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Noise bars (?) on DV footage


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#1 Simon Roel

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Posted 30 March 2008 - 07:44 AM

Hello there,

I worked as a B-camera operator on a no-budget short film. Despite the odds, the producer/director was able to get assistance from the Danish Army. They supplied a PMV and a couple of jeeps and we got to shoot in a military area. So a pretty big operation considering the budget of the production.
The A-camera operator and I shot on a Canon XL1S and a Canon XL1, respectively. We used regular, fresh JVC miniDV tapes.

The problem:
There is no footage on the 4 tapes we shot on. Only short fragments (no longer than 30 seconds) spread out across the tape that, when played back through both Canon cameras and a third DV camera, are filled with grey bars across the frame (cf. attached grabbed frame). There is almost no sound; only a few noises and a few very crystal clips of audio, though these are clipping.
The A-camera operator and I made sure that the cameras were recording and that the time code was running before we yelled 'speed' to the director.
We had no problems throughout the day. The XL1S had to be cleaned with a cleaning tape at one point, which we did.

Possible causes:
? We have been going through what happened throughout the entire day - what we did, the circumstances of the shoot etc. And there seems to be no clear reason why the tapes shouldn't have worked properly. We recorded on the same type of miniDV tape from the very same batch later that night, without altering ANY of the settings on the two cameras, and that footage worked fine.
Could it be some kind og climatic issue or could it be static electricity or something that scrambled the footage? We were shooting in a military area, but the director had done that another time using the very same XL1S at the very same location, and that worked fine.

We would simply like to know the reason why this happened so we can avoid it in the future (of course, we'd like to be able to obtain the footage from the tapes, but that seems unlikely). If you've experienced anything similar or can give os a clue as to what happened, please respond - I'd be most grateful.

Kind regards,
Simon

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Edited by Simon Roel, 30 March 2008 - 07:48 AM.

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#2 Phil Rhodes

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Posted 30 March 2008 - 09:18 AM

I suspect you're stuffed. The alternating stripes are produced by either damage to or clogging of one half of the set of heads on the drum. If it's dirt then it could presumably have been cleaned, if it's damage or wear it'll be a new set of heads, but either way if you have several other cameras refusing to play the stuff back then the problem will be baked into the tape and is probably not recoverable, at least not without a hugely expensive data forensics effort.

Your cameras must be pretty long in the tooth by now. About the only comfort I can offer you is that if these cameras both worked before (you did presumably test them before taking them out on an important shoot?) you may be able to clean the heads out now - possibly you got a bad batch of tape.

The only other thought is that you ran into some sort of military RF problem. Any type of radar or ECM system can and will affect unshielded electronics, but I would not have expected it to produce the results you have here - the alternating stripe is something that I think more or less all DV devices do at some point in their lives, and is highly characteristic of a head problem.

P

PS - In the example you gave, the even head passes were bad. Does it ever switch around, so that the odd passes are bad? This would point strongly to bad tape clogging heads.
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#3 Freya Black

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Posted 30 March 2008 - 10:44 AM

Did you try to play back all 4 tapes off the same camera at first?

You should have played the tapes back off the cameras they were shot on as this would have avoided cross contamination if there was a problem with the camera.

If this is what did happen then I would still treat the camera you used for this operation as suspect.

As you may know, it is thought that different brands of tape can interact with one another. For this reason people often recommend to stick to the same brand of tape over the life of the camera. It's a bit of an extreme measure but there we are. I suspect being a low budget production, you scrounged up cameras wherever, which means that people using different cameras could have used different brands and...

I'm just having a guess at some possiblities.

I've had the same problem as you once, and it was heartbreaking and really, really embaresing.
I think that was last year actually. The year that everything I touched turned brown.

love

Freya
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#4 Simon Roel

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Posted 30 March 2008 - 11:46 AM

Hi Phil,

Thanks for answering.
To your questions:
Yes, we did test the cameras before the shoot - they worked fine and recorded without any trouble. We also tested the cameras afterwards, away from the military area - no problem, they recorded and played back just fine. And we didn't change a single setting between the end of the shoot and this post-shoot test.
I don't know if the passes switch - I'll have to ask the director to check that out. However, if the heads were causing the problem, wouldn't they cause the same problem after the shoot (which they didn't)?

It's still mysterious, in my opinion.

Best,
Simon

I suspect you're stuffed. The alternating stripes are produced by either damage to or clogging of one half of the set of heads on the drum. If it's dirt then it could presumably have been cleaned, if it's damage or wear it'll be a new set of heads, but either way if you have several other cameras refusing to play the stuff back then the problem will be baked into the tape and is probably not recoverable, at least not without a hugely expensive data forensics effort.

Your cameras must be pretty long in the tooth by now. About the only comfort I can offer you is that if these cameras both worked before (you did presumably test them before taking them out on an important shoot?) you may be able to clean the heads out now - possibly you got a bad batch of tape.

The only other thought is that you ran into some sort of military RF problem. Any type of radar or ECM system can and will affect unshielded electronics, but I would not have expected it to produce the results you have here - the alternating stripe is something that I think more or less all DV devices do at some point in their lives, and is highly characteristic of a head problem.

P

PS - In the example you gave, the even head passes were bad. Does it ever switch around, so that the odd passes are bad? This would point strongly to bad tape clogging heads.


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#5 Phil Rhodes

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Posted 30 March 2008 - 12:43 PM

> However, if the heads were causing the problem, wouldn't they cause the same problem after the
> shoot (which they didn't)?

You'd have thought so, yes. It's very irregular.

It certainly is heads-related - that pattern of breakup confirms it, really. The fact that there's occasional bursts of partial picture implies that something is changing rapidly. It could be more or less anything really but since they work now I suggest it was probably a particularly bad bit of luck with the tape or weather conditions. Sufficient condensing humidity could put water on the heads but I would expect the camera to detect that and complain - and condensing humidity in that part of the world at this time of year is not something I would expect to be a problem unless you were in and out of heated buildings or vehicles a lot.

The only alternative I can think of, that being RF interference, is tricky to believe. How long does it keep looking like your example picture? Is the visible picture disturbance synchronised in any way to the apparent orientation of the camera?

Can you go back and try it again? Ask the military what equipment they were running?

Were the cameras subjected to any concussion, as from a field gun being fired or some other explosion?

P
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#6 Hal Smith

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Posted 30 March 2008 - 07:12 PM

Looks to my eyes like you might have been close to a high powered search radar. The timing of the bands correlate with the pulse repetition rates close to those used by long range radars. Also the fact that you had nothing on the tape most of the time mixed with some footage with the banding is indicative of a large radar slowing rotating. When it was pointed at you, nothing, when it pointed away from you the inteference was dropping to the point where your cameras worked a bit.

Many moons ago I stuck my head up through a hatch in the radome floor of a working 3 million watt USAF radar site on the north coast of Alaska. I heard a very loud BEEP inside of my head for about 2 seconds until I realized what I was hearing was the radar - and down the hatch I went as fast as possible. My guess is one of the fillings in my teeth was detecting the radar signal!
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#7 Phil Rhodes

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Posted 30 March 2008 - 07:54 PM

The timing is characteristic of a head failure. For whatever reason - clogging, RF intrusion, aliens - it's a head related issue. DV heads rotate at 9000RPM, which, given two record heads on the drum, dictates twelve stripes per frame at 25fps.

Phil
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