Video signals rely on a complex, synchronized timing of information across all broadcast, playback and recording environments. Tiny changes in this timing can therefore affect the stability in a video signal, causing what are known as ‘time base errors.’ These errors take on the form of “flagging” or flagwaving at the top of the screen, “skewing” error at the bottom of the screen, or an interspersed, horizontal “tearing” of the image throughout the picture area.
Several causes of time base error include sudden changes in tape tension, uneven capstan rotation, and/or variations in speed throughout the tape path. The degradation of tape itself can also contribute to time base error. Tape that has been stretched or otherwise compromised can suffer from longitudinal error (often seen in environments where tape cleaning machines are used) and transverse error, or inconsistencies in a tape’s width.
Can it be fixed?
A Time Base Corrector (TBC) will help counteract time base error by storing lines of video and retrieving them again at a rate matching a signal’s “correct” horizontal sync. Time base error can also be corrected during playback by adjusting a video deck’s tape tension using the skew control. Poorly maintained video decks that suffer from such things as variations in tape speed, head speed and uneven capstan rotation can contribute to time base error.
Three conditions caused by time base error - flagwaving, skew error, and tearing. Source: Global Village flyer, ca. 1980.
Johannes Gfeller, Agathe Jarczyk, and Joanna Phillips, “Lengthwise Tape Expansion or Shrinkage” in Compendium of Image Errors in Analogue Video (edited by Swiss Institute for Art Research, Zürich: Scheidegger & Spiess, 2012), p.60, videos 9 and 10.