Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1984-10-22
1989-01-17
McElheny, Jr., Donald
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
G11B 708, G11B 2110
Patent
active
047992069
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to tracking control apparatus for recording disc reproducing apparatus which reproduces successively recorded information from a rotatably driven disc-like recording medium (which will be referred to as a disc hereinbelow), by way of a pickup means, wherein the pickup means is scanned with satisfactory tracking along an information track on a disc, while the tracking characteristic is maintained stable irrespective of application of external impact thereto.
TECHNICAL BACKGROUND
Since spiral information tracks recorded on a disc are apt to be eccentric with respect to a center hole of the disc, means for causing a reproducing means to scan along information tracks, which means is called a tracking servo, has hitherto been required in order to reproduce tracks of such a disc.
Since such tracking servo is well known, a detailed description of the structure thereof is omitted.
FIG. 1 is an explanatory diagram for the principle of a servo system using three beams, which is a system of a tracking servo for an optical disc used for obtaining a reproduced signal by way of a light beam. In FIG. 1, the reference T.sub.1, T.sub.2 and T.sub.3 are information tracks of a disc; 2 is a light beam for reading; and 1 and 3 are auxiliary beams for detecting tracking signal, and the difference between reproduced outputs of the auxiliary beams 1 and 3 is detected using a differential amplifier so as to obtain a tracking signal 4 shown in FIG. 1. In the above, since the information tracks T.sub.1, T.sub.2 and T.sub.3 are moving relative to the light beams 1, 2 and 3 due to the eccentricity of the disc, the tracking signal 4 shown in FIG. 1 is obtained. A range "A" within the tracking signal 4 indicates that the beams 1, 2 and 3 are scanning the track T.sub.2, a range "B" indicates that a portion between the tracks T.sub.1 and T.sub.2 is being scanned.
FIG. 2 is a structural diagram of the tracking servo. The reference 10 is a disc; 11 is a motor for rotating the disc 10; 12 is a reading lens; 13 is a light source used so that the disc 10 is irradiated by auxiliary beams 1, 2, 3 for tracking as shown in FIG. 1 using a diffraction grating 14. The reference 15 is a beam splitter; 16 is a tracking mirror, and these elements are used for controlling the position of the light beam so that the reading light beam, which is focussed on the disc 10 by way of the reading lens 12, is always on a track. The references 17 and 18 are photo detectors for receiving reproduced outputs of the auxiliary beams 1 and 3 shown in FIG. 1, and the tracking signal 4 of FIG. 1 is obtained at an output of a differential amplifier 19. The reference 20 is an equalizer which is a filter for correcting the characteristic of the tracking servo system. The reference 21 is a D.C. amplifier which supplies a current corresponding to the output from the equalizer 20 so as to effect tracking control wherein the deflecting angle of the tracking mirror 16 is controlled.
Assuming that the rotational speed of the disc 10 is 1800 rpm, the eccentricity of the disc 10 has a harmonic component of a fundamental wave of 30 Hz, and therefore, the tracking mirror 16 has to be operated so as to scan an information track of the disc 10 by the reading light beam 2 of FIG. 1 by tracking the eccentricity. As a result, at least the band of tracking servo is required to cover all the harmonic components of the above-mentioned eccentricity. For instance, assuming that 30 Hz component of eccentricity is 100 micron and 1 kHz component is 2 micron, the reading light beam 2 has to follow the information track of the disc 10 with an accuracy of .+-.0.1 micron. Therefore, the gain of the tracking servo at 30 Hz is required to be 100/0.2=500, and the gain at 1 kHz is required to be 2/0.2=10 (wherein 0.2 means .+-.0.1 micron). In this way the tracking servo band width and gain of the same are selected.
FIG. 3 illustrates a block diagram of the tracking servo system described with reference to FIG. 2. The reference R is a signal indicative of the posit
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Journal of the Audio Engineering Society, vol. 28, No. 6, Jun. 1980, pp. 429-432, New York, US; K. Okada et al.: "A New PCM Audio Disk Pickup Employing a Laser Diode", FIG. 1; p. 340, paragraph 2.2.
Matsushita Electric - Industrial Co., Ltd.
McElheny Jr. Donald
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