Carrier wave reproducing circuit in synchronized detection system of medium speed facsimile

Claim 1

In a signal demodulator circuit for demodulating a modulated input signal containing black information and white information, said demodulator of the type including a synchronized detector for receiving said modulated input signal and detecting said modulated input signal, a carrier reproducing circuit for generating a reproduced carrier from said modulated input signal and applying said reproduced carrier to said synchronized detector, said modulated input signal including signal portions representing said black information and signal portions representing said white information, the improvement characterized in that: said carrier reproducing circuit includes a phase synchronizing loop connected to receive said modulated input signal for providing said reproduced carrier to said synchronized detector; and disabling means for disabling a phase locking of said phase synchronizing loop in response to a predetermined level of representation of said black information, said loop continues to provide an output even when not phase locked; and said phase synchronizing loop comprises: a voltage controlled oscillator (6), a phase comparator (14) whose inputs are connected to receive said modulated input signal and to receive the output signal of said voltage controlled oscillator, and a loop filter (5) whose input is connected to the output of said phase comparator and whose output is connected to the control terminal of said voltage controlled oscillator through which the output signal of said comparator is applied to the control terminal of said voltage controlled oscillator for controlling the oscillation frequency of said oscillator, said disabling means comprising: switching means (21) provided between the output of said phase comparator and the input of said loop filter; and signal detection means for detecting said predetermined level of representation of said black information and controlling said switching means in accordance with said detected level; and said signal detection means comprises: a band-pass filter (22) of a narrow band connected to receive said modulated input signal for extracting frequency components applied thereto near the carrier frequency of said modulated input signal; and a level detector (23) for controlling said switching means based on said level derived from the output signal of said band-pass filter.