An audio signal is typically visualized as a bar chart or wave. The bar or wave is high when the sound is loud and low when it is soft. We recognize that a repetitive sound, such as a metronome's beat, makes repetitive peaks and valleys in the visualization. When audio has multiple channels (such as a stereo or surround-sound recording), each channel can be considered a separate signal and can be visualized as a separate bar chart or wave.

Similarly, in a video, every channel of every pixel can be considered a separate signal, rising and falling (becoming brighter and dimmer) over time. Imagine that we use a stationary camera to capture a video of a metronome. In this case, certain pixel values will rise and fall at a regular interval as they capture the passage of the metronome's needle. If the...