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Aliasing - A new perspective out of Box

Started by Bhanu Prakash Reddy July 16, 2003
Lasse Langwadt Christensen wrote:
>
...
> > you can get free rotating hubs, as fas as I can tell it's basically an > extra hub plate with a bearing so, when you driving the look stationary > and when you stop they keep rotating for while.
What makes them rotate then?
> > I think they are quite expensive, but the buyers are probably the type > that spends more money on spoilers, stereo, blue neon light and such > than they do on the car ..... >
Oh, nuts! There's no point in making what any yahoo can have for money. It's back to the self-levelling hollow cylindrical hood ornament, with a visible hint of rifling inside. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
> Now comes my doubt...some times it appeared to me that the tyres are > rotating in the opposite direction of > the car....Why is it so???
I think more simple explanation: camera do sampling with any angle at predecessor pozition. This angle will arise accordingly speed, our brain interpolate to analog form. When this angle more 360 degrees - we see slow speed, little less - move to oppozite direction. Cheers Victor
After posting this yesterday, I took a walk down Main Street last night
(Concord, NH) because they're having a NASCAR race near here this
weekend and have cars, etc. setup on Main Street to look at.  What did I
see but a pickup all tricked out with free rotating hubs!  And I know
they were free because the truck was parked and you could spin the hubs
...


Rick Armstrong wrote:
> > > I swear that someone must be making hubs that free rotate from the > > wheel. > > Hi Ron, > > I've seen this too; it's a new fancy-schmancy kind of free-spinning hubcap > accessory thingy. It's really weird to see a car pull up to a stop light, > come to a halt, then to see the "wheels" keep spinning. > > Rick Armstrong > (reply address is bogus)
Lasse Langwadt Christensen <langwadt@ieee.org> wrote in message news:<3F15E268.4030901@ieee.org>...
> Paavo Jumppanen wrote: > > > snip > > > > > > Yes, it is aliasing and mechanical engineers may also refer tp it as > > the strobascopic effect. > > > > As for why ti appears to be moving backwards, that depends upon the > > proximity of the rotational speed to the frames per second. If > > identical or harmonically related the tyre will appear stationary. If > > the rotational speed is slower it will appear to be rotation in the > > right direction but at a speed that is the difference between the > > frame rate and the rotational speed. If the rotational speed is > > slightly faster it will appear to be rotating backwards with speed > > that is the difference between the rotational speed and the frame > > rate. > > > > I beleive that's why you can't have flourecent lights in places with > rotating machines, someone might think it isn't rotating or rotating > very slow and grap it... > > And I'm sure that illusion of the thing rotating backwards can be > somehow related to negative frequencies > > -Lasse
They do put flourescent lighting in workshops and such (at least I have) it's just not recommended to wire them with a single phase (all lights pulsing at the same frequency). Usually, people install 2-3 phases (120 degrees apart) throughout the building, so that you avoid the strobascopic effect.
"Col Brown" <cb135@hotmail.com> wrote in message
news:a254af6b.0307180751.43c50302@posting.google.com...

> They do put flourescent lighting in workshops and such (at least I > have) it's just not recommended to wire them with a single phase (all > lights pulsing at the same frequency). Usually, people install 2-3 > phases (120 degrees apart) throughout the building, so that you avoid > the strobascopic effect.
Some two lamp balasts run the two lamps 90 degrees out of phase to minimize the overall flicker. -- glen
Col Brown wrote:
> > Lasse Langwadt Christensen <langwadt@ieee.org> wrote in message news:<3F15E268.4030901@ieee.org>... > > Paavo Jumppanen wrote: > > > > > snip > > > > > > > > > Yes, it is aliasing and mechanical engineers may also refer tp it as > > > the strobascopic effect. > > > > > > As for why ti appears to be moving backwards, that depends upon the > > > proximity of the rotational speed to the frames per second. If > > > identical or harmonically related the tyre will appear stationary. If > > > the rotational speed is slower it will appear to be rotation in the > > > right direction but at a speed that is the difference between the > > > frame rate and the rotational speed. If the rotational speed is > > > slightly faster it will appear to be rotating backwards with speed > > > that is the difference between the rotational speed and the frame > > > rate. > > > > > > > I beleive that's why you can't have flourecent lights in places with > > rotating machines, someone might think it isn't rotating or rotating > > very slow and grap it... > > > > And I'm sure that illusion of the thing rotating backwards can be > > somehow related to negative frequencies > > > > -Lasse > > They do put flourescent lighting in workshops and such (at least I > have) it's just not recommended to wire them with a single phase (all > lights pulsing at the same frequency). Usually, people install 2-3 > phases (120 degrees apart) throughout the building, so that you avoid > the strobascopic effect.
Not exactly. The single-lamp fluorescent fixtures I have in my garage operate from the only phase I have available there, and they show a stroboscope effect. I use an incandescent gooseneck near the lathe, but I would have no problem otherwise. In larger spaces, I would use or recommend two-lamp fixtures. Those have ballasts so designed that the bulb currents are in quadrature. Running some fixtures from each of three phases in a large shop balances the load, but individual fixtures are usually far enough apart so than any one area is illuminated mainly by a single one. Most of those fixtures are two-lamp with quadrature balasts. That's common here in the US. Has anyone but me noticed that when one bulb in a two-lamp fixture burns out, the flicker becomes noticeable, and that unless both bulbs are replaced, residual flicker remains? Fluorescent lamps lose brightness as they age. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org> wrote:

