Lengthwise Tape Expansion or Shrinkage


Comparable to skew error, lengthwise distortion is often very pronounced. “The track angles on a severely stretched tape will be too shallow, on a severely shrunk tape they will be too steep: in both cases they will no longer correspond to the scanning angle of the playback heads… causing the playback heads to lose contact with segments of the helical tracks.”1

Unlike skew error, the error is not due to playback equipment. Also, audio on a tape with helical scan will likely be affected. “If FM or PCM audio is recorded on the helical track, even mild tape expansion or shrinkage may render it distorted or totally inaudible.”2

“Factors that can cause tape to expand include excessively high temperatures, the specific composition of the tape substrate, and excessive mechanical tension as a result of e.g. excessive tape pack tension, playback on defective equipment, or Sticky Shed Syndrome.”3 Running a tape through a cleaning machine under certain conditions can also stretch tape and introduce skew error. “Factors that can cause tape to shrink includes—again—excessively high temperatures, because the tape substrate is pre-stretched during production and if heated up again, may contract. Excessive climate fluctuations may also cause tape to shrink.”4

Can it be fixed?

Again, this error is similar to skew error. Some VTRs have skew controls that allow for minor adjustments in tape tension, most notably, several models of U-matic machines and some 1/2” open reel models. If possible, use this together with tracking control to adjust tape tension and head alignment. Otherwise, skew error may be minimized through the use of a good TBC. It may also require an alignment adjustment or back tension adjustment in the player equipment, but be careful: this kind of tinkering should be done by a trained technician and not performed on equipment used for recording.

References

1. 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 && 10.
2. Gfeller et al., “Lengthwise Tape Expansion or Shrinkage.”
3. Gfeller et al., “Lengthwise Tape Expansion or Shrinkage.”
4. Gfeller et al., “Lengthwise Tape Expansion or Shrinkage.”

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