In digital audio recording, the voltage of the analog audio waveform is "sampled" in a manner analogous to the manner in which motion picture film captures motion as a series of still photographs. Because it is necessary to sample audio at very high frequencies to accurately represent the highest (treble) frequencies present; the consistency of the timing of the conversion is paramount to good performance.
During reproduction, the timing of the DA conversion (or “de-sampling”) is equally important to accurate results. Any difference in the timing of the sampling or de-sampling will result in distortion of the waveform, which causes it to differ from the original in both the time and amplitude domains. The way this distortion is perceived can be very different than the perception of harmonic or intermodulation distortions common to analog audio.
Relatively long-term variations in the sample frequency can result in the perceived pitch of the audio changing, while short-term variations such as jitter can result in the generation of non-musical distortions.
Before the advent of digital audio, time domain distortions resulted from differences between the timing of the recording and that of the playback. Examples include off-speed playback, which could result from recording, playback, or both being at something other than the nominal speed. Periodic “pitch” variations could be caused by things like the center hole of a vinyl record being off-center or the speed of analog tape varying. A common form of long term variation in analog tape recording is referred to as “wow” and could result from tape tension varying as the reels rotated. Deformation of the tape in the form of stretching could effectively slow the playback in the stretched section of tape.
Short term variations referred to as “flutter” could cause modulation distortion as the tape vibrated in the vicinity of the magnetic playback head. Factors such as the tendency for the tape to vibrate like the bow of a violin as it passed stationary guides or the heads, or bad bearings in guide rollers could result in flutter.