Interesting example of aliasing...

Started by Eric Jacobsen June 3, 2007
Got this from a friend, who was mostly interested in it because it
looks cool, and we both play an on-line game that includes this type
of helicopter.

http://www.break.com/index/helicopter-blades-stop-moving.html

It's a pretty interesting example of visual aliasing.   I wonder what
the rotation would have looked like if one of the rotors had been
painted a different color.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
Eric Jacobsen wrote:
> Got this from a friend, who was mostly interested in it because it > looks cool, and we both play an on-line game that includes this type > of helicopter. > > http://www.break.com/index/helicopter-blades-stop-moving.html > > It's a pretty interesting example of visual aliasing. I wonder what > the rotation would have looked like if one of the rotors had been > painted a different color.
That's fun. I wonder if it was staged, by getting the rates tuned up for it, or just fortuitous? That tail rotor is pretty slow rotating too, which adds nicely to the effect. The way the rotor stays still for so long is a good illustration of how a helicopter really locks down the speed of the rotor. Steve
Steve Underwood <steveu@dis.org> writes:

> The way the rotor stays still > for so long is a good illustration of how a helicopter really locks > down the speed of the rotor.
Not necessarily - maybe the rotor speed was varying and the camera and the rotor were in phase lock... (Well! You never know about these things...) -- % Randy Yates % "My Shangri-la has gone away, fading like %% Fuquay-Varina, NC % the Beatles on 'Hey Jude'" %%% 919-577-9882 % %%%% <yates@ieee.org> % 'Shangri-La', *A New World Record*, ELO http://home.earthlink.net/~yatescr
On Jun 3, 8:35 pm, Steve Underwood <ste...@dis.org> wrote:
> Eric Jacobsen wrote: > > Got this from a friend, who was mostly interested in it because it > > looks cool, and we both play an on-line game that includes this type > > of helicopter. > > >http://www.break.com/index/helicopter-blades-stop-moving.html > > > It's a pretty interesting example of visual aliasing. I wonder what > > the rotation would have looked like if one of the rotors had been > > painted a different color. > > That's fun. I wonder if it was staged, by getting the rates tuned up for > it, or just fortuitous? That tail rotor is pretty slow rotating too, > which adds nicely to the effect. The way the rotor stays still for so > long is a good illustration of how a helicopter really locks down the > speed of the rotor. > > Steve
I think Randy is on to the right idea. I bet somebody used the audio to create a shutter trigger for a ccd camera. I recall an audio triggered stroblight that was used for diagnosing large mechanical transmission systems. The device had a microphone and the audio was FFTed and displayed on a screen. The user could move the cursor over to the desired frequency of interest and this selected a frequency that was used to trigger the strobe light. A phase adjustment was also available. It was really cool! The transmission case was opened and the gears were run. The strobe was synched to the sound of interest and the phase was adjusted to effectively rotate the bad spot on a single gear into sight. This made finding bad cags and bearings rather simple in an operating system. Clay

Clay wrote:

>>>Got this from a friend, who was mostly interested in it because it >>>looks cool, and we both play an on-line game that includes this type >>>of helicopter.
That looks like a soviet "Hind", right?
> > I think Randy is on to the right idea. I bet somebody used the audio > to create a shutter trigger for a ccd camera.
Locking the CCD of the image tracker of a missile on the sound of the chopper seems like quite obvious idea. That should improve the immunity to the decoys. VLV
Clay wrote:
> On Jun 3, 8:35 pm, Steve Underwood <ste...@dis.org> wrote: >> Eric Jacobsen wrote: >>> Got this from a friend, who was mostly interested in it because it >>> looks cool, and we both play an on-line game that includes this type >>> of helicopter. >>> http://www.break.com/index/helicopter-blades-stop-moving.html >>> It's a pretty interesting example of visual aliasing. I wonder what >>> the rotation would have looked like if one of the rotors had been >>> painted a different color. >> That's fun. I wonder if it was staged, by getting the rates tuned up for >> it, or just fortuitous? That tail rotor is pretty slow rotating too, >> which adds nicely to the effect. The way the rotor stays still for so >> long is a good illustration of how a helicopter really locks down the >> speed of the rotor. >> >> Steve > > > I think Randy is on to the right idea. I bet somebody used the audio > to create a shutter trigger for a ccd camera. I recall an audio > triggered stroblight that was used for diagnosing large mechanical > transmission systems. The device had a microphone and the audio was > FFTed and displayed on a screen. The user could move the cursor over > to the desired frequency of interest and this selected a frequency > that was used to trigger the strobe light. A phase adjustment was also > available. It was really cool! The transmission case was opened and > the gears were run. The strobe was synched to the sound of interest > and the phase was adjusted to effectively rotate the bad spot on a > single gear into sight. This made finding bad cags and bearings rather > simple in an operating system.
Taking a second look, I guess its too steady to be anything but hard locked, probably acoustically. Its probably someone trying to demo their autosyncing camera. You might be surprised how little the rotational speed of a helicopter blade varies over a range of conditions, though. I've developed electronics for helicopters and had very steady sustained patterns in signals which turned out to be effects from the main rotor. Steve
Vladimir Vassilevsky wrote:
> Clay wrote: > > >>>Got this from a friend, who was mostly interested in it because it > >>>looks cool, and we both play an on-line game that includes this type > >>>of helicopter. > > That looks like a soviet "Hind", right? > > > > > I think Randy is on to the right idea. I bet somebody used the audio > > to create a shutter trigger for a ccd camera. > > Locking the CCD of the image tracker of a missile on the sound of the > chopper seems like quite obvious idea. That should improve the immunity > to the decoys.
A missile.. you mean, the kind you launch from a F/A-18 from 25 kilometers away and that travels at supersonic speeds? Lock them on the sound of helicopters?
Steve Underwood <steveu@dis.org> writes:

