Transmultiplexer Via Spread-Spectrum Modulation

A new scheme is derived for the digital TDM-FDM conversion (transmultiplexer) of N voice channels. In the complex version it simply consists of 1) a polyphase filter and 2) a modulator with carrier W_2Nt²=exp(j2πt²/2N) where t is 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 bank N modulators or the N-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 of N=64 voice channels.