Abstract: In accordance with the triangle method the two input signals whose sum or difference frequencies are to be formed, are compared with each other as triangular signals, and in the case of equality and equal slope a difference pulse and for the opposite direction a sum pulse is formed. The triangular signals, according to the invention, are each produced by a counter, which counters are driven by the input signal or by a signal whose frequency is a corresponding multiple of the input signal frequency. When one of the two final counts of each counter is reached, an associated memory is reset, which reverses the counting direction of said counter and at the same time indicates the direction of the slope of the triangular signal. The counts are compared in parallel in a digital comparator and the output signal of said comparator in conjunction with the final-count or transfer memories of the counters produces the pulses for the corresponding output signal.