Weird Video Problem at DVinfo.net

Gary Burlingame · February 3rd, 2009, 05:13 PM

I was shooting a company party at a bowling alley recently. After the employees bowled, the CEO projected a "Jeopardy" like game onto a huge TV screen that hung above the bowling lanes. The feed for this game was coming from a laptop computer and the employees played along using remote transponders to signal their ability to answer the questions that were projected onto the screen. Most of the time I kept my PD 170 zoomed into the TV screen while my camera-mounted shotgun microphone captured the audio.

When I got home and captured the footage to my computer, to my horror I noticed that there were horizontal, drop-out like lines (but they looked like analog dropouts, not digital ones) that appeared anytime my camera was showing the TV screen (and they only showed up within the confines of the TV screen). They disappeared when I zoomed out and panned to the participants, then reappeared when I zoomed back to the screen. There were also dropouts on the audio track, yet none of these problems were noticeable while I was taping (I was using the flip out LCD & headphones for monitoring).

Does anyone know what could have caused this and what one could do to prevent it from happening again? After the Jeopardy game was over and the TV screen resumed showing regular televised programming (i.e. live sporting events), the "dropouts" disappeared. The only thing I can think of is that this must have something to do with the video signal generated by the laptop (conflicting refresh rate?). I was shooting with a shutter speed of 60 and the exposure & focus were set on manual. The way I see it, this will be a nightmare to edit (I will probably use freeze frames instead of live footage to hide the dropouts) but the audio will be a different story.

Does anyone have any ideas on what caused this, how to avoid it in the future, and how I can fix it in post? Thanks!

Edward Phillips · February 3rd, 2009, 05:51 PM

Were the audio drop outs only during the TV shots as well or throughout the shoot?

Gary Burlingame · February 3rd, 2009, 05:57 PM

Thanks, Edward. They only occurred during the TV shots.

Don Bloom · February 3rd, 2009, 06:52 PM

shooting a computer screen requires some different settings but i can't remember them right off hand. However to prove the point take your camera and shoot something that's playing on the computer and you'll see what I mean. Something about the refresh rate vs. frame rate and shutter speed IIRC. I haven't shot a computer screen for a very long time so I can't help you with the settings but I bet if you google it you'll find them.
As for fixing it, probably not much you can do.

Don

Mike Rehmus · February 3rd, 2009, 09:53 PM

Bad news: you cannot fix it. What your are seeing is the vertical retrace interval. Our eyes don't see it because it occurs at 60 times per second (television frame rate) which is above our flicker fusion frequency.

The reason for the problem: The frame rate of the display and the camera were not synched. Professional cameras have a control to allow one to slide the camera into synch with the display. Or one can feed the pro camera synch from the display system. Both ways avoid the dark bar.

Edward Phillips · February 4th, 2009, 09:13 AM

Mike, just out of curiosity, how does this flicker affect the audio recording?

Gary Burlingame · February 5th, 2009, 05:09 PM

Thanks for all of your feedback. I recaptured the footage using a firewire cable instead of a USB connected break out box and the audio problem disappeared. So it looks like I had 2 separate issues here, so I apologize for the red herring about the audio dropouts.

I have posted a sample clip of the problem footage here:
Jeopardy Game Footage on Vimeo

I'm still curious to know if the video problem was indeed caused by the refresh rate of the laptop that was feeding the big screen, and what, if anything, I could have done to avoid the problem. As I said before, none of the interference was visible on my LCD while taping and I didn't know I had a problem until I got home and played the footage back.

Thanks again.

R Geoff Baker · February 15th, 2009, 10:24 AM

Hard to judge from the sample posted whether your problem was actually what I first assumed -- a mismatch between the frame rate you shoot at & the display rate of a computer monitor. The typical result of the mismatch is a rolling bar that cycles through the computer screen, slowly climbing the screen then restarting as it vanishes from the top. This bar is not apparent on an LCD computer monitor, as the refresh of such a monitor is handled a little differently -- consequently, as a shooting concern this problem has faded.

The simplest 'almost' fix is to shoot with a longer shutter speed -- if you force 1/30 instead of the standard 1/60 you will see a lot less of the rolling bar. This has implications for things other than computer monitors, so you may want to play around with this solution.

Kent Frost · February 15th, 2009, 11:42 AM

Are you certain that wasn't actually happening on the TV screen when you were shooting it? That's kinda what it looks like to me.

Gary Burlingame · February 15th, 2009, 02:12 PM

No, Kent. Everything looked fine--that's what's so weird about this.

