Fundae Session 1: Stroboscopic Effect

Temporal aliasing is the technical term for a phenomenon also known as the "stroboscopic effect" or the "wagon-wheel effect."

The last name comes from the observation that in video or motion pictures, the spoke wheels on horse-drawn wagons sometimes appear to be turning backwards.

The effect occurs when a) the view of a moving object is represented by a series of short samples rather than a continuous view, and b) the moving object is engaging in repetitive or cyclic motion at rate that is close to the sampling rate.

It is a particular example of a range of phenomena called aliasing which occur when something continuous is represented by a series of short or instantaneous samples—in this case, the consecutive frames of a motion picture or video signal.

For example, consider the stroboscope as used in mechanical analysis. This is a "strobe light" that is fired at an adjustable, variable rate. Let's say you are looking at a moving part that rotates at 60 revolutions per second. Now say that instead of illuminating it with a continuous flash, you illuminate it with a series of very short flashes of light 60 times per second. Each light catches the object at the same position in its rotation. Since at 60 flashes per second the persistence of vision smooths out the visual experience, it appears as if the object is standing still. If you illuminate it at 59 flashes per second, each flash will catch it at a slightly different part of its rotation and it will seem to be rotating slowly; it will take 59 flashes = one second before the flash catches it at the starting point again, and the object will look as if it is rotating once a second. If you illuminate it at 61 flashes per second, each flash catches it a little earlier in its rotation and the object look as if it is rotating backwards.

In the case of a television or movie camera, action is captured as a series of brief snapshots and stroboscope effects can occur..

The reason it is seen so often in motion pictures of spoked wheels is as follows. The wheels of a vehicle don't turn at 24 revolutions per second unless the vehicle is going awfully fast. But if you have precisely-built twelve-spoked wheels, every spoke looks the same as every other spoke and they are all perfectly spaced. So if each wheel turns at only TWO revolutions per second, which is very reasonable, and you film at 24 frames a second, each frame will catch the spokes in the "same" position and the wheel seems to be standing still. Really the spoke that's at the 12-o-clock position in each frame is a different spoke each time, but they all look the same. If the wheel is turning a little slower than 2 revolutions per second, the position of each spoke is a little further behind in each frame and the wheel seems to be turning backwards.

The reason it is called "aliasing" is that in electrical engineering, when a continuous audio signal is replaced by series of samples—say, a 24.1 Hz signal is sampled at 24 samples per second—the result looks the same as if an 0.1 Hz signal were sampled at 24 samples per second, so 0.1 Hz is said to be an "alias" for 24.1 Hz.

Via Wikipedia

Die Dulci Fruere