Abstract: The invention relates to filtering a periodic digital signal having N logic levels and having noise superposed thereon. The instantaneous probability of existence of each of these logic levels is converted into a corresponding one of N signals each having its own maximum amplitude when the modulated value of the received instantaneous signal is equal to the normal value for a corresponding logic level, which amplitude falls off by application of a weighting coefficient as the received instantaneous value moves away from the normal value of that logic level. The signal relating to each looked-for logic level is integrated over the duration of an elementary modulation period in the received digital signal, and the recognized logic level is the level corresponding to the largest value integral.