Abstract: Apparatus for indicating the timing relationship between a reference point of a repetitive input signal, e.g. the horizontal sync point of a video signal, and a signal element that repeats at a higher frequency than the input signal, e.g. an element of the color burst, comprises a phase-locked oscillator for generating a continuous wave signal that is in phase with the signal element. The continuous wave signal is used to generate a train of pulses having a repetition frequency that is equal to the repetition frequency of the signal element and is selectively variable in phase relative to the continuous wave signal, and this train of pulses is used to modulate a display of the repetitive input signal in a dimension other than one that is used in a normal waveform display, e.g. intensity.