| 摘要 |
PURPOSE:To reduce the error of the revolving speed of a disk even in case the clock signal to be detected out of the disk is omitted, by holding the output voltage of a rotational frequency detecting means at the relevant time point in response to the output of a clock omission detecting means. CONSTITUTION:When the omission of a clock is detected by a detecting circuit 12, a voltage signal (e) of the relevant time point is held by a sample holding circuit 8 in response to the change of an output signal (g). The holding voltage (d) of the circuit 8 is inputted to a differential amplifier 7. While a switch 3 is changed to the B side when the clock is omitted and the error signal between the voltage (d) received from the amplifier 7 and the signal (e) serves as the control input of a motor driving circuit 4. In other words, a 2nd servo control means functions to maintain the revolving speed of a disk when the clock is omitted. When a clock signal (b) is detected, the switch 3 is set back to the A side. Then a 1st servo control means works. In such a way, the error of the revolving speed is extremely reduced.
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