The Impact of Some CCITT Recommendations on Transmultiplexer Design

A short description of the transmultiplexer structure is given, and a brief review of some significant CCITT recommendations is presented. The paper briefly explains the influence of these recommendations on the design of the main subblocks of a back-to-back configurated transmultiplexer, the main subblocks being the two baseband filters, the two o²DFT processors and the two channel filters (polyphase networks). The paper emphasizes how the above recommendations influence the design of the channel filters. As a consequence, the crosstalk mechanism is treated in detail and filter requirements based on these mechanisms are presented. The impact of the noise requirements is discussed and computer simulation results are presented. It is concluded that the channel filters which are realized as direct-form IIR filters could be designed using 16 bit wordlengths. Computer simulation results have shown that the o²DFT processor could also be designed using 16 bit wordlength when an optimum scaling strategy is used; however, the noise margin will be unsatisfactorily low in this case, and the number of bits is consequently increased to 20.