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Sony VX2100 / PD170 / PDX10 Companion
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Old February 3rd, 2009, 05:13 PM   #1
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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.
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Old February 3rd, 2009, 05:51 PM   #2
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Were the audio drop outs only during the TV shots as well or throughout the shoot?
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Old February 3rd, 2009, 05:57 PM   #3
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Thanks, Edward. They only occurred during the TV shots.
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Old February 3rd, 2009, 06:52 PM   #4
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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.

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Old February 3rd, 2009, 09:53 PM   #5
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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.
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Old February 4th, 2009, 09:13 AM   #6
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Mike, just out of curiosity, how does this flicker affect the audio recording?
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Old February 5th, 2009, 05:09 PM   #7
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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.
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Old February 15th, 2009, 10:24 AM   #8
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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.
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Old February 15th, 2009, 11:42 AM   #9
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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.
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Old February 15th, 2009, 02:12 PM   #10
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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.
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