E Effect Before reaching the focused point, a brake pulse is generated and the speed of the focus search operation is slowed down, so the movement of the objective lens when passing the focused point position is slowed down, and the detection of the focused point position is delayed. becomes easier.

F. Example In FIG. 1, a pickup device 1 includes an objective lens, a drive coil for driving the objective lens, an information (HF) signal, a focus error signal obtained by an astigmatism method, and a tracking error. It includes a photo sensor etc. that detects and generates signals etc. Output from pick-up device 1
The HF signal, focus error signal, and tracking error signal are amplified by a preamplifier circuit 2. The focus search drive signal (sine waveform signal,
(triangular waveform signal, etc.) is sent to the pickup device 1 via an adder circuit 4, a first switch means 5, and a drive circuit 6.
The voltage is applied to the drive coil of the objective lens provided in the interior, and vertical movement of the objective lens (focus search operation) is started.

During such a focus search operation, the focus error signal output from the preamplifier circuit 2
The voltage waveform of SF is as shown in FIG. The figure shows a waveform when the objective lens moves from a position close to the disk to a position far from the disk, with the left side of the horizontal axis indicating a position close to the disk, and the right side indicating a position far from the disk.

Now, after starting the focus search operation,
When the first level detection circuit (which may be composed of a comparison circuit) 7 detects that the focus error signal SF has reached a predetermined level l (see FIG. 2), the brake is activated in response to this detection output. Pulse trigger circuit 8
is activated, and the brake pulse generation circuit 9 outputs the brake pulse SB. The brake pulse SB is applied to the adder circuit 4 and then to the drive coil of the pickup device 1.

After such a brake pulse is generated, the moving speed of the objective lens decreases, and the slope of the focus error signal becomes gentle. The situation is as shown in FIG. In FIG. 3, waveform a shows the focus error signal SF' in the conventional focus search operation, and waveform c shows the focus error signal SF' in the conventional focus search operation, and waveform c shows the focus error signal SF' in the present invention when the brake pulse SB shown in waveform b is applied. It shows the cass error signal SF. waveform a
It can be seen that the slope of waveform c is gentler than that of waveform c.

Now, after the brake pulse is generated, the focus error signal SF shifts to the range of change shown between B and D in FIG. The area between B and D becomes the in-focus point detection range. Between B and D, the focus error signal becomes the predetermined level l again, but since the first level detection circuit 7 is configured to detect the rise to the predetermined level l, the focus error signal becomes the predetermined level l again between B and D. The brake pulse trigger circuit 8 is activated by the fall of the brake pulse SB, and the brake pulse SB is not generated.

The point in time when the focus error signal passes the zero cross point c (focus point position) is detected by a second level detection circuit (which may be configured with a comparison circuit) 10 that detects the reference (0) level. That is, the second level detection circuit 10 is a zero-cross detection circuit,
The point in time when the objective lens passes through the focal point position in the direction away from the disk is detected. When the focused point is detected in this manner, the switch trigger circuit 11 is activated, thereby opening the first switch means 5 to open the focus search loop and closing the second switch means 12. The focus servo loop is closed.

In this way, the focus search operation is completed,
Shift to focus servo operation. The focus servo circuit 13 is a circuit for performing gain control, phase compensation, etc. of the focus servo system.

Furthermore, in order to prevent malfunctions due to noise etc., the brake pulse trigger circuit 8 and the switch trigger circuit 11 are detected by the pickup device 1.
Preferably, it is driven when the HF signal is above a predetermined level. Therefore, in this embodiment, the HF signal SHF output from the preamplifier circuit 2 is
When the third level detection circuit 14 detects that the level of the signal is higher than a predetermined level, the circuits 8 and 11 are set to a drivable state.

G. Effects According to the present invention described above, by applying a brake pulse, the moving speed of the objective lens near the in-focus position is suppressed, and the in-focus position can be reliably detected. The moving speed at other positions is relatively fast, and the focus search operation time in cycles in which the in-focus position cannot be detected is shortened, so there is an advantage that the focus search operation time is shortened as a whole.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a focus search circuit according to the present invention, FIG. 2 is a diagram showing a focus error signal, and FIG. 3 is a diagram showing a focus error signal and a brake pulse. 1 is a pickup device, 3 is a focus search drive signal generation circuit, 5 is a first switch means (focus search termination means), 7 is a first level detection circuit,
9 is a brake pulse generation circuit, and 10 is a second level detection circuit (zero cross detection circuit).

Claims (1)

[Scope of Claims] 1. A focus search circuit configured to apply a focus search drive signal to an objective lens drive circuit in order to move the objective lens relative to the disk and detect the focused position, the focus search circuit comprising: a level detection circuit that detects that the cass error signal has reached a predetermined level; and a brake pulse generation circuit that is driven to apply a brake pulse to the objective lens drive circuit in response to the output of the level detection circuit. , a zero-crossing detection circuit that detects zero-crossing of the focus error signal;
A focus search circuit characterized by comprising a termination means for terminating the focus search operation in response to the output of the zero-cross detection circuit.