Abstract: A digital device has an input for receiving successive samples of input d in digital form representative of phase values .zeta.. A sequence of the most recent input samples is stored in an input memory. A carrier phase source provides a signal representative of the phase .theta. of a carrier wave. A function memory stores values of a function k.sub.i .multidot. sin (.zeta..sub.n-i +.theta..sub.n) and is addressed by first selection means which select a group corresponding to a coefficient k.sub.i and second selection means which select a member of the group corresponding to the value of (.zeta. + .theta.). During a calculation period the sum of all values of the function is calculated for i varying between l and a predetermined number r. Each such sum forms a sample of the output wave which is converted to analogue form and filtered to remove unwanted sampling components.