large LED screen flicker

Pete Raynell · May 1, 2016

I recently completed a job that required some shots to be done in a retail store, this store had an 12ft x 6ft LED screen feature wall playing the companies latest TVC. The director requested this be used in some shots out of focus to add some movement to the BG.

On the recce I noted that the screen may produce flicker and that a camera test should be done prior to shooting, and any adjustments to the screens refresh rate be made accordingly, however due to time and budget this was unable to be done.

On the day (Shooting Alexa, 25fps, 180 degree shutter) In Australia (50Hz), the LED screen produced noticeable horizontal banding consistent with a screen refresh rate being set to 60Hz not 50Hz.

Switching the Alexa to 30fps confirmed my suspicions (or so I thought) by removing the banding from the LED screen but producing flicker in all the small prac lights in the store that where our main light source.

We where on a tight schedule and I had about 10 minuetes to try and eliminate the screen flicker problem, with the Alexa set back at 25fps.

As the screen was controlled by software remotely I spoke with a technician over the phone who switched the screen to a refresh rate of 50p, this helped though did not completely remove the screen flicker. I got the technician to switch to a multitude of different refresh rates hoping to find the sweet sport, as well as trying different shutter angles in the short amount of time I had with no luck eliminating the problem.

The best result we could get was actually to set the screens refresh rate on the software to 60i. Though with banding still slightly present the decision was made to frame the screen out and use a different BG.

The finished spot looks great and the client is very happy but I am left bugged by this issue and wondering If I have missed something?

Has anyone come across this issue with large led screen?

The screen was made up of what appeared to be 12 - 3X2ft panels, they where all connected via ethernet to what appeared to be a group of video hubs (controlled remotely)

Could it be the changes being made in the software where not correct as other settings may have been in play from the video hubs hardware or the screens them self?

Or have I missed something really simple here! I thought I would share in case someone can point out an oversight I may have made.

Pete.

Satsuki Murashige · May 1, 2016

This article on rolling shutter by Art Adams may be useful to you: http://www.provideocoalition.com/rolling-shutter-and-flickering-hmis/

Phil Rhodes · May 1, 2016

Further to issues of rolling shutter, which are certainly very relevant here, there's the problem of the LED display itself. They invariably use PWM to control the brightness of individual pixels and this might work on any scheme of the designer's choosing, quite independently of the video settings which are exposed to the user.

Some of them are designed to be camera safe, but unless you have been specifically advise this is the case, it's unlikely. There may be no solution, so don't feel bad about it.

P

Pete Raynell · May 3, 2016

Reading up a little it seems that adjusting the screens brightness may have been a solution as it affects the on/off rate of the PWM LED that causes the flicker.

http://www.flatpanelshd.com/focus.php?subaction=showfull&id=1362457985

This would explain why another shot (an ECU of a smart phone) presented similar problems but when the phones screen brightness was turned up to 100% the flicker was no longer present.

Pete.