| 摘要 |
A drop-out detector for detecting the absence of a pulse in either of first and second pulse trains, especially for use with the spindle servo error generator of a videodisc mastering machine. Prior attempts at providing a drop-out detector function for the spindle servo error generator rely on the use of RC based timing circuits to create time windows within which to look for the presence or absence of a reference or tach pulse, as the case may be. Such arrangements provide satisfactory performance in connection with constant angular velocity type discs, since reference and tach frequencies remain constant throughout the mastering process. However, problems arise in utilizing such RC based circuits in connection with constant linear velocity type discs. The present invention overcomes these problems by operating on a pulse width modulation technique for controlling the spindle motor speed. In such an arrangement, rather than to phase compare two AC signals, one from the reference oscillator and one from the tachometer, a three-stage shift register is provided whose output is pulse width modulated by the difference in time relationship between two pulse train inputs, one pulse train representing a reference standard and the other representing the tachometer output. A detector indicates when the states of the first and third stages of the shift register are the same which signifies an improper relationship between the tachometer pulses and reference pulses. |
