ADSL and VADSL splitter design and telephony performance

The effect of introducing ADSL and VADSL splitter filters into the telephony transmission system is discussed, first with passive filters and then with active filters. The advantages of the active approach are demonstrated through examples of both passive and active splitter designs. The discussion is prefixed by a review of telephony transmission in the access network, with particular emphasis on performance and metrics used for its assessment. The various definitions of return loss and loudness ratings are explained and the implications for subjective performance given. Particular attention is drawn to impedance considerations and the reasons that complex reference impedances are used in many countries