Hi

I have worked with LED walls before with Alexa, and had to genlock/TC lock them to get rid of refresh artefacts and double lines. However, now I'm on a mini, and I see horizontal banding with sawtooths on the edges..

the bands move with the camera (so this is sensor-related), change thickness with shutter and backlight, and I cannot get rid of them with genlock or anything else. If the speeds doesnt sync, they keep moving up on the screen


I hope this is the correct place to ask, but this to me seems like an issue between the moving shutter and the duty cycle of the LEDs...and thats a bit over my head.

thank you
Balazs Rozgonyi
Video Assist Hungary

Attaching a picture of the problem:

Firstly check the source hertz and frame rate if the source material. I’ve had this when mac a set to 60hertz. Also I remember using a 179.9 degree shutter that was the only angle that Bandubg disappeared. 

hope this helps,
dave 

Thanks Dave

I have full control over source, and it’s 23,976. Player, converter and led controller is genlocked together. If they aren’t, the bands move up and down.

I have played around the shutter and obviously it changes the size of the bands. There are rates when they almost disappear. It’s 153.5 for 23.986. However when moving the camera it’s still very apparent. Also seems to be changing by content.

Balázs Rozgonyi
Video Assist Hungary

That’s a head scratcher. Do you have another camera you can point at the wall, to test if it is still apparent? 

If if you have not already done it, maybe a hard reset of the camera will clear it up.

How is your power supply?  
--
Guy Mastrion, DP, Creative Director, Professor

Hey Balazs

Just trying to wrap my head around this as I might be shooting back projected material soon but was thinking of opting for projectors instead of LED walls.

One thing I noticed is that the banding is curved and not absolutley horizontal. One test you might do is to feed a image to the wall, a still not a video, with a hard edge between black and white vertical and then an image with a hard edge black and white horizontal and test again, and then once again a still with some kind of gradient that is high res and doesn't have any banding in the original. I just thought that might eliminate the possiblility of two opposing codecs/frame/sample rates and see if it might be to do with the physical lines of the LED wall interfering with the Alexa sensor read pattern.

Also for the LED wall arent there some seperate controls that are nothing to do with the computer/server that is generating your footage?

Good luck

Jonathon Sendall
DP, London UK.

One thing I noticed is that the banding is curved and not absolutley horizontal.

The Alexa is a global shutter camera. And these move when you pan the camera up and down, so this is caused by the readout, or more precisely the wall changing during readout, I think. That’s why it’s angled, imho. But I have not seen curves. Some duty cycles in the wall controller seem to change the angle.

Balazs Rozgonyi
Video assist hungary

 

Art Adams

Cinema Lens Specialist

 

 

ARRI Inc.
3700 Vanowen Street
Burbank, CA 91505
Phone:  (818) 841-7070
Fax:     (818) 848-4028

Get all the latest information from www.arri.com, Facebook, Twitter, Instagram and YouTube.

 

 

This message is confidential. It may also be privileged or otherwise protected by work product immunity or other legal rules. If you have received it by mistake, please let us know by e-mail reply and delete it from your system; you may not copy this message or disclose its contents to anyone. Please send us by fax any message containing deadlines as incoming e-mails are not screened for response deadlines. The integrity and security of this message cannot be guaranteed on the Internet.

---

This email has been scanned for email related threats and delivered safely by Mimecast.
For more information please visit http://www.mimecast.com

---

Yes, sorry, that’s what I meant, obviously. Beg pardon. 

 

Art Adams

Cinema Lens Specialist

 

 

ARRI Inc.
3700 Vanowen Street
Burbank, CA 91505
Phone:  (818) 841-7070
Fax:     (818) 848-4028

Get all the latest information from www.arri.com, Facebook, Twitter, Instagram and YouTube.

 

 

This message is confidential. It may also be privileged or otherwise protected by work product immunity or other legal rules. If you have received it by mistake, please let us know by e-mail reply and delete it from your system; you may not copy this message or disclose its contents to anyone. Please send us by fax any message containing deadlines as incoming e-mails are not screened for response deadlines. The integrity and security of this message cannot be guaranteed on the Internet.

---

This email has been scanned for email related threats and delivered safely by Mimecast.
For more information please visit http://www.mimecast.com

---

Just stumbled across this thread, but may have some input. Refresh rate is not actually as important in LED walls as the scan rate is.

LED panels are capable of a much faster refresh than camera shutter speeds, so as long as you’re genlocked, that will be no problem. However, the scan rate of the panel, expressed as a fraction such as 1/12, 1/16, 1/32 etc. shows what percentage of the screen is lit up at any given time. If this number is too low, the shutter of the camera will catch the screen mid-refresh, and you’ll see artifacts. In general, you want a scan rate of 1/12 or better (1/8 is the best I’ve seen, from Silicon-Core.)

This effect is not necessarily a factor for in person viewing, so the vast majority of LED panels don’t optimize it, and many manufacturers don’t even list it in the specs. It’s also a fundamental relationship between the panel, shutter speed, and camera sensor read speed, which means if you’re stuck with that display, you may be out of luck unfortunately.

-- 
Andy Jarosz
MadlyFX Special Effects & Props
Andy@...
708.420.2639
Chicago, IL

We just wrapped an LED shoot in Thailand where we experienced a sawtooth artifact effect.  we spent many hours troubleshooting and ultimately we believe it was due to the orientation of the panels themselves - they were installed 90deg from their native arrangement and we think the top-to-bottom refresh of the camera was conflicting with the now left-to-right refresh of the display. Panels arranged correctly did not display the sawtooth artifact, so be cognizant of your installations.

hopefully this helps future efforts =)

Sean Porter
DP
Oregon

Was there a good reason to install them in a different orientation?
Do you recall the led manufacturer?
Who teched your wall and who was responsible for the content?

LED Walls should have a Hz rate locked in step with your camera. At least that’s how we do it on the tv show I work on

Jason M. Norman
I.A.T.S.E 728
Lighting Console Programmer
Network Systems Specialist
Lucid Lighting Co.
Los Angeles, CA
(c) 360.481.5379