what about google? This defines simplex, half duplex, full duplex: http://physinfo.ulb.ac.be/cit_courseware/datacomm/dc_014.htm This defines full duplex for sound cards (the same way) : http://www.techtv.com/callforhelp/answerstips/story/0,24330,2236390,00.html Does anybody have a link which defines full duplex as "combining two channels in the same direction"? (whatever - multiplexing, adding, modulating...) Wolfgang