>Bhanu Prakash Reddy wrote: >> >> > >> > It isn't because the brain is sampling, but because the movie or TV camera >> > is sampling. >> > >> > Yes, it is aliasing just like is often described here. >> > >> > Also note that the aliased motion may appear to be reversed. >> > >> > -- glen >> >> Even if u see really (not in a movie) a fast moving vehicle, u observe >> the same situation with its tyres.In that case who is sampling...brain >> only right??? >> >> -bp$ > >No. You won't see the effect in uniform light. At night, reflections >from parts of the wheel when they assume specific ankles can act like a >synchronized stroboscope, making the wheel appear stationary. > >Jerry
At night artificial lighting creates a shutter effect since the lights turn off 120 times a second with 60Hz power and 100 times a second with 50 Hz power. This creates a visual sampling system because you only get to sample the object when the lights are actually on. Although one doesn't perceive the flicker, one does perceive the aliasing effects that can happen. I've personally witnessed this visual aliasing many times, understood it, and therefore never gave it much more thought beyond initial interest. Once a few years ago while driving home in the daylight I observed the same visual aliasing in the wheel of the car in my side rear-view mirror. It took me a while to wrap my brain around that one since it was natural daylight illumination. I was beginning to worry that I was on "The Truman Show" for a while until I realized that the mechanical vibration in the rear-view mirror was producing the same shuttering effect achieved with AC electric artificial lighting at night. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
On Wed, 16 Jul 2003 15:12:02 -0400, "Clay S. Turner"
<physicsNOOOOSPPPPAMMMM@bellsouth.net> wrote:

>This brings to mind several interesting effects. The first is the speed of a >nerve impulse depends on its strength. So trying to catch a ball when it is >dark causes us to miss the ball since its true position is ahead of its real >position. Remember playing baseball late during the summer and trying to >catch a fly ball. By the time you manage to get the glove up, the ball has >already hit you in the face.
I take it you weren't often picked for the all-star team? ;) Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Eric Jacobsen wrote:
> > On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org> wrote: > > >Bhanu Prakash Reddy wrote: > >> > >> > > >> > It isn't because the brain is sampling, but because the movie or TV camera > >> > is sampling. > >> > > >> > Yes, it is aliasing just like is often described here. > >> > > >> > Also note that the aliased motion may appear to be reversed. > >> > > >> > -- glen > >> > >> Even if u see really (not in a movie) a fast moving vehicle, u observe > >> the same situation with its tyres.In that case who is sampling...brain > >> only right??? > >> > >> -bp$ > > > >No. You won't see the effect in uniform light. At night, reflections > >from parts of the wheel when they assume specific ankles can act like a > >synchronized stroboscope, making the wheel appear stationary. > > > >Jerry > > At night artificial lighting creates a shutter effect since the lights > turn off 120 times a second with 60Hz power and 100 times a second > with 50 Hz power. This creates a visual sampling system because you > only get to sample the object when the lights are actually on. > Although one doesn't perceive the flicker, one does perceive the > aliasing effects that can happen. > > I've personally witnessed this visual aliasing many times, understood > it, and therefore never gave it much more thought beyond initial > interest. Once a few years ago while driving home in the daylight I > observed the same visual aliasing in the wheel of the car in my side > rear-view mirror. It took me a while to wrap my brain around that > one since it was natural daylight illumination. I was beginning to > worry that I was on "The Truman Show" for a while until I realized > that the mechanical vibration in the rear-view mirror was producing > the same shuttering effect achieved with AC electric artificial > lighting at night. > > Eric Jacobsen > Minister of Algorithms, Intel Corp. > My opinions may not be Intel's opinions. > http://www.ericjacobsen.org
I see that too, but people claim to see the effect with illumination by headlights! Then it's reflection at specific orientations. The explanation I rejected was that the eye itself sampled. Tain't so. If it were, we would observe beats. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Glen Herrmannsfeldt wrote:
> "Col Brown" <cb135@hotmail.com> wrote in message > news:a254af6b.0307180751.43c50302@posting.google.com... > > >>They do put flourescent lighting in workshops and such (at least I >>have) it's just not recommended to wire them with a single phase (all >>lights pulsing at the same frequency). Usually, people install 2-3 >>phases (120 degrees apart) throughout the building, so that you avoid >>the strobascopic effect.
Almost all fluorescent light fittings for office environments have three tubes, but in much of the world they are all wired to the same phase. What a waste. When they are wired to the three phases, the overall flicker is pretty minimal (as opposed the often bloody awful flicker of a single tube). You still get people whining about it, though. I guess it shows how much of the complaint about flicker is psychological (or is it just the lazy, looking for an excuse?).
> Some two lamp balasts run the two lamps 90 degrees out of phase to minimize > the overall flicker.
That is also to keep the power factor reasonable. In some places I think it is a requirement. Regards, Steve