Tape is susceptible to expansion and shrinkage, see Lengthwise tape expansion or shrinkage. These dimensional changes impact the length and angle of the recorded tracks. On playback, the loss of correspondence between the track position and the playback head results in a skewed picture. “A skew error shows as a hooking in the picture at the top [or bottom] of the TV monitor. If the picture hooks to the left, the video track on the tape is longer than the playback track length of the machine. If the picture hooks to the right, the video track on the tape is shorter than the playback length of the machine.”1 Audio may also be affected.
“Even if there is no actual image error, it is possible to see, at the bottom of the uncropped image (i.e. in underscan mode), a slight, jittery line displacement immediately below the head switching point. If the symptom is very pronounced, the skewing will also appear along the top edge of the image, where it becomes visible as a disruptive skew error.”2
Can it be fixed?
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. 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.
Example(s)
Skew is visible at the top of the screen in this short clip.
References
1. Charles Bensinger, The Video Guide (2nd ed.), (Santa Barbara, California: Video-Info Publications, 1981), 124. ↩
2. Johannes Gfeller, Agathe Jarczyk, and Joanna Phillips, “Skew Error” in Compendium of Image Errors in Analogue Video (edited by Swiss Institute for Art Research, Zürich: Scheidegger & Spiess, 2012), p.100, video 52. ↩