Horizontal synchronization is a timing signal used in video interfaces to indicate the end of one scan line and the beginning of the next. In analog CRT displays, it triggers the electron beam to return to the left side of the screen. In digital displays, it remains a fundamental timing reference to keep the pixel data aligned with the physical grid of the panel.
The image is tearing diagonally because the HSYNC line has a cold solder joint and keeps dropping out.
If a monitor loses HSYNC but keeps the vertical sync signal, the display usually scrambles into diagonal, unreadable lines.
Related terms
Scan line
A single horizontal row of pixels or phosphors drawn across a display screen.
Cathode ray tube
A vacuum tube containing one or more electron guns and a phosphorescent screen used to display images.
TCON
A timing controller is an integrated circuit that dictates the precise row and column addressing for a display panel.