February 3rd, 2009, 05:13 PM	#1
Gary Burlingame Regular Crew Join Date: Dec 2006 Location: Bellevue, WA Posts: 36	Weird Video Problem I was shooting a company party at a bowling alley recently. After the employees bowled, the CEO projected a "Jeopardy" like game onto a huge TV screen that hung above the bowling lanes. The feed for this game was coming from a laptop computer and the employees played along using remote transponders to signal their ability to answer the questions that were projected onto the screen. Most of the time I kept my PD 170 zoomed into the TV screen while my camera-mounted shotgun microphone captured the audio. When I got home and captured the footage to my computer, to my horror I noticed that there were horizontal, drop-out like lines (but they looked like analog dropouts, not digital ones) that appeared anytime my camera was showing the TV screen (and they only showed up within the confines of the TV screen). They disappeared when I zoomed out and panned to the participants, then reappeared when I zoomed back to the screen. There were also dropouts on the audio track, yet none of these problems were noticeable while I was taping (I was using the flip out LCD & headphones for monitoring). Does anyone know what could have caused this and what one could do to prevent it from happening again? After the Jeopardy game was over and the TV screen resumed showing regular televised programming (i.e. live sporting events), the "dropouts" disappeared. The only thing I can think of is that this must have something to do with the video signal generated by the laptop (conflicting refresh rate?). I was shooting with a shutter speed of 60 and the exposure & focus were set on manual. The way I see it, this will be a nightmare to edit (I will probably use freeze frames instead of live footage to hide the dropouts) but the audio will be a different story. Does anyone have any ideas on what caused this, how to avoid it in the future, and how I can fix it in post? Thanks! Last edited by Gary Burlingame; February 3rd, 2009 at 06:23 PM.

February 3rd, 2009, 09:53 PM	#5
Mike Rehmus Wrangler Join Date: May 2002 Location: Vallejo, California Posts: 4,049	Bad news: you cannot fix it. What your are seeing is the vertical retrace interval. Our eyes don't see it because it occurs at 60 times per second (television frame rate) which is above our flicker fusion frequency. The reason for the problem: The frame rate of the display and the camera were not synched. Professional cameras have a control to allow one to slide the camera into synch with the display. Or one can feed the pro camera synch from the display system. Both ways avoid the dark bar. __________________ Mike Rehmus Hey, I can see the carrot at the end of the tunnel!

February 15th, 2009, 11:42 AM	#9
Kent Frost Regular Crew Join Date: Jul 2006 Location: Springfield, Missouri Posts: 117	Are you certain that wasn't actually happening on the TV screen when you were shooting it? That's kinda what it looks like to me. __________________ "Your world is all these elements; of light and sound, of taste, smell, and touch. Woven together in many dimensions on the fabulous loom of your brain. Your brain; the most complicated thing in the world, which you yourself grew...without even thinking about it."

February 15th, 2009, 02:12 PM	#10
Gary Burlingame Regular Crew Join Date: Dec 2006 Location: Bellevue, WA Posts: 36	No, Kent. Everything looked fine--that's what's so weird about this. Last edited by Gary Burlingame; February 15th, 2009 at 03:45 PM.

February 3rd, 2009, 05:51 PM	#2
Edward Phillips Regular Crew Join Date: May 2008 Location: Memphis, TN Posts: 118	Were the audio drop outs only during the TV shots as well or throughout the shoot?

February 3rd, 2009, 05:57 PM	#3
Gary Burlingame Regular Crew Join Date: Dec 2006 Location: Bellevue, WA Posts: 36	Thanks, Edward. They only occurred during the TV shots.

February 3rd, 2009, 06:52 PM	#4
Don Bloom Inner Circle Join Date: Jan 2003 Location: Chicago, IL Posts: 6,609	shooting a computer screen requires some different settings but i can't remember them right off hand. However to prove the point take your camera and shoot something that's playing on the computer and you'll see what I mean. Something about the refresh rate vs. frame rate and shutter speed IIRC. I haven't shot a computer screen for a very long time so I can't help you with the settings but I bet if you google it you'll find them. As for fixing it, probably not much you can do. Don

February 4th, 2009, 09:13 AM	#6
Edward Phillips Regular Crew Join Date: May 2008 Location: Memphis, TN Posts: 118	Mike, just out of curiosity, how does this flicker affect the audio recording?

February 5th, 2009, 05:09 PM	#7
Gary Burlingame Regular Crew Join Date: Dec 2006 Location: Bellevue, WA Posts: 36	Thanks for all of your feedback. I recaptured the footage using a firewire cable instead of a USB connected break out box and the audio problem disappeared. So it looks like I had 2 separate issues here, so I apologize for the red herring about the audio dropouts. I have posted a sample clip of the problem footage here: Jeopardy Game Footage on Vimeo I'm still curious to know if the video problem was indeed caused by the refresh rate of the laptop that was feeding the big screen, and what, if anything, I could have done to avoid the problem. As I said before, none of the interference was visible on my LCD while taping and I didn't know I had a problem until I got home and played the footage back. Thanks again.

February 15th, 2009, 10:24 AM	#8
R Geoff Baker Major Player Join Date: Jan 2005 Location: Ottawa, ON Posts: 471	Hard to judge from the sample posted whether your problem was actually what I first assumed -- a mismatch between the frame rate you shoot at & the display rate of a computer monitor. The typical result of the mismatch is a rolling bar that cycles through the computer screen, slowly climbing the screen then restarting as it vanishes from the top. This bar is not apparent on an LCD computer monitor, as the refresh of such a monitor is handled a little differently -- consequently, as a shooting concern this problem has faded. The simplest 'almost' fix is to shoot with a longer shutter speed -- if you force 1/30 instead of the standard 1/60 you will see a lot less of the rolling bar. This has implications for things other than computer monitors, so you may want to play around with this solution.