Music technology (electric) remained so until the 1920s, when several recent radio-related developments in electronics converged to revolutionize the recording process. These included improved microphones and auxiliary devices such as electronic filters, all dependent on electronic amplification to be of practical use in recording. In 1906, Lee De Forest invented the "Audion" triode vacuum tube, an electronic valve that could amplify weak electrical signals. By 1915, it was in use in long-distance telephone circuits that made conversations between New York and San Francisco practical. Refined versions of this tube were the basis of all electronic sound systems until the commercial introduction of the
European Triode Festival European Triode Festival Since 2000 each year the "European Triode Festival" (ETF) has been held in various venues in Europe (the first three events were called "Aarhus Triode Festival" aka "ATF"). The ETF focuses on do-it-yourself audio valve amplifiers using triode vacuum tubes, and brings together high end vacuum tube audiophiles from all over the world (from as far away as Finland and New Zealand). At the festival, participants demonstrate their (all home-built) equipment (analogue/digital/amplifiers/speakers/etc.) and discuss aspects of high-quality vacuum tube sound reproduction. The event is a private venture of in total 300 audio lovers, of which some 120