a waveform storage and display system particularly useful in digital oscilloscopes is provided in which successive waveforms of a repetitive signal are compared with a reference waveform , and upon detection of a difference or change within the waveform , the changed waveform is automatically stored . in addition to the conventional acquisition and display waveform memories , a reference - waveform and a changed - waveform memories are provided . once a reference waveform is selected and stored , comparison of subsequent successive waveforms to detect changes is achieved under microprocessor control . several display modes to provide a visual indication of changes may be implemented .