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LEDs versus CCTV camera A bit of an odd one....

#1 User is offline   GridGirl 

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Posted 22 November 2008 - 01:27 PM

Got a bit of a weird one going on at the moment; we have several LED strips (which we're using for the first time) installed to light a set wall, and I looked up at my monitor (calling from backstage) during the tech to see them randomly chasing - if more than one colour was on, they were chasing between colours, but with only one colour on they were chasing on and off. As you do, I asked the operator what exactly was going on, as I was sure we hadn't plotted a random LED chase, and he was rather surprised by my question and answered that they weren't chasing at all. Sure enough, when I stood up and poked my head around the corner, the wall was lit nicely with solid non-chasing colours! We've established that it isn't a TV problem, as all five stage feed monitors (which are a mixture of LCD monitors and ordinary old TVs) are doing exactly the same thing, so we've come to the conclusion that it must be the camera. Has anyone ever struck this before, or know what causes it, and if there's a way to stop it?! The camera is a fairly standard security camera, full colour with auto-irising (I think!) to cope with varying light levels, and we've never had anything like this before. Nobody can figure out why it's doing it, and over 12 weeks I may get a little annoyed at it so would love to know if there are any solutions to it!

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#2 User is offline   thelxbloke 

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Posted 22 November 2008 - 01:32 PM

This is most likeley down to the way the LED's do their dimming this combined with the ?scan rate? of the camera would probably cause this...

Please correct if I'm talking rubbish!

HTH
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#3 User is offline   The Gaffa 

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Posted 22 November 2008 - 01:58 PM

Sounds like the scan rate on the camera, we have a similar problem with two of our cctv cameras when they're in night mode.
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#4 User is offline   Andrew C 

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Posted 22 November 2008 - 02:00 PM

I'd say that they were beating. The scan rate of the camera is near to a harmonic of the refresh rate of the LEDs (or visa versa). As to what you could do about it without changing one or the other however...
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#5 User is offline   lxdad 

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Posted 22 November 2008 - 03:59 PM

Most LED fixtures use Pulse Width Modulation to make the LEDs brighter or dimmer. This is fine to your eye because Persistance of Vision will make your brain fill in the blanks. However cameras do not work like your brain and they will see a flickering lightsource if the frequency of the LEDs is an integer multiple of the frame rate.
Good LED fixtures have a capacitor arrangement to do some 'smoothing' to get rid of this flicker or simply run at a very high frequency.

If you are planning to use LED fixtures on camera it is always important to either use ones you know will work, or get the fixtures in front of a professional camera BEFORE you do the fitup!

This information is simply from my experience, I am not an electrical engineer, so hopefully someone more knowledgable can fill in some of the blanks in my explanation.

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#6 User is offline   adamharman 

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Posted 22 November 2008 - 11:24 PM

View Postlxdad, on 22 Nov 2008, 3:59 PM, said:

Most LED fixtures use Pulse Width Modulation to make the LEDs brighter or dimmer.

Good LED fixtures have a capacitor arrangement to do some 'smoothing' to get rid of this flicker or simply run at a very high frequency.


They won't have a capacitor for that because 'smoothed' PWM would be DC which would be rather pointless. Higher frequency seems much more likely.

I think the way the cameras work is the sensor effectively "takes" a photo, and this is then read off, so you're getting a view of an extremely short time period rather than the 1/25 sec that you'd expect from the frame rate. The LEDs are probably flashing fast enough that in a full frame time you'd get exposure from all the colours. Your LED strips seem to be flashing each colour in turn, hence the different effect depending on how many colours are on.

There may be a type of camera which can deal with it, but I don't know enough to say for certain.

#7 User is offline   RobOwen 

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Posted 23 November 2008 - 04:32 PM

Also, if cheaper chips are used they might have had only one PWM output. This would mean multiplexing the outputs (RGB) using transistors and then rapidly switching through the colours, hence the chasing between them. This is the technique commonly used for controlling banks of seven segment displays (switching the common as the pin count would be excessive driving four numbers at the same time)
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#8 User is offline   jason5d 

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Posted 26 November 2008 - 06:28 AM

You need to use a better camera.

Try a good camcorder if you can get hold of one
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#9 User is offline   GridGirl 

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Posted 26 November 2008 - 11:35 AM

View Postjason5d, on 25 Nov 2008, 6:28 PM, said:

You need to use a better camera.

Try a good camcorder if you can get hold of one


Good camcorders have been tried in the past; we can't get a wide enough angle lens (our stage is VERY wide and the house is shallow, so you can't put the camera far enough back), plus they don't always react well to wildly differing light levels. a professional video camera is much more expensive than we want to be spending, plus in every other situation the existing camera is perfect for our needs. We had some professional video guys in taping the show on Monday, and they didn't have any problem; when asked, they thought it was definitely to do with frame and scan rate. So, it just looks like I have to put up with another 11 1/2 weeks of chasing LEDs on my monitor! To tell the truth, I'm now at the point where I can switch off to it, my brain has just got used to it!
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#10 User is offline   Vanjast 

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Posted 27 November 2008 - 06:36 AM

As mentioned You get a 'beat' frequency when you have 2 (or more) sources close to the same frequency (or a multiple thereof).

Another problem you'll have with this stuff is...

- The LED's are most probably 'driven' with a base PWM frequency (should usually in the KHz range - You have some cheapy ones where this frequency is very low = major problems)
- Added to this will be the Duty Cycle frequency (the dimming part) which starts at the base frequency and goes upwards as far as the resolution of the PWM driver - this can be quiet high. The effect of this, is that at a certain dimming level the 'Beat' becomes more pronounced and fades away on either side. Where this happens depends on the frame rate of your camera.

What you should probably get is a 'DC driven' LED light[s]. There are plenty high power LEDs nowdays on the market, so it's possible that there is one floating around somewhere - Sorry I cannot help further as I have not looked into this side of LEDs, but maybe googling might help.
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This post has been edited by Vanjast: 27 November 2008 - 06:41 AM


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