| 摘要 |
PURPOSE:To perform automatic balance control over a signal level by controlling the gain of a variable gain amplifier so that the amplitude difference of the pilot frequency signal in the input signal of a differential amplifier is minimum. CONSTITUTION:When the amplitude level detected by the phase sensitivity amplitude detecting circuit 75 of an amplitude detecting circuit 72 is larger than a reference voltage Vr, a switch 71 is closed with the H-level output of a comparator 76 and a closed loop for phase control is formed. The output of an amplitude adjusting circuit 65 is inputted to a phase sensitivity amplitude detecting circuit and a signal output corresponding to the phase difference of the pilot frequency signal of frequency f2 from the reference signal of a band-pass filter 77 is obtained from a low-pass filter 67. The delay time of a delay circuit 41 is so controlled that the phase difference between the reference signal and pilot frequency signal is 90 deg., and then the residual component of the pilot frequency signal of frequency f2 appearing at the differential output owing to the phase difference is minimized and the noise component in the differential output is also minimized. Consequently, a photomagnetic reproducing device is held in an invariably optimum state. |
