A new scheme is derived for the digital TDM-FDM conversion (transmultiplexer) ofNvoice channels. In the complex version it simply consists of 1) a polyphase filter and 2) a modulator with carrierW2Nt2=exp(j2πt2/2N) wheretis the normalized discrete time, both working at the rate of the TDM (or FDM) flux. The polyphase filter is quite different from the polyphase network encountered in other transmultiplexer schemes: it is one-input one-output with a constant phase in each sideband of the resulting FDM signal. The modulator replaces the bankNmodulators or theN-point DFT processors of usual schemes. The derivation of the scheme is carried out in general, and its complexity is discussed with reference to the standard format ofN=64 voice channels.

Starting from the general principle of frequency shifting a 4 kHz bandwidth digital filter to perform the TDM-FDM conversion, a new decomposition of the filter is introduced. This decomposition allows for the use of a DFT of much lower dimension than the number of channels to be multiplexed, together with a few modulators (multipliers) working at the highest bit rate, and a tree configuration of filters and simple modulators. The filters are of a few different types and can be designated to have very simple coefficients, so avoiding the use of true multipliers; furthermore, wave digital filters, with their known advantages, can be used for some of the more severe filtering steps.