A description is given for a delta modulation system wherein the sampling interval is varied as a function of the characteristics of the signal to be coded. The system thus operates asynchronously as compared with the usual delta modulator, and permits the transmission of fewer samples during periods of small signal variation. The algorithm used for sampling interval variation is common to both transmitting and receiving ends of the system, and so signal reconstitution at the receiver is possible without the explicit transmission of interval information. Some results are presented in the form of line printer generated images.