Vertical banding with external power? Anyone seen it?

arsenix

« on: 17 / May / 2016, 19:45:42 »

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I have an aerial rig that I have been developing and recent testing is showing some noticeable vertical banding in the photos. The same banding shows up on all three cameras in the rig, so it must be some kind of conducted noise. The period of the bands is ~10 pixels. The cams also have USB attached (with PTP live) and are triggered through the battery temp terminal. So in theory there could be noise coming in through avenues other than the power supply I suppose.

I had looked at the power supply in the system with an o-scope and didn't see anything really significant. I would assume that these cameras have a reasonable amount of built in noise filtering?

Just wondering if anyone else had seen similar noise before with Canon cams externally powered (or otherwise).
I attached is a cropped image of boring grass, with exciting banding

James

Cropped_banding.JPG (160.03 kB, 1282x958 - viewed 56 times.)

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waterwingz

12125

Re: Vertical banding with external power? Anyone seen it?

« Reply #1 on: 17 / May / 2016, 20:17:09 »

Quote from: arsenix on 17 / May / 2016, 19:45:42

I have an aerial rig that I have been developing and recent testing is showing some noticeable vertical banding in the photos. The same banding shows up on all three cameras in the rig, so it must be some kind of conducted noise. The period of the bands is ~10 pixels.

Very unusual for a random noise source to product vertical stripes like that. But the clock frequency from one camera would be pretty close to the clock frequency of the other two so they could "beat" together and give that sort of pattern. See below ..

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The cams also have USB attached (with PTP live) and are triggered through the battery temp terminal. So in
theory there could be noise coming in through avenues other than the power supply I suppose.

If you have the battery 3rd terminals simply connected in parallel then you are effectively joining the electronics of the three cameras together, introducing a potentially huge source of noise as the various clock circuits interact with each other. At a minumum, a small decoupling capacitor at each camera 3rd terminal contact to local ground would be a good idea. Also, a separate series resistor to whatever switch arrangement you have built is a good idea.

How are your connections currently wired?

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I had looked at the power supply in the system with an o-scope and didn't see anything really significant. I would assume that these cameras have a reasonable amount of built in noise filtering?

Probably not a good assumption. The cameras are really designed to work from batteries - a very noise free source of power.

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Ported : A1200 SD940 G10 Powershot N G16

arsenix

Re: Vertical banding with external power? Anyone seen it?

« Reply #2 on: 17 / May / 2016, 20:50:29 »

Quote from: waterwingz on 17 / May / 2016, 20:17:09

If you have the battery 3rd terminals simply connected in parallel then you are effectively joining the electronics of the three cameras together, introducing a potentially huge source of noise as the various clock circuits interact with each other. At a minumum, a small decoupling capacitor at each camera 3rd terminal contact to local ground would be a good idea. Also, a separate series resistor to whatever switch arrangement you have built is a good idea.

How are your connections currently wired?

I do in fact have the third terminals connected together currently. There is a large cap buffering this signal, but it is central and there are flying leads going out the cameras. There are flex leads connecting each of the cameras to a central wiring board. I don't currently have filtering at each camera on these terminals, but maybe this would be useful. I can experiment with this. I hadn't thought noise from the battery temp terminal would end up in the images. This is fairly easily t

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Probably not a good assumption. The cameras are really designed to work from batteries - a very noise free source of power.

Very true. There are quite a few capacitors on the camera PCB's... and it gets regulated down, but despite this something is making it through. I'll try adding some additional filtering close to the cameras and see if that fixes things. Your comment about the unlikely vertical banding also resonated with me. I'm assuming the clock frequency on these cams in the 10's of Mhz. The period on that noise would indicate the noise getting in there is a few Mhz then... which I would think would be trivially filtered by the camera's input capacitors. Guess not!

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waterwingz

12125

Re: Vertical banding with external power? Anyone seen it?

« Reply #3 on: 17 / May / 2016, 21:24:08 »

Quote from: arsenix on 17 / May / 2016, 20:50:29

I do in fact have the third terminals connected together currently. There is a large cap buffering this signal, but it is central and there are flying leads going out the cameras.

Large capacitors - especially electrolytic - are essentially useless for filtering high frequency noise. Regardless of where they are positioned. And the other name for "flying leads" is "antennas".

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There are flex leads connecting each of the cameras to a central wiring board. I don't currently have filtering at each camera on these terminals, but maybe this would be useful. I can experiment with this. I hadn't thought noise from the battery temp terminal would end up in the images.

Not just useful, but all most certainly essential. I'd suggest a 0.1 µF ceramic cap mounted as close to the battery 3rd terminal connection as possible.

If you want to do a little reading, here's a couple of articles I found with a quick google.
https://www.intersil.com/content/dam/Intersil/documents/an13/an1325.pdf
http://www.allaboutcircuits.com/technical-articles/clean-power-for-every-ic-part-2-choosing-and-using-your-bypass-capacitors/

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Very true. There are quite a few capacitors on the camera PCB's... and it gets regulated down, but despite this something is making it through. I'll try adding some additional filtering close to the cameras and see if that fixes things. Your comment about the unlikely vertical banding also resonated with me. I'm assuming the clock frequency on these cams in the 10's of Mhz. The period on that noise would indicate the noise getting in there is a few Mhz then... which I would think would be trivially filtered by the camera's input capacitors. Guess not!

Again, my guess is the noise is being coupled by that 3rd terminal battery connection - not the power supply.

And when I originally said the clock frequencies would "beat" together that's exactly what happens - you end up with a lower frequency noise source.

Easy enough to test - disconnect the 3rd terminal connections and take a few single shots.

« Last Edit: 17 / May / 2016, 21:54:11 by waterwingz »

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Ported : A1200 SD940 G10 Powershot N G16

arsenix

Re: Vertical banding with external power? Anyone seen it?

« Reply #4 on: 26 / May / 2016, 19:17:50 »

I did some experiments and so far haven't figured it out.

BTW when I said "big caps" I didn't mean really giant.

[fast forward a few hours of fooling around trying various capacitors, indoor/outdoor tests etc]

I tried adding 1uF caps (ceramic, low ESR) right at the camera boards, both on the power feed to the cameras as well as on the third battery terminal. Neither seems to have had much effect, or both at once. Maybe slightly improved, but still bad enough that it is hard to judge. I took a closer look at the camera PCBs too and they have quite a bit of input capacitors on the power line. At least 3 or 4 fat 1210 sized SMD caps.

It is definitely a noise issue since it only happens when the cameras are being flown on my vehicle (a multicopter). I did double check this to make absolutely sure just now. I also can't even replicate it with the vehicle running on a test stand (which is highly strange). Maybe it only occurs at short exposure times (it is brighter outside)? Also tried higher ISO inside on test stand (exposure times similar to outdoor flights)... no bands.

Just took it outside to take some photos in bright sunlight, even when on the ground... BANDS! Maybe this is some kind of light leakage? The lenses are de-cased, but are also facing downwards and sitting on the belly of a vehicle. Sounds like I am getting somewhere finally!

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reyalp

13745

Re: Vertical banding with external power? Anyone seen it?

« Reply #5 on: 26 / May / 2016, 22:01:07 »

Quote from: arsenix on 26 / May / 2016, 19:17:50

Just took it outside to take some photos in bright sunlight, even when on the ground... BANDS! Maybe this is some kind of light leakage? The lenses are de-cased, but are also facing downwards and sitting on the belly of a vehicle. Sounds like I am getting somewhere finally!

"De-cased" ?
Did you remove or disable the mechanical shutter?

Did you try waterwingz suggestion of

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disconnect the 3rd terminal connections and take a few single shots.

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