The paper deals with the use of a digitally controlled voltage source (d.c.v.s.) as a general-purpose interface element. This concept has been successfully applied to bidirectional analogue-digital (a.d.) and radix conversions. The element may also be controlled from a digital computer to synthetise ‘analogue’ functions. A further application involves the use of a series of d.c.v.s. elements to generate a power series of terms from which sine, exponential, logarithmic and other functions may be constructed using a binary counter to provide the digital input. Methods of generating repetitive or nonrepetitive functions are outlined which can be given precisely controlled frequencies and/or amplitude. The D.C.V.S. element may be given to independent digital inputs as well as an analogue input, and this allows the generation of very complex waveforms. Relatively unsophisticated linear- and logic-circuit packages will achieve accuracies within 0.1% in the applications described.