David Thompson


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February 15, 2011



See, this is what happens when people listen to jazz.


Is this String Theory?


If you watch a guitar string vibrate with a TV screen in the background you will get a similar effect.
I just tried it with my computer monitor and it didn't work. But our old cathode ray TV does it. I think it has something to do with the scan rate.

Simen Thoresen

Woha! This makes me believe that the old western cart-wheels actually did rotate backwards.

Similarly, propellers doing silly things;



It's a form of Aliasing http://en.wikipedia.org/wiki/Aliasing
Simen (above) has it right, its the same as the 'wagon wheel effect' in films. Funnily enough, your own eyes can alias spatially at the edges of your vision; if you look at a black/white grille (like the top of an electric heater perhaps) at the very corner of your eye, you might notice the apparent spacing of the grille get larger. This is due to the reduced density of light sensing cones at the edges of the retina, giving effectively a lower sampling frequency.

Rob Huck

Yup. Aliasing. Never seen it with a vibrating music string, before. You need some ultra-high sample rates to capture that. Incredible.

David Bouvier

Actually, you don't need a high capture rate - you just need the frequency of vibration to be close to an integer multiple of the sampling rate.

Say a sampling rate of 25 fps and a note a little below middle C at 251 hz, and you would see a sampling-effect vibration frequency of 1 hertz.

As the gap increases you get odder harmonic effects ("beat patterns") where the magnitude of the underlying vibration itself appears to osscilate, then returning to simple slow vibrations as the frequency approaches the next multiple of the sampling rate.

Of course if the vibration is precisely a multiple the sample rate you see no motion at all.


Sine waves, frequency and frame rates. Oh, and throw in the number of pixels in the CCd of the camera. All easily explained, even to this moron.

If it's sorcery, it's the electronic kind. Nice tune, tho'.

Capt. Craig

Here you go! http://www.break.com/index/helicopter-blades-stop-moving.html

Rob Huck

You're right, David B. Having an ultra-high sample rate could help prevent aliasing, not cause it. Having an insufficient sample rate leads to aliasing, particularly, as you say, when the rate approaches a harmonic.

My bad. It's been a while since I've thought about signal processing.

Thanks for the correction.


I just read that the brain has frame rates too, but each portion of the brain has different rates, and one part of the brain coordinates the incoming signals so that we can perceive cause and effect in the right order.

The theory is that in the schizophrenic brain, the signals don't get processed in the right order, so a schizophrenic might hear something before they see it, on the TV, for example, which would provide the illusion that the TV people are extracting content from your brain.


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