JPS595968B2 - information reproducing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an information reproducing apparatus using a photodetection method, and more particularly to an information reproducing apparatus equipped with a tracking pull-in circuit.

For information on optical detection type video disc playback devices, see Philips Technical Review; 033, 1973.
It is well known from Mo. 7.

FIG. 1 is a schematic diagram of this playback device. Laser light emitted from a laser light source 1 passes through a lens 2, a half mirror 3, a mirror 4, an optical deflector 5, and an objective lens 6, and hits the surface of the disk by approximately 1 μφ. Squeezed. The laser beam reflected by the surface of the disk travels backward through the objective lens 6, the optical deflector 5, the mirror 4, and the half mirror 3, and is detected by the photodetector 8. In this playback device, in order to track the track recorded on the disc, the amount of deviation of the light spot from the track center position is detected by optical means, and the light deflector 5 is driven so that the light spot is always on the track. Automatic control is applied to keep it in the center position. However, if the light spot is completely off the track, that is, when starting the device or playing from the middle of the disc, or if the light spot becomes unable to track the track due to some kind of disturbance, the tracking pull-in may occur. It is very difficult to immediately perform a tracking operation without taking any special measures, unless the automatic control system, especially the optical deflector 5, has good frequency response. This becomes a particular problem when the deviation between the center of the disk and the center of rotation, that is, the amount of eccentricity of the disk is large. for example,
The eccentricity of the disk is 100 μ, the rotation speed of the disk is 30 revolutions per second, and the size of the light spot on the disk surface is 1 μ.
φ, If the width of the recorded track is 1 μ, the time the light spot crosses the track is at least O, 05 ms,
The longest is about 1ms. Therefore, in order to draw the light spot onto the track, the Jizakiura control system must have flat frequency characteristics up to 1 KH2 or more and a system with good phase characteristics. For this purpose, the mechanical resonance frequency of the optical deflector needs to be several KH2 or more, but such an optical deflector is very expensive. However, since a low cost reproduction device is desired, it is necessary to realize tracking pull-in using a low-cost optical deflector, such as a low-resonance galvanometer device, even if the frequency characteristics are not good. Therefore, an object of the present invention is to develop an optical system that uses a slightly different frequency of disk rotation and a signal that gradually attenuates, in order to enable tracking even when the optical deflector has a low mechanical resonance. An object of the present invention is to provide an information reproducing device having a tracking pull-in circuit for driving a deflector.

The present invention will be explained in detail below with reference to Examples.

FIG. 2 is a block diagram for realizing tracking pull-in. The signal from the photodetector 8 is divided into two parts, one of which is input to a video signal demodulation circuit 9 and the other to a waveform shaping circuit 10 for tracking acquisition. In this waveform shaping circuit 10, a low frequency component of the output signal of the photodetector 8 is extracted through a low frequency filter of several tens of KHz. This signal
The tracking is input to the deviation detection section 11, and a certain level of detection is performed. If the signal is always higher than the set level, it is determined that tracking is on, and if it is lower than the set level, it is determined that tracking is off. moreover,
A pulse generating circuit 12 generates a gate pulse of about 1 to 5 seconds and a tracking pull-in pulse when tracking is off. The gate pulse is returned to the tracking error detection section 11, and the tracking error detection section 11 is not operated during the period when the gate pulse is output. Next, the tracking pull-in pulse is input to the waveform shaping circuit 13, and the waveform is shaped into a signal suitable for tracking pull-in. For example, the width of the pulse for tracking pull-in is about 15 milliseconds, and this pulse is input into the resonant circuit. The resonant frequency of this resonant circuit is set to a frequency slightly different from the rotation speed of the disk, and the Q value of the resonance is determined so that the amplitude becomes about 1/100 of the initial state in a few seconds. . The tracking servo system also transmits the signal from the photodetector 8 to the tracking servo circuit 15.
and drives the optical deflector 5. The tracking pull-in signal is added to the tracking servo signal by an adder 14 to drive the optical deflector 5. The signal from the tracking servo circuit 15 is a high frequency signal of several hundred Hz or more since tracking has never been done before, so the signal of the adder 14 is a high frequency signal of several hundred hertz or more superimposed on the solid line in FIG. Be what you are. However, since the mechanical resonance frequency of the optical deflector 5 is low, the movement of the optical spot on the disk is approximately as shown by the solid line in FIG. Therefore, the light spot moves on the disk while being slightly out of phase with the period of disk rotation and with the amplitude decreasing little by little, so a state in which the light spot remains on the track for approximately one cycle always occurs. . In this state, the signal from the tracking servo circuit 15 becomes a low frequency signal of about several Hz, and the optical deflector 5 starts to move based on this signal, and tracking pull-in is performed. After the tracking begins, the tracking servo circuit 15 tracks the track, and the signal from the pull-in circuit exists as an error. However, after the tracking operation is performed, this error becomes an error that affects the output portion of the tracking servo system, so it is sufficiently compressed and its influence on the servo system disappears within a few seconds. As described above, the tracking pull-in method of the present invention is very effective as a means for operating the tracking servo system normally when tracking is out of order.

The width of the tracking pull-in pulse generated by the pulse generation circuit 12 does not need to be limited to 15 milliseconds, but may vary depending on the z-resonance frequency F of the waveform shaping circuit 13.

In this case, it is optimal if the time is about one second.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram of a photodetection type video disc playback optical system, Fig. 2 is a block diagram of signal processing to realize the tracking pull-in method according to the present invention, and Fig. 3 is a signal processing diagram for tracking pull-in. This is an outline of the waveform. DESCRIPTION OF SYMBOLS 1... Laser light source, 2... Lens, 3... Half mirror, 4... Mirror, 5... Optical deflector, 6...
Objective lens, 7... Disk, 8... Photodetector, 9
... video signal demodulation circuit, 10 ... waveform shaping circuit,
11... Track deviation detection section, 12... Pulse generation circuit, 13... Waveform shaping circuit, 14... Addition circuit, 15... Tracking servo circuit.

Claims (1)

[Claims] 1 means for irradiating a light spot onto information formed along a track on a rotating recording medium; a light detection means for converting light modulated by the information into an electrical signal; and a tracking means for moving the recording medium so as to be scanned along the track, the information reproducing device comprising a tracking means for moving the recording medium so as to be scanned along the track. , an information reproducing device characterized in that the tracking means is driven by a signal from the pull-in circuit.