Method and apparatus for analog graphics sample clock frequency verification

A method of converting an analog video signal for a video image formed as a sequence of frames of video lines of pixels into a digital video signal, the method comprising:

• sampling the analog signal with a clock with controllable frequency and selectable phase;
• coupling an analog-to-digital converter for receiving the analog signal to the clock;
• using the analog-to-digital converter to convert the analog signal into the digital video signal at sample points controlled by the clock;
• selecting a sequence of phases for the clock;
• using the digital video signal, computing a measure of the difference between a pixel from one frame and a corresponding pixel in another frame for each phase in the sequence of selected phases;
• and
• verifying the frequency of the clock using a characteristic of the measure of the difference between successive frames.

A digital display device comprising a video signal conversion circuit constructed to receive an analog signal of a video image formed as a sequence of frames of video lines of pixels and to produce a digital signal representative of the video image, the video signal conversion circuit comprising:

• a clock with controllable frequency and selectable phase for sampling the analog signal;
• an analog-to-digital converter coupled to the clock for receiving the analog signal and converting it at sampling points controlled by the clock into the digital signal;
• a computation circuit to compute for each phase in a sequence of selected phases of the clock a measure of the difference between a pixel from one frame and a corresponding pixel in another frame, using the digital signal;
• and
• a verification circuit to verify the frequency of the clock using a characteristic of the measure of the difference between successive frames.