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Jon Diez Dominguez

SYNC WITH LED VIDEO-WALLS AND FILM CAMERAS

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Hi,

I am preparing a project with LED video-walls and camera tracking for realtime 3D backgrounds, very similar to the technology being used in "The Mandalorian" series (Disney +).

We are considering shooting on film cameras (Kodak stock). For budgetary reasons we still do not know if we will use S16mm or 35mm.

I am concerned about the synchrony between the camera and the LED video-walls. Using digital cameras we can all see immediately if there is any sync problem that causes flickering or subtle differences between the modules that conforms the video-walls. But I'm not sure what will happen using a film camera.

Does anyone have experiences with 16 or 35 film cameras and LED video-walls? Have you used any system to synchronize the shutter of the camera with the screens? And using different camera speeds or shutter angles?

 

Thanks for your help.

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You'd need to check the spec of the video wall refresh rate, but I know that you mostly no longer need sync boxes to film displays. Discovered this a while ago while doing a test and rolled on some stuff with a TV in the background. The TV was showing normal (Im guessing 29.97) playback, but because its a 120hz tv its showing repeats of those frames longer (frame smoothing was off). So it refreshed so fast in combination with displaying the whole image at once (vs CRTs that drew lines with an electron gun) that there was no roll bar or flicker.

Unfortunately the only way to be sure is to test. But if the walls are running at 120 or 144hz I suspect you'd be fine. 

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1 hour ago, Robin Phillips said:

You'd need to check the spec of the video wall refresh rate, but I know that you mostly no longer need sync boxes to film displays. Discovered this a while ago while doing a test and rolled on some stuff with a TV in the background. The TV was showing normal (Im guessing 29.97) playback, but because its a 120hz tv its showing repeats of those frames longer (frame smoothing was off). So it refreshed so fast in combination with displaying the whole image at once (vs CRTs that drew lines with an electron gun) that there was no roll bar or flicker.

Unfortunately the only way to be sure is to test. But if the walls are running at 120 or 144hz I suspect you'd be fine. 

You need to be very careful with this. The TV you encountered was extremely likely an LCD type which tend to shimmer slightly, if at all. An LED video wall operates on completely different principles and may create very different, and much more noticeable, problems.

Unless the people supplying the LED video wall have experience with camera safety, I would be very, very cautious about anything they tell you about that particular wall. Test extensively, or use a company with experience working on a film set.

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Without genlock, you're stepping into some risky territory with an LED wall, refresh rate aside.

For your LED volume, are you using nDisplay and Unreal?

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Honestly, even with genlock there are all kinds of PWM issues with LED video displays. Definitely a test twice situation.

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1 minute ago, Phil Rhodes said:

Honestly, even with genlock there are all kinds of PWM issues with LED video displays. Definitely a test twice situation.

True.

@Jon Diez Dominguez, test test test test!

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Posted (edited)

1000% test, and try to break it in your test--shoot close to the screens, at sharp angles, etc. to account for every scenario. If you're forced to not have genlock, use a 144 degree shutter and limit vertical camera moves. Make sure the screens you use are high quality--Roe BP2 or BO2 ideally--and that they have a low scan rate.

Video panels usually run at either 1920 or 3840hz (they know that's confusing) so you have refresh rate for days, but the actual time that the screen takes to refresh is a large factor. The shutter "actuation" speed of film cameras may not play nice with it.

For what it's worth, we did the video that is the background of loledvirtual.com without genlock on a 8K Weapon and it turned out fine, but there were some shots with flicker that had to be fixed in post.

Edited by Andy Jarosz

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I just wrote a piece about a short film called Megan, shot by the estimable Markus Förderer, which used LED displays to simulate helicopter flight. It works incredibly well, casting appropriate interactive light on the helicopter and its occupants. It was done without genlock but there's a concern over using video on the wall which has a lot of movement in it, so that double-exposed frames become objectionably obvious, so be careful about that.

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Does anyone know if Arri ESU-1 (External Synchronization Unit) can be used with genlock? Or is it only useful with analog video sources?

Jon, would love to hear how your tests turn out if you decided to test film cameras with led walls!

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Lacking any other info, I'd assume that a synchronisation device would be useful for synchronising the camera to an external video source. This might be useful for ensuring that the display shows precisely one frame of video per film frame, and that would be something you might want (though it has been done successfully without that). I would not assume that it would solve all potential problems, though. Some displays (or more properly the display driver electronics) might retime your signal, and the pulse width modulation used might still create flicker, even if you're seeing frame-for-frame video across the whole display.

In short there are many ways for people to engineer LED video walls and it's impossible to predict how any particular piece of equipment might behave. Test.

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Hi, I've written an iPhone app that can measure light frequency directly called FlickerFinder. Essentially you point the phone towards the light, it will autoexpose to reveal and measure the flicker, then give you a suggested shutter speed. It's in the final stages, I just need some help calibrating the rest of the iPhones. https://flickerfinder.com

Alternatively if you know the frequency you can use a typical flicker free calculator app: https://apps.apple.com/app/id1514983816

Hope this helps.

6999hz7.jpg

flickercalc2.png

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