This invention relates to a servo circuit performing servo controls such as a focus servo and a tracking servo for controlling relative position between a recording medium and a pickup head in an information recording and/or reproducing apparatus such, for example, as a Compact Disc player for the Compact Disc Digital Audio System, a video disc player and a tape recorder and, more particularly, to a servo circuit of this type capable of reducing electric power consumption without degrading servo faculty by changing the gain in the servo loop in accordance with disturbances such as external vibrations and eccentricity or warping of the disc.
In a Compact Disc player to be mounted in an automobile, for example, floating of the mechanism unit from a source of vibration and increase in the servo capacity in reading a signal from the disc is required for satisfying anti-vibration property of the player. For increasing the servo capacity, it is necessary to increase servo gain of a focus actuator control and a tracking actuator control of a pickup head within a range in which stability of the servo circuit can be secured.
If the servo gain is increased, however, electric current flowing through these actuators increases and thereby generates considerable heat in the pickup head. As a result, the output of a laser beam oscillator in the pickup head changes and this adversely affects the servo gain characteristics with resulting shortening of the life of the laser beam oscillator. Besides, increase in electric current results in a greater load applied to the power source and a larger power consumption.
Such inconveniences arise not only in a Compact Disc player but also in other information recording and/or reproducing apparatus such as a tape recorder in which the position of a pickup head with respect to a recording medium is controlled by means of a servo loop.