how this effect is possible???

[jtnichol]

Wow that is cool! Thanks for posting! Boy those Framerates are too tightly synced or out of sync....lol

[-bart-]

In that's not a rolling shutter effect.
It is too perfect to be 'accidental'.
Microphone, BPF, VCO, divider, PLL, and camera with genlock crammed together will do the trick.

[Stoney3K]

In that's not a rolling shutter effect.
Is to perfect to be 'accidental'.
Microphone, BPF, VCO, divider, PLL, and camera with genlock crammed together will do the trick.

It's harder to pull off than you think. When you have the camera genlocked to the rotor RPM, you'll need to constantly keep dropping shutter angle as the helo takes off and the rotor spools up, to avoid introducing motion blur.

Lower shutter angles mean less exposure, so you're gonna need a LOT of light (or a really fast lens) and some lightning fast iris to compensate and maintain the lighting conditions like they did in this clip.

[mixedgas]

Bart is sort of correct.

Its internal in the cameras. We ran into that in some Sony cameras where I used to work. Sony has a system in the DSP in camcorders that moves the camera off stroboscopic events by a fraction of a frame. When we tried to film stoboscopic illumination in the lab, the camera would move off the repetitive bright events. I had to induce dither in our strobe circuit to compensate.I took a sync stripper chip and used it to lock our strobes to the camera. I had to add a slipping counter to the strobe side of the sync chip.

Every other camera in the lab I could genlock or lock the strobes to the video with a sync chip. But not the Sony camcorders.


Then a guy with a broadcast grade Sony came to visit, and he switched on a mode that was "LOCK TO FLICKER", and low and behold, he could film locked to the strobe... So I suspect the camera had the lock to flicker mode, was set to rolling shutter, and the choppers FADEC engine control is constant RPM.

The turbines try to run at pretty close to constant speed and 90 to 95% RPM in flight in most Helos.The rotor speed, NR does change. I learned that building the projection optics for a helo sim. I spent quite a bit of time flying the sim to check my work.
I'm sure Stuka can confirm that.

If Not convinced, Read Shawn Coyle's book, "Cyclic and Collective", Shawn is a personal friend.
Shawn covers the helo hardware very well.

Some times, as in the case of the C130 rotors, its the rolling shutter. In the case of the Hind, I suspect Fadec plus the camera control DSP. No one set out to lock to the Hind, it just happened to be the right combination of camera software and a constant speed rotor (well, as close as you can get to one)

The Hercules turbo-props run at a constant speed, they change the pitch of the props to vary thrust. So its probably easy to find some wierd shutter rates that sample the motion.

Steve

[Cade]

Hmm,....
The helo blades synced with the shutter speed looks cool, and Shawn Coyle's book still doesn't help to clarify (for me) that we are NOT catching the "tail-ends" of the beams:



Here's the source (un-slowed) video that I shot at the BALEM last month:

http://fs13n4.sendspace.com/dl/50caf...c/IMG_0146.MOV

or

http://www.mediafire.com/?7z3c3qrhdddzg6a

[mixedgas]

Your not catching the beams in "Space time", your seeing a "undersampling" effect from the CCDs vertical rows.

Google "Video Interlace" and Undersampling.

Then understand in modern CCDs that there is more then one kind of interlace, as the move to digital video allowed the chip designers to do all sorts of neat tricks on the silicon.\

You do not have a CCD that can gate in the 9 nanoseconds or so that you think your seeing, it would have to be clocked at well over 3 Ghz to do that. Only a few specialized military or lab CCDs are capable of Ghz gating. Those that can are nearly blind and not color capable.

If you had a camcorder that fast, I would have ultrafast laser researchers (and I am one, as of next tuesday) busting down Best Buy's doors to get them. It would revolutionize biological research.
Steve

[Cade]

Your not catching the beams in "Space time", your seeing a "undersampling" effect from the CCDs vertical rows.

Google "Video Interlace" and Undersampling.

Then understand in modern CCDs that there is more then one kind of interlace, as the move to digital video allowed the chip designers to do all sorts of neat tricks on the silicon.\

You do not have a CCD that can gate in the 9 nanoseconds or so that you think your seeing, it would have to be clocked at well over 3 Ghz to do that. Only a few specialized military or lab CCDs are capable of Ghz gating. Those that can are nearly blind and not color capable.

If you had a camcorder that fast, I would have ultrafast laser researchers (and I am one, as of next tuesday) busting down Best Buy's doors to get them. It would revolutionize biological research.
Steve

Steve,

Thanks for the response. I'm glad that next Tuesday you will finally be a laser researcher.

