A digital transmission system including a digital signal generator (GEN) able to generate a rectangular output signal (SX2, SX2ʹ) , e.g. in the audible frequency range. The frequency and the amplitude of this output signal are respectively determined by the frequency (500/16,000 Hz) of a square wave (SQ1, SQ1ʹ) and by the duty cycle of a rectangular wave (SE0) both waves being created in the generator under control of an incoming digital word, the rectangular wave chopping the square wave. The frequency content of the spectrum of the output signal is identical to that of the square wave.