Abstract: The outputs of two filter channels for accepting two keying frequencies are compared to produce the output. Each channel uses finite impulse response transversal filters; preferably with an impulse response of duration comparable with the symbol period of the incoming signal. Each channel has a pair of filters for accepting phase quadrature components (or, alternatively, quadrature synchronous demodulators followed by filters); the filter coefficients differ from sinusoidal by (or, in the alternative case, are formed by) a modification function which is the same for both filters of the pair.