A vehicle ignition signal is sensed and digitized for subsequent display on a display device, the digitization comprising sampling the sensed signal at a number of predetermined time intervals, all shorter in duration than an interval determined by the display device resolution, to determine a signal transition to a maximum spark value, to determine the maximum spark value, and to determine the sample within each display resolution interval having the greatest absolute value displacement from that of the determined sample from the previous resolution interval, the determined sample and the maximum value sample being displayed on the display device.