Abstract: The invention relates to a detector of the mean level of a signal particularly intended to indicate whether an expected alternating signal is absent or present. This detector uses an analog comparator, a digital counter and a converter for establishing an analog signal to be compared with the expected rectified signal. The counter content oscillates round the mean value of the rectified signal. The counter serves as a digital integrator for the sign of the difference between the input signal and the content of the counter, in such a way that on average the input signal is just as often above as below the counter content. The digital - analog conversion can take place with the aid of switched capacitors.

Abstract: A sampled first order high pass filter having a configuration that eliminates the output offset voltage normally associated with known filters of this general type. It is constructed according to the diagram of the accompanying figure with two operational amplifiers, four capacitors and eight switches actuated at each period in two separate phases a and b. The filter is particularly useful in digital telephone transmission. The filter is placed just up-stream of an analog-digital converter sampled at the same frequency as the filter and the second amplifier serves as a comparator for conversion outside the phases a and b.