> Clay wrote: >> On Jun 3, 8:35 pm, Steve Underwood <ste...@dis.org> wrote: >>> Eric Jacobsen wrote: >>>> Got this from a friend, who was mostly interested in it because it >>>> looks cool, and we both play an on-line game that includes this type >>>> of helicopter. >>>> http://www.break.com/index/helicopter-blades-stop-moving.html >>>> It's a pretty interesting example of visual aliasing. I wonder what >>>> the rotation would have looked like if one of the rotors had been >>>> painted a different color. >>> That's fun. I wonder if it was staged, by getting the rates tuned up for >>> it, or just fortuitous? That tail rotor is pretty slow rotating too, >>> which adds nicely to the effect. The way the rotor stays still for so >>> long is a good illustration of how a helicopter really locks down the >>> speed of the rotor. >>> >>> Steve >> I think Randy is on to the right idea. I bet somebody used the audio >> to create a shutter trigger for a ccd camera. I recall an audio >> triggered stroblight that was used for diagnosing large mechanical >> transmission systems. The device had a microphone and the audio was >> FFTed and displayed on a screen. The user could move the cursor over >> to the desired frequency of interest and this selected a frequency >> that was used to trigger the strobe light. A phase adjustment was also >> available. It was really cool! The transmission case was opened and >> the gears were run. The strobe was synched to the sound of interest >> and the phase was adjusted to effectively rotate the bad spot on a >> single gear into sight. This made finding bad cags and bearings rather >> simple in an operating system. > > Taking a second look, I guess its too steady to be anything but hard > locked, probably acoustically. Its probably someone trying to demo > their autosyncing camera. You might be surprised how little the > rotational speed of a helicopter blade varies over a range of > conditions, though. I've developed electronics for helicopters and had > very steady sustained patterns in signals which turned out to be > effects from the main rotor.
Hey Steve, My original comment was intended as a joke. I didn't think it was likely that someone was ACTUALLY phase-locking (frequency-locking?) the camera, but maybe they were. I personally have no clue - my post was supposed to be all in fun. So you've worked on helicopter electronics? Cool! The closest I can come to that is when I worked on a small piece of DSP (linearization) code that was part of the IMU for the Minute-Man missle that goes in a helicopter. -- % Randy Yates % "Ticket to the moon, flight leaves here today %% Fuquay-Varina, NC % from Satellite 2" %%% 919-577-9882 % 'Ticket To The Moon' %%%% <yates@ieee.org> % *Time*, Electric Light Orchestra http://home.earthlink.net/~yatescr
Michel Rouzic wrote:
> Vladimir Vassilevsky wrote: >> Clay wrote: >> >>>>> Got this from a friend, who was mostly interested in it because it >>>>> looks cool, and we both play an on-line game that includes this type >>>>> of helicopter. >> That looks like a soviet "Hind", right? >> >>> I think Randy is on to the right idea. I bet somebody used the audio >>> to create a shutter trigger for a ccd camera. >> Locking the CCD of the image tracker of a missile on the sound of the >> chopper seems like quite obvious idea. That should improve the immunity >> to the decoys. > > A missile.. you mean, the kind you launch from a F/A-18 from 25 > kilometers away and that travels at supersonic speeds? Lock them on > the sound of helicopters? >
I think he intended that as a joke. Its probably a good enough idea to win a lucrative defence research contract, though. :-) Steve
Randy Yates wrote:
> Hey Steve, > > My original comment was intended as a joke. I didn't think it was > likely that someone was ACTUALLY phase-locking (frequency-locking?) > the camera, but maybe they were. I personally have no clue - my post > was supposed to be all in fun. > > So you've worked on helicopter electronics? Cool! The closest I can > come to that is when I worked on a small piece of DSP (linearization) > code that was part of the IMU for the Minute-Man missle that goes in a > helicopter.
I only did one short project for helicopters. It was thermal imaging for police work, like spotting a warm body in a shallow grave. I used to fly radio controlled helicopters, as a hobby. Helicopters are fun machines to play with. :-) Steve