Importance

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Abstract

Scanning apparatus for an optical record carrier includes a position control loop, a focus control loop, and an adjustment circuit for stabilizing a control parameter of each loop such as the gain and frequency transfer characteristic thereof. To detect the value of the control parameter of a control loop a calibration signal of relatively high frequency is intermittently introduced therein. This makes possible more sensitive detection of the loop response than if a low frequency calibration signal were employed. Despite the higher frequency, the power consumption of the apparatus due to the calibration operation is relatively low because the calibration signal is only intermittent. The adjustment circuit detects the signal produced by a control loop in response to the calibration signal and by comparison therewith derives an adjustment signal for adjusting the control parameter of such loop. The adjustment signal is stored in digital form, whereby it is retained without drift even during off intervals of the calibration signal. Switching means are provided for enabling the loop adjustment circuit to be selectively connected to either control loop. Such switching means may take the form of a programmable digital control unit.

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First Claim