Modem for telephone system

Modem comprising a modulation section for converting a modulated IF bit stream into phase and a demodulation section for converting a phase-modulated IF signal converted into a look-alike into the bit stream from which this IF signal was converted. In the bitstream, each given number of successive bits defines a symbol. In the modulation section, each symbol is phase modulated by MDDP conversion (modulation by differential phase shift). The symbols are digitally filtered by an RIF filter (13) to provide a filtered signal which, converted into a modulation signal having a modulation frequency centered on a predetermined frequency and which deviates therefrom according to the value of the symbol.