Abstract: In a demodulation device for demodulating a phase-modulated wave, a baseband signal producing circuit produces a baseband signal having particular information which is dependent on the phase-modulated wave. Responsive to the baseband signal, a phase information producing circuit produces a phase difference signal. A reception logic circuit detects the baseband signal to produce an alarm signal when the particular information is not detected from the baseband signal. In response to the alarm signal, a reset signal producing circuit produces a reset signal. With reference to presence and absence of the reset signal, a selector selects, as a control signal, one of the phase difference signal and a cancelling signal which is supplied to the selector. In accordance with the control signal, a wave producing circuit produces a reproduced and a phase-shifted carrier wave which are used to make the baseband signal producing circuit produce the baseband signal.

Abstract: A motor speed control device for use in an image forming apparatus with a differential phase detection circuit for outputting differential phase data between the clock pulse and a frequency proportional signal representative of the motor speed and a differential speed detecting circuit for outputting differential speed data between the length of the clock signal and the length of the speed signal. A pulse width modulated signal obtained from the two signals controls the speed of the motor.