Abstract: Digital signals which are sent each with their own frequency are corrected for frequency-dependent attenuation of the transmitting medium in a filter (5) so as to maintain a desired relationship between the amplitudes of the signals after the filter. Difference circuits (6) which have output signals (US1, US2, US3) control the filter means, obtain signals (Uf0) on their first inputs from a common voltage supply circuit (9) and signals (Uf1, Uf2, Uf3) on their second inputs from a voltage supply circuit (10) for each of the output signals. A decoder (18) sends a control signal (S0, S1, S2, S3) associated with the respective digital signal. The control signals reset switches (17) whereby the voltage supply circuits (9, 10) obtain signals from an amplitude detector (13) corresponding to the respective frequency in the transmitted signal. Each of the output signals (US1, US2, US3) fed to the filter (5) by the difference circuits (6 ) thus corresponds to a digital signal.