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Old January 12th, 2007, 12:33 PM   #1
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green dots when using +18db.. dropped pixel look

Hi,

I have recently discovered a problem with some footage i have filmed in a dark situation. I had to use the gain control and knock it up to +18 and now i notice there is 2 green dots resembling dead pixels on an lcd screen on the footage. I can now faintly see the same green dots when i connect my xl1 to a tv with the lens cap on and knock the gain up to +18 or +30. (the green dots are only visable when the image is dark/black)

I am completely stumped and luckily had a back up camera filming this particular footage.

If anyone has any help or experience regarding this it would be very much appreciated
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Old January 12th, 2007, 01:05 PM   #2
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ok so after doing some research I see it is probably a hot pixel. This one week after it arrived back from a canon authorised dealer for a check up ("no fault found" despite the fact I still have the same sticky zoom handle as before i sent it in)

Has anyone in the UK had any experience of getting this problem repaired? and is a CCD replacement the only way?
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Old January 13th, 2007, 07:57 AM   #3
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If you read the back of the XL1s manual, page 109, "About the CCD" you will see a note that effectively states that hot pixels can become apparent at high gain settings (or slow shutter speeeds).

What is going on is that it is the nature of CCD's that all pixels have a "dark current" - some output even when there is no light hiting them. And the amount of dark current varies from pixel to pixel due to manufacturing variances. When you add gain this dark current is amplified as is the currrent caused by light. In most cases with complex image content this is not noticeable.

Now for a very simple analysis. If the you have a pixel with baseline dark current of nominally, say, 2 IRE units among pixels with a baseline dark current of 1 IRE, all are well below NTSC Black (7.5 IRE) and not noticeable. However, if you add 18 db of gain the hot pixel increases 16 IRE while the others increase to only 8 IRE and it becomes very noticeable relative to the others that are still pretty much in the black.
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Old January 13th, 2007, 01:29 PM   #4
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thanks Don I think I understand that. What causes it in the first place? Prolongued overexposure to light? I have a UV filter too would that have helped?
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Old January 14th, 2007, 07:46 AM   #5
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I suspect that it is possible for individual pixels to age at different rates, possibly due to damage at the molecular level caused by exposure to radiation; e.g., spending time high altitude such as in an airplane during a sun spot storm ther the atmosphere does nto provide protection. Space satellites are hardened againts just this sort of damage. This would be in addition to normal variances in drk current that happened when the CCD was made and thus appear later in the camcorders life.

I would expect damage caused by something coming though the lens in a shot (e.g., shooting the sun) to cause more wide spread dammage to the CCD.

The point of course is - can you see the spot in that I'll call normal shots with full frame complex image content, as oposed can you find the spot looking for it in special case shots (e.g., maximum gain and lens cap on, or extensive very dark backgrounds)?

Perhaps in the unusual shots like on a black field with high gain you can save the shot by applying a one-pixel mask in post to hide the spot.
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Old January 14th, 2007, 02:10 PM   #6
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Don,
In a well lit situation without using a + gain you cannot see them at all, it is only in a darkened situation they are visible and made a lot worse by boosting the gain
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