The advent of the single-chip microprocessor has made possible the use of digital signal processing in low-cost communications circuitry. The all-digital approach has been used in a high-performance data modem for the transmission of burst messages through the voice channel of v.h.f. narrow band f.m. radio sets. The techniques used in this novel design of software modem are illustrated by reference to the version using coherent quadrature phase shift keying (q.p.s.k.) modulation and constraint length 48 convolutional encoding with sequential decoding.