Pop stars with more charisma than talent have long enjoyed the benefits of software that corrects out-of-tune notes in recorded performances. But that software has worked only with notes that sound one at a time, as in a vocal line or a sax solo. New technology from the German company Celemony, however, can pull apart notes played at the same time--on different strings of a guitar, for instance--and modify them individually. The software even works in real time, for live variation of recorded loops.

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