Noise suppression circuit for two-way telephone system - has filter comprising subtractor extracting outputs of impedance transformers at outputs of two integrators

The noise suppression circuit has the control ac voltage derived from the voice channel rectified and passed to a filter to remove the component caused by interference or background noise. The filter's output is rectified to control the gain of the channel. The filter has two input parallel integrators (I1, 12) with different time constants. Two operational amplifiers (OP1, OP2) acting as impedance converters are connected to the outputs of the integrators. A subtractor (OP3) subtracts the outputs of the operational amplifiers from one another and its output is the output of the filter. The time constants are such that the voltage at the output of the first integrator follows the amplitude variations of the LF input to the filter whilst the output voltage of the second integrator is similar to the average of the filter input voltage and is not affected by these amplitude fluctuations.