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Old Jun 9, 2009, 3:12 PM   #1
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New guy here. I use a camcorder for evaluating my golf swing. I have heard that some (all/few/not sure) camcorders (mini-DV) have some form of delay or scan or (not sure about the right term) where the top of a single frame is sampled much before the bottom of the frame. For my purposes this would mean that when (for example) the shaft of the club is vertical and looks like it is flexx'ed forward (bottom end more advanced that the top end), this may not be the case as these two sections of the image were not sampled at the same time.

My questions are:

1) What is the proper technical name of this phenomenon (assuming that it is real)

2) What is the typical delay from top to bottom of a frame (for cases where this is an issue).

3) Other than taking two 'identical videos' with the camera rotated at 90 degrees, how would you go about determining the extent of this effect?

Thanks.

dave
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Old Jun 9, 2009, 3:26 PM   #2
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Hi and welcome Dave.

I'm not sure about everything for for question one I believe it is called progressive scan.

Question 2 I believe it will depend on the technology/manufacturer.

Question 3, I've not idea at all.

Hopefully someone else will know more.
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Old Jun 9, 2009, 3:48 PM   #3
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Many camcorders have a high speed shutter option that renders better slow motion videos and still images. That may significantly reduce the problem you're seeing.
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Old Jun 9, 2009, 3:50 PM   #4
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Mark - thanks for the response.

From what I know there is 'interlaced' and 'progressive scan'

Interlaced is basically two fields (odd scans and even scans) that are sampled at somewhat different times (and then combined).

Progressive scan does it 'all at once'.

Neither of these seem (from what I know) to create the problem that I am referring to. But I have read just enough references to 'that, that, the other' to make me wonder.

Thanks again.

dave
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Old Jun 9, 2009, 3:52 PM   #5
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Quote:
Originally Posted by TCav View Post
Many camcorders have a high speed shutter option that renders better slow motion videos and still images. That may significantly reduce the problem you're seeing.
I'm just not sure that shutter speed necessarily avoids the problem. Shutter speed is about how much exposure a given pixel/CCD element/whatever gets. It doesn't necessarily tell you whether or not the bottom pixels were scanned at the same time as the top ones.

Thanks.

dave
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Old Jun 9, 2009, 7:47 PM   #6
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The problem that I am referring to is "the rolling shutter" problem.

A Google/whatever search on that term yields useful results.

And, as with most things, "it varies".

dave
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Old Jun 10, 2009, 3:22 AM   #7
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My mistake, I thought that the technical term for rolling shutter was progressive scan but this isn't correct. From what I can tell CCD cameras don't suffer from rolling shutter but CMOS ones do. I have Canon SX1 and 500D both with CMOS and both with rolling shutter, not a problem for what I do and I was aware of the limitation before I got them.
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Old Jun 10, 2009, 6:36 AM   #8
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When the high speed shutter option is selected, the image is exposed, then scanned, and then another image is exposed. The resulting video is a sequence of still images, not on-the-fly scans of what the sensor is exposed to.
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Old Jun 10, 2009, 8:44 AM   #9
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Quote:
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When the high speed shutter option is selected, the image is exposed, then scanned, and then another image is exposed. The resulting video is a sequence of still images, not on-the-fly scans of what the sensor is exposed to.
Is that always true? The question came up on a golf discussion board regarding the following image.

http://www.pbase.com/bret/image/113518953/original

The question (obviously) being "is the shaft really that much flexxed forward at impact"? The camera used was a Canon 5d2.

dave
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Old Jun 10, 2009, 9:04 AM   #10
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Quote:
Originally Posted by DaveLeeNC View Post
Is that always true? The question came up on a golf discussion board regarding the following image.

http://www.pbase.com/bret/image/113518953/original

The question (obviously) being "is the shaft really that much flexxed forward at impact"? The camera used was a Canon 5d2.

dave
You don't need to know anything about photography just a little physics to know that the head will be trailing the shaft so this is simply due to rolling shutter.

I'm sure most golfers will have seen swing sequences when the shaft is bent the other (correct) way.
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