Dihedral Maladjustment refers to a condition resulting from a miscalibration of the playback heads. It is similar to skew in that it is a timing error resulting in a “bending” of the video image, however, skew is displayed as a uniform bending in one direction (left or right), and dihedral maladjustment displays as a fanning or V-shaped distortion, generally at the top of the image. Proper calibration of video heads requires that they are positioned 180 degrees apart, resulting in a full helical scan of the tape from rotation to rotation. The head switch controls which head is active with each scan of the tape. If the heads are slightly out of 180 degree phase, recorded video fields will be read too early or too late, resulting in a distortion of the image. The v-shaped skewing occurs because of the toggled field mispositioning, with lines read too late positioned to the right, and lines read too early positioned to the left.1
Can it be fixed?
Dihedral maladjustment can be recorded into a video image, or it can occur as a result of poorly calibrated record machine. It may be possible to eliminate the dihedral artifact with a TBC, or in cases where it is severe, the playback heads on a machine can be calibrated to mistime correctly for the recorded program.
1. Johannes Gfeller, Agathe Jarczyk, and Joanna Phillips, “Dihedral Maladjustement” in Compendium of Image Errors in Analogue Video (edited by Swiss Institute for Art Research, Zürich: Scheidegger & Spiess, 2012), p.93, video 45. ↩