Timebase correction
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Timebase correction is a technique to reduce or eliminate errors present in all analog recordings on mechanical media, including video tape recorders and videocassette recorders, caused by mechanical instability. If the mechanism ran at an absolutely constant speed, and never varied from moment to moment, or from the time of recording to the time of playback, then the timing of the playback signal would be exactly the same as the input. However, as laws of physics prevent mechanisms from doing this, the timing of the playback always differs from the original signal. The discrete nature of video signals - division into lines and fields - means that any such timing distortion must appear as timebase errors.
Video signals are more sensitive than audio signals to mechanical error, due to their wider bandwidth. Even back in 1956, let alone today, professional reel-to-reel audio tape recorders relying on mechanical stability alone had no audible pitch distortion, and no need for timebase correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals. Timebase correction fixes (or at least reduces) these errors by inserting a variable delay in the video stream, then adjusting this delay in real time so that the output signal appears at precisely the right rate and time.
Since video is written and read in diagonal tracks, using heads on a spinning drum to read or write a moving tape, there are many potential causes of timing errors. Causes of longitudinal error (error in the long direction of the tape) include variations in the rotational rate of the capstan drive and jamming of tape in the machine; transverse error (error in the cross tape direction) is caused by variations in the rotational speed of the scanning drum and differences in the angle between the tape and the scanning heads (usually addressed by video "tracking" controls). Longitudinal errors are similar to the ones that cause wow and flutter in audio recordings. Since these errors are not so subtle and since it is standard video recording practice to record a parallel control track, these errors are detected and servos are adjusted accordingly to dramatically reduce this problem.
[edit] Reference
Extensive guide to TBCs and their selection
A digital synchronizer for a video-tape recorder, Bucciarelli, F.V.; Proceedings of the IEEE, Volume 61, Issue 4, April 1973 Page(s):506 - 507
