A method is taught for providing a reference video signal to a video display
a video interface from a video display adapter for a host computer. The method
includes the step of providing a horizontal sync line to carry a horizontal sync
signal that occupies pre-defined time intervals as a representation of a horizontal
sync pulse. Another step is signaling to the video display adapter that a reference
amplitude pulse will be sent on a video data line. A further step is sending the
reference amplitude pulse on a video data line during the pre-determined time intervals
when the horizontal sync signal occupies the horizontal sync line.