Circuit arrangement for generating the vertical synchronizing

Abstract

In magnetic video-tape equipment of the helical-scan type the vertical synchronizing information is situated at the beginning of a track near the edge of the magnetic tape. When the edge portion of the magnetic tape does not cooperate correctly with the head in the case of a faulty magnetic tape or apparatus, the vertical synchronizing information is not scanned and the resulting picture will roll in a vertical direction. However, since the beginning of a track coincides with the edges of the head-change signal, the vertical synchronizing information, in accordance with the invention, is always derived from the head-change signal, i.e. even during normal undisturbed operation, and is added to the CVBS signal, the signal scanned from the magnetic tape being blocked during this time. For this purpose the edge of the head-change signal is delayed and this delay is controlled so that normally the generation of the vertical synchronizing information from the line-pulse signals which are synchronized with the preceding track starts halfway between two such line-pulse signals or their active edges. Also when the permissible phase shift to the head change signal relative to the line-pulse signal upon each head change is taken into account the vertical synchronizing information always starts an equal number of half-line periods after the beginning of a track, thereby precluding a vertical picture jump, not even by half a line period.