Motor speed control for video disc reader - has servo amplifier and comparator circuit acting on photocell motor speed sensor output and reference signals

摘要

The video disc motor is controlled to maintain a constant speed by utilising a servo loop in which the phase of a signal from the reader is compared with a reference. A signal from a photocell sensing the motor shaft rotation is applied to a high gain differential input amplifier (IC1) operating as a comparator. The square wave output is applied to a transistor (T1) and diode circuit (D1-4) to convert frequency to current for application to a second amplifier (IC2). This amplifier compares the value of current with a reference voltage to produce a corrected error signal which is applied to a differential input power amplifier (IC3) to drive the motor (101). A switching stage (F1) allows two disc formats to be utilised.