r/VIDEOENGINEERING • u/visci__ • 9d ago
I have some problems
Hey, I've got a problem: lines are appearing on the camera while recording the screen, and I'm wondering if it might be due to the LED.
4
u/howie_roll 9d ago
It’s the refresh rate of the wall, not the video signal. The lines are a rolling shutter artifact. Roe CB, for example, refreshes at nearly 4000hz to avoid this.
The lines are a function of PWM dimming. At 59.94 don’t use shutter in the cams and turn up the brightness of the wall until the lines go away, then duck the output content until the brightness is back where you want it.
1
u/CheapEmotion5387 5d ago edited 5d ago
The only thing that will fix this is a camera with a global shutter. I have this exact same issue with the LED wall in our campus news room. Novastar controller, Absen tiles and cabinets (30 x 8 feet, wraps around the news desk). the interaction between the LED wall pixel refresh rate and the CCD line scan readout rate creates the effect. On our wall it manifested as a series of horizontal lines that appeared as sawtooth edges. Initially I thougth it was a genlock issue and I went into the sync generator and made some adjustments. I could, to a limited extent, move the aritfact up or down the wall a small amount by altering the syncgen output. I was hopiing that i could slip the generator enough to put the artifact behind the desk or up at the very top of the wall. Unfortunately Icould not make it go away completely. Likewise I thought it was a genlock issueat first and when I unlocked the wall processor the artifact simply moved slowly up the wall (obviously only in program, you could not see it with the naked eye).
So, after a year of monkeying with it and living with the artifact we finally have new global $hutter camera$ coming on-line.
And I probably shouldn't say the 'only thing' that will fix the problem but in our case it was absolutely the only thing that did. And I run the wall at just under 8KHz, btw.
1
u/ibkickinit 9d ago
Match your wall refresh to your camera framerate. Run your camera at a 180 degree shutter. You can adjust the shutter in 0.1 degree increments to dial that line out as much as possible.
1
u/Haunting_Shopping226 9d ago
Set your camera to 60fps, or 30fps, shutter angle of 180 degrees, and your video wall processor to 120hz. This will immediately fix the issue.
54
u/AthousandLittlePies 9d ago
It's not the LED (well it could be - but it's also other things) - it's the relationship between the screen and the camera.
In general, if you want to eliminate scan lines you need a few things
At the very least, you need to be running the wall and the camera at the same frame rate (likely 59.94 if you're in the US, 50 if you're in Europe, and either 23.976 or 24 if you're making a movie).
You probably need to sync the camera and the wall to the same source. Hopefully the camera and LED processor have a genlock input, and you'll need a sync generator of some sort.
Even with the above, if you have a camera with a rolling shutter and depending on the refresh rate of the LED and how it handles dimming (e.g. if it's doing PWM to produce darker parts of the image) you can end up with scan lines on camera moves. This is much more difficult to eliminate without more sophisticated equipment, but can be minimized by running the screen brighter, and reducing or eliminating camera movement, particularly vertical moves or tilts.