A synch roll
, or sync roll
is a phenomenon that occurs when the synchronization between a video player's head and the video encoded on the tape is lost and gradually resynchronized. It usually occurs when there is a gap, drop out, or misalignment of the videotape's control track
that is used to tell the VCR/VTR where the beginning of each frame begins. When the control track suddenly reappears, the head may already be in the middle of reading a frame of video, in which case the speed adjusts to synchronize with the video on the tape, causing a vertical 'rolling' or 'scrolling' of the picture on the television.