Strictly speaking, isochronous means, “having a periodic nature.” Sine waves are isochronous. In audio-speak, however, isochronous usually refers to two signals which, while produced at the same sampling rate, are not synchronized. In this case, due to small variations in the clocks which drove the sampling frequency, the two signals will eventually drift out of synchronization, even if started at precisely the same time. Isochronous signals are a problem when attempting to play back audio, video, and/or control signals which require timecode-accurate alignment for the very accurate long-term synchronization of sound and vision. See also FireWire, coherence, incoherent.