An act named Echtzeit had one of the standout tracks on a compilation titled Music to Listen to Music By, released a year and a half ago on Private Elektro Records, which is based in Leipzig, Germany (website here). Echtzeit has resurfaced with a new name, C:/, and two new tracks, both hosted for the past month by Private Elektro (here). The downloads couldn’t be more different from each another. One is close to eight minutes of overlapping sine waves and beats, all of whose internal clocks are slightly out of step, which leads to wonderful rhythmic surprises, the audio equivalent of optical illusions. At times a light keyboard melody, reminiscent of the 1980s synthesizer renaissance, arrives to lend a song-like coherence. The other track, a lengthy drone piece, dispenses with rhythm almost entirely, unless you count the waves of rumbling that constitute the work’s lower register and the occasional chimes that define its upper reaches.