| 摘要 |
PURPOSE:To attain an excellent focus servo at all times, by performing focus control with a focus control signal whose level is increased or decreased with the lapse of time sequentially until the next detecting output appears in response to an output generated through the detection of the fact that an output defference between a peak hold means and an envelope detection means is in excess of a prescribed threshold value. CONSTITUTION:A comparator 46 is inverted from a positive from a positive to a negative level, when an output of the envelope detecting circuit 44 is increased sequentially, the output is reduced again via an envelope maximum value and when a position (x) representing the amount of focus shift is a point C, since a difference between an envelope voltage Ve of the envelope detecting circuit 44 and a hold voltage Vp of a peak hold circuit 44 is a threshold voltage e0 of a comparator 46, then an output of an integration circuit 49 starts decreasing, a DC component of a differential amplifier 11 is again decreased and changed in the direction of a point B. A DC component inputted to a drive circuit 12 fluctuates between the points C and B and control is attained so as to move an output voltage of the envelope detecting circuit 44 between voltages represented in broken lines 32, and the excellent focusing is maintained by setting suitably the threshold voltage e0. |