To clarify. I recorded this video on an iPhone4 (which uses a CMOS sensor), rather than a CCD.
I was unaware that there was "interlacing: on CMOS devices. ... To clarify: I am fully aware that the CMOS sensor in my phone does NOT use interlacing, so it must be "undersampling". But enough nit-picking on my part, I'm not trying to make any enemies here.

Your point remains the same; the appearance of the "tail-ends" of the laserbeams is a phenomenon of the slow framerate of my camera, compared to the enormous velocity at which the lasers are propagating (c*).
Understood.

Please clarify: Will I not see this effect if using a "film" camera?

-Cade


*c here denotes the speed of light


-time to give Time Warp a call-

[Stoney3K]

[QUOTE=Cade;224185]Please clarify: Will I not see this effect if using a "film" camera?

-Cade
[/COLOR]

A true film (movie) camera will expose a single frame every time the shutter is opened, with the exposure time being determined by the 'angle' (of a butterfly shutter) of a full shutter rotation where the shutter is open. After all, a film's frame rate is fixed at 24 frames per second. Any motion that will occur during that exposure time, will affect the entire frame at once, and cause motion blur.

On a CMOS device, the chip is being scanned continously and linearly from top to bottom during the exposure. If something were to move (e.g. a laser beam being scanned or camera moved with respect to the beam), it will affect the exposure of certain areas of the chip in a different manner as they're being scanned. This causes the 'rolling shutter' artifacts.

The propeller is a good example. As the propeller moves in front of a the camera's imaging chip, the line of pixels that's being transferred to memory moves along with it. In layman's terms, a single 'slit' of the image is being stored in small segments of time, instead of transferring the entire image to memory at once as would happen with a film camera. As the 'slit' moves down the chip, the object being recorded may have moved with respect to the camera. This causes the eventual image (put together from a lot of slits stacked on top of each other) to get warped if the object is moving fast enough.

With laser beams, the effect has nothing to do with photons propagating through space, but is caused by the scanning beam moving with respect to the camera. On one partial exposure, it can be in frame, where as on another, it may have shifted slightly because the galvos were moving, or it was blanked or moved out of frame altogether. This causes 'parts' of the beam to be visible on the image, where other parts are not, since the laser beam was scanning elsewhere when that part of the frame was stored.

A camcorder with a 'film' setting will not change this, since the way the image is scanned doesn't change. With a real (photographic) film camera, you won't see the effect, neither would you notice it with a static, unscanned beam.

Neat trick: If you make a scan that's slow enough, you can actually curve the laser beams on a CMOS camcorder.

[dash8brj]

I am assuming the video with the prop, that the engine is attached to a dehavilland dash 8. (it sure looks like one). With that in mind...

At cruise the dash 8's prop is spinning at ~900 rpm. 1450 rpm at takeoff and 1450 rpm near touchdown feathered for instant recovery if needed. This setting also gives the quickest transistion to beta prop at pretty much full power, which aids in slowing the aircraft on touch down. Multiply the 900rpm prop shaft by 4 for the 4 blades attached and you get 3600 passes a minute. divide by 60 and you end up with 60. 60 passes per second. Not sure if this helps anyone explain the phenomenon with the propeller.

But, if the camers is shooting at 29.97 frames per second (typical NTSC scan frequency) then thats just under half the framerate of the prop. At 25FPS (PAL standard) its quite a bit less than the half prop framerate.

[mixedgas]

I am assuming the video with the prop, that the engine is attached to a dehavilland dash 8. (it sure looks like one). With that in mind...

At cruise the dash 8's prop is spinning at ~900 rpm. 1450 rpm at takeoff and 1450 rpm near touchdown feathered for instant recovery if needed. Multiply the 900rpm prop shaft by 4 for the 4 blades attached and you get 3600 passes a minute. divide by 60 and you end up with 60. 60 passes per second. Not sure if this helps anyone explain the phenomenon with the propeller.

But, if the camers is shooting at 29.97 frames per second (typical NTSC scan frequency) then thats just under half the framerate of the prop. At 25FPS (PAL standard) its quite a bit less than the half prop framerate.

Thats perfect, fits exactly, 2 or 4X oversampling when you factor in fields, frames and compression, = the right amount of blur for the bending.

And I checked, the P3 is a 13,000 constant output shaft RPM, not prop RPM, there is a "dual speed gearbox" between the prop and the output shaft. This puts the prop at ~1200 RPM, NOT THE 9600 RPM I said. My long term memory is a bit leaky these days. Mea Culpa.

Thank you, Captain!

Steve