Abstract: A method of combining and separating a low data rate digital channel with or from the high data rate digital channel of a transmission link in which the low data rate digital channel S1(t) is combined with the high data rate digital channel signal S2(t) in a composite transmission S(t) by overmodulation or by amplitude superposition, and in which the high data rate digital channel signal S2(t) is regenerated from the composite transmission S(t) by comparing the instantaneous amplitude value of the signal S(t) with a level (d, d') close to one-half of the minimum useful amplitude of the component of the high data rate digital channel signal S2(t) in the composite transmission S(t), the method being characterized in that the low data rate digital channel signal S2(t) is extracted from the composite transmission S(t) by subtracting the regenerated high data rate digital channel signal S2(t) from the composite signal after both the composite transmission signal S(t) and the regenerated high rate digital channel s