Abstract: Correlation between a waveform displayed on a waveform display device and the corresponding portion of a video image displayed on a monitor is achieved by brightening a portion of a video input signal from which the waveform and video image are produced. A horizontal sweep signal is compared with voltages representing the left and right edges of the waveform display device screen to determine the portion of the video input signal that is "on-screen". The on-screen signal is combined with a line select signal to produce a control signal. The control signal is used to level shift the video input signal to provide a "bright-up" feature as part of the video image on the monitor, indicating the portion of the video image being displayed as a waveform on the screen of the waveform display device.