Transmultiplexer Realization with Multistage Filtering

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.