A communication apparatus is used to interface signals with any one of a plurality of telephony networks having different impedances. The apparatus includes memory, a processor, a digital-to-analog (D/A) converter, and an interface port. Different sets of impedance control values are stored in the memory....http://www.google.com.tw/patents/US7020274?utm_source=gb-gplus-share專利 US7020274 - Universal apparatus and method for interfacing signals with telephony networks
