Abstract: A video adapter converts an analog image signal output from a personal computer to a digital signal. The clock frequency, phase data, and display position data for the A/D conversion are set automatically according to the analog image signal to adjust for any offset. An A/D converter analog/digital converts the analog image signal from a digital image signal source. An image start/termination coordinate detection circuit detects the video signal Sid start and end coordinates of the digital video signal from the A/D converter using the synchronization signals and clock, and then generates coordinate data. A clock generation circuit generates the clock input to the A/D converter based on the input from the display control circuit. A delay circuit delays the horizontal synchronization signal in the input analog image signal based on the phase data from the display control circuit and further outputs the result to the clock generation circuit.