Oscilloscope

An oscilloscope is the simplest kind of CRT display: the beam repeatedly moves from left to right at a user-set speed, and as it does so, it's jiggled up and down by an input signal.

This allows it to draw a waveform on the screen. If the waveform in question repeats at the same frequency as the beam, you can see a single cycle of it at a time, animated in realtime as its timbre and volume change.