Re: Possibly unrelated questions

Rune Allnor wrote:

On 17 Feb, 13:46, "John E. Hadstate" <jh113...@xxxxxxxxxxx> wrote:

"Richard Owlett" <rowl...@xxxxxxxxxxxxx> wrote in message

news:13rg6q7a1esbs64@xxxxxxxxxxxxxxxxxxxxx


glen herrmannsfeldt wrote:

I was watching the usual car
commercial where the (aliased) wheels
are turning slowly backwards and
realized that sometimes negative
frequencies do exist.

Never thought of stroboscopic effects in terms of sampling
theory. Are wheels going "backwards" and stop/slow
motion effects generally really an aliasing effect or
artifact of not
meeting a Nyquist related criterion?

I slightly disagree with Rune's answer, "sort of". Assuming
your question is posed to need a Yes/No answer, I think the
answer is "absolutely".


Well, technically speaking you are right. However, sampling
and aliasing in more than one dimension is not at all trivial
to understand. It is common to use severe spatial undersampling
in seismic applications. However, because of the way the spatial
spectra combine with the time dimension, a unique 2D spectrum
can be obtained.

The movie in question is a 3D signal (time + two spatial
dimensions), so I would prefer not to get too involved with
the interpretation of the spectrum with coresponding
aliasing effects.

Rune

I'd think it would be time + 1 spatial as the effect is seen if camera is paned to keep the axle in the same position in the frame. The spatial dimension would then be the angular position of the wheel.
.

Relevant Pages

  • Re: Possibly unrelated questions
    ... Are wheels going "backwards" and stop/slow ... and aliasing in more than one dimension is not at all trivial ... spectra combine with the time dimension, a unique 2D spectrum ...
    (comp.dsp)
  • Re: Possibly unrelated questions
    ... Are wheels going "backwards" and stop/slow ... your question could also be interpreted to require an answer of "aliasing effect" vs. "artifact of not meeting a Nyquist related criterion". ...
    (comp.dsp)
  • Re: Possibly unrelated questions
    ... Never thought of stroboscopic effects in terms of sampling theory. ... Are wheels going "backwards" and stop/slow ... It gets really interesting when a frames of film are re-sampled for the TV frame rate. ...
    (comp.dsp)
  • Re: Possibly unrelated questions
    ... for the simplified mathematics that we use to describe it. ... I was watching the usual car commercial where the wheels ... are turning slowly backwards and realized that sometimes negative ... It serves as a familiar example of temporal aliasing. ...
    (comp.dsp)
  • Re: Do you think NI can fix my PLL?
    ... But there is another dimension to the project to explore syncing two wheels together. ... When one loop is locked to another, it seems to need a bandwidth 5 to 10 times higher than the first loop. ...
    (sci.electronics.design)