Vertical blanking interval

In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field[1][2] and the beginning of the first visible line of the next frame or field.

[6] In raster cathode-ray tube (CRT) displays, the blank level is usually supplied during this period to avoid painting the retrace line—see raster scan for details; signal sources such as television broadcasts do not supply image information during the blanking period.

Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, often alternating between fairly steep diagonals from right to left and less-steep diagonals back from left to right, starting in the lower right of the display.

This is especially true in video game systems, where the fixed frequency of the blanking period might also be used to derive in-game timing.

On some very early machines such as the Atari 2600, the programmer is in full control of video output and therefore may select their own blanking period, allowing arbitrarily few painted lines.