Onboard digital transmultiplexers and regenerative transponders in satellite systems employing multiple spot-beam antennas will play an important role in realising future cost-effective communications networks for use in developing countries and where isolated user terminals require access to the global telecommunications network. In the paper, a new fast algorithm for the realisation of a complex input/complex output fast transform based digital transmultiplexer (TMUX) is presented with application to a quadrature multicarrier modulation and demodulation system. The novel approach adopts a splitting of the transform processor into two conjugate operations, and a decomposition of the phase offsets is performed leading to a saving in the number of arithmetic operations required. A reduction in the amount of control circuitry required for implementation is also facilitated. An experimental four-channel QPSK modulator based upon the new technique has been implemented for hardware simulation purposes using a TMS320C25 DSP based system, and performance evaluation results are presented where 16 bit integer DSP processing is employed.