1. A method for improving the signal-to-noise ratio in a digital communication system employing a differential coder using a feedback path at a transmitter thereof for producing, where i is an interger, prediction error samples e.sub.i from successive input data samples s.sub.i in an input data stream and employing digital interpolation at a receiver thereof to reconstruct data samples s.sub.i in an output data stream representative of the input samples s.sub.i, said method comprising the steps of: processing at the transmitter at least two successive n-bit input data samples s.sub.i supplied thereto for producing, from a corresponding prediction error sample e.sub.i being a linear function of at least one order of said successive n-bit input data samples, an encoded prediction error sample e.sub.i ' for transmission over a data channel, periodically robbing from the data channel a number of encoded prediction error samples e.sub.i ' and replacing in the feedback path of the differential coder each input data sample s.sub.i affected by said robbing by an estimated input sample E(s.sub.i '), where E(s.sub.i ') is a linear function of at least two orders of previously supplied input data samples s.sub.i ', reconstructing said input data signals at the receiver by decoding successive prediction error samples e.sub.i ' transmitted over said data channel, said decoding being performed by establishing a predicted received sample s.sub.i on the basis of the autocorrelation function of at least one order of previously decoded samples s.sub.i ', decoding each received data sample as s.sub.i '=e.sub.i '+s.sub.i estimating the received data sample E(s.sub.i ') as a function of at least two previously decoded data samples s.sub.i ', after receiving at least one additional prediction error sample e.sub.i+1 ',. .. and decoding at least one additional data sample s.sub.i+1 ', re-estimating the data sample E(s.sub.i ') by delayed interpolation thereby to generate a re-estimated data sample E((s.sub.i ')), and replacing the decoded data sample s.sub.i ' in the output data stream with the re-estimated data sample E((s.sub.i ')) thereby to reconstruct the transmitted input data signal s.sub.i with an improved signal-to-noise ratio in the communication system.