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 Posted: May 08, 2012 - 04:46 PM |
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Joined: Sep 12, 2009
Posts: 2402
Location: Sacramento, CA
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| I need to make some videos of some PWM-driven RGB LEDs, but I'm finding that I get artifacting between the PWM rate and the video rate. The effect is vertically rolling horizontal bands of more/less brightness. Anybody have any experience with this? Any video cameras (consumer, not professional) that are better for this kind of work? |
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Posted: May 08, 2012 - 05:36 PM |
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Joined: Sep 15, 2006
Posts: 172
Location: Northern Colorado
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The way I've seen it handled is to sync the object you are trying to record with the camera. An example is videoing a TV picture. If not synced one usually sees a scrolling horizontal band. That is like what you are seeing. This capability is only found on professional equipment. The only solution I can think of for consumer equipment is to play with the PWM frequency and/or maybe shutter speed on the camera if you have that option.
Good luck
Roger |
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Posted: May 08, 2012 - 05:54 PM |
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Joined: Sep 12, 2009
Posts: 2402
Location: Sacramento, CA
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Thanks, yes I had played around with the PWM rate and sure enough, the bands went from slowly going down to quickly going up.  Unfortunately I'm using the AVR internal clock, so there's no way I'll get multiple AVRs perfectly synched. However, maybe "close enough" will be close enough. |
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Posted: May 09, 2012 - 02:18 AM |
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Joined: Sep 15, 2006
Posts: 172
Location: Northern Colorado
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do you have the ability to edit your video frame by frame? you could "photo shop" it to look the way you want it.
Roger |
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Posted: May 09, 2012 - 02:34 AM |
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Joined: Feb 19, 2010
Posts: 507
Location: Montreal, QC, CA
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You could try running the PWM at a much higher frequency. It should alleviate the flickering effect a bit if you keep the camera's frequency 3 or more harmonics lower than your PWM frequency. That also means you will use more power though.
In terms of video editing, a simple short persistence of vision filter should do it. |
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Posted: May 09, 2012 - 02:53 AM |
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Joined: Dec 11, 2007
Posts: 6849
Location: Cleveland, OH
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I've not tried this, but what have you got to lose?
I would put the camera in manual mode for its exposure and focusing, essentially turning off all internal micro controlled adjustments. I would also film it in a bright environment, so that the LEDs are not the brightest component of the image. (Yes, I know, it won't look as impressive..., but at least try this for starters). As you noted consumer grade gear doesn't have an external synch option.
If you had a scope and time you could probe around the video camera and see if you could get a frame synch signal of sorts to use in your AVR to reverse synch, perhaps.
JC |
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