I shot a video of my stopwatch in 24pa mode, then opened in 24p vegas timeline with 2:3 pulldown allowed for 24p. I observe 24 frames for each elapsed second (as expected), but the time between each frame varies (in units of .01 sec) as follows: .09, .01, .10, .00, .10, .00, .09, .01, .00, .09, .01, etc, etc up until 24 frames and 1.00 sec have elapsed. I expected to see on the video a uniform time between each frame of approx .042 sec (1/24 sec). I realize that the above intervals average to .042 sec/ frame, but the intervals are not uniform. Can anyone explain this?
frame to frame timing in 24pa mode
- Thread starter kmcgrath
- Start date
Re: frame to frame timing in 24pa mode
I'm not sure what you mean by "between each frame." One frame should run into the next, right?
I'm not sure what you mean by "between each frame." One frame should run into the next, right?
Re: frame to frame timing in 24pa mode
Also, keep in mind that your stopwatch, while accurate, is still digital and based on multiples of two, so 1/1000 of a second is probably actually 1/1024 of a second. Could account for a discrepancy when you're looking at 24 fractions of a second.
Or something like that. Digital watches have come a long way since the Timex or Texas Instruments boxes of 1978, but they still have to obey binary rules. The more fractional you get inside a second, the less accurate it will be.
Also, keep in mind that your stopwatch, while accurate, is still digital and based on multiples of two, so 1/1000 of a second is probably actually 1/1024 of a second. Could account for a discrepancy when you're looking at 24 fractions of a second.
Or something like that. Digital watches have come a long way since the Timex or Texas Instruments boxes of 1978, but they still have to obey binary rules. The more fractional you get inside a second, the less accurate it will be.
Re: frame to frame timing in 24pa mode
The "time between each frame" is the time difference I observe on the stopwatch from two adjacent video frames. Maybe my understanding of the acquisition of each frame is the problem. I was assuming that each frame was captured in an "instant" in time, which clearly cannot be the case. I'm not really sure how video is captured during the course of time when the "shutter is open". In other words, are all the pixels captured at once during this time (not possible I think), or are they scanned similar to how an NTSC tv plays back. Maybe that would explain why the time I see on the stopwatch in each frame is not uniformly spaced. Could it be that what I am seeing on my digital stopwatch is somehow a "smear" of the digits during the time the "shutter is open", and I am not really seeing the single time period to .01 sec that I think I am seeing when I look the digits on the stopwatch in each frame in vegas timeline?
The "time between each frame" is the time difference I observe on the stopwatch from two adjacent video frames. Maybe my understanding of the acquisition of each frame is the problem. I was assuming that each frame was captured in an "instant" in time, which clearly cannot be the case. I'm not really sure how video is captured during the course of time when the "shutter is open". In other words, are all the pixels captured at once during this time (not possible I think), or are they scanned similar to how an NTSC tv plays back. Maybe that would explain why the time I see on the stopwatch in each frame is not uniformly spaced. Could it be that what I am seeing on my digital stopwatch is somehow a "smear" of the digits during the time the "shutter is open", and I am not really seeing the single time period to .01 sec that I think I am seeing when I look the digits on the stopwatch in each frame in vegas timeline?
Re: frame to frame timing in 24pa mode
The frame is acquired all at once, yes. It's progressive. An NTSC TV is interlaced.
The frame (at defaults) is read f0r 1/50 of a second, 24 times per second. Hmm. That would be .02. Hmm. Look at the numbers you're seeing there. .09 in 1; .10 the next, difference of of .01 between them; HIGHLY unlikely the start of your frame corresponds exactly to a hundredth of a second, with could account for the difference. I haven't extrapolated the math here, but I bet it works out.
The frame is acquired all at once, yes. It's progressive. An NTSC TV is interlaced.
The frame (at defaults) is read f0r 1/50 of a second, 24 times per second. Hmm. That would be .02. Hmm. Look at the numbers you're seeing there. .09 in 1; .10 the next, difference of of .01 between them; HIGHLY unlikely the start of your frame corresponds exactly to a hundredth of a second, with could account for the difference. I haven't extrapolated the math here, but I bet it works out.
Re: frame to frame timing in 24pa mode
Maybe the math does work out. If the +/- error in my time measurements is guestimated as the time the shutter is open (,02 sec) plus the time the shutter is closed between acquisitions (.0217 sec), then the total error would be about .042 sec. If I add or subtract this amount from each of my time measurements from each frame, I could come up with the expected "time between frame" value. So maybe all is well after all. I was getting a bit nervous that my camera was progressively acquiring on some random timescale! Maybe I should redo my experiment using a shorter shutter speed to minimize the time elapsed during a frame acquisition and stopwatch measurement...
Maybe the math does work out. If the +/- error in my time measurements is guestimated as the time the shutter is open (,02 sec) plus the time the shutter is closed between acquisitions (.0217 sec), then the total error would be about .042 sec. If I add or subtract this amount from each of my time measurements from each frame, I could come up with the expected "time between frame" value. So maybe all is well after all. I was getting a bit nervous that my camera was progressively acquiring on some random timescale! Maybe I should redo my experiment using a shorter shutter speed to minimize the time elapsed during a frame acquisition and stopwatch measurement...
Barry_Green
Moderator
Re: frame to frame timing in 24pa mode
I think you're onto it, kmcgrath. The CCD doesn't scan, as I understand it... it's a surface full of accumulating capacitors, so as soon as the "shutter" opens, the entire frame starts accumulating light. So yes, you'd see 1/50th of a second's worth of smearing digits, if your stopwatch were able to update its display at a rate faster than 1/50th of a second.
You could try your experiment again but manually set the shutter speed to some absurdly high number, like 1/1000. That would go a long way towards minimizing any smear and should provide more measurable results.
I think you're onto it, kmcgrath. The CCD doesn't scan, as I understand it... it's a surface full of accumulating capacitors, so as soon as the "shutter" opens, the entire frame starts accumulating light. So yes, you'd see 1/50th of a second's worth of smearing digits, if your stopwatch were able to update its display at a rate faster than 1/50th of a second.
You could try your experiment again but manually set the shutter speed to some absurdly high number, like 1/1000. That would go a long way towards minimizing any smear and should provide more measurable results.
Re: frame to frame timing in 24pa mode
Given that any motion during the time that the "shutter" is open will be smeared on the final image, why is it that video often times appears so "sharp" even when the individual frames making up the video are quite blurred. I guess the answer must be that the sharpness is only a perceived thing due to the nature in which we process the information??? On a possibly related note, what is the use of "motion blur" icon in the video track in Vegas and how should be it be properly applied in 60i and 24p formats? I clicked on the help info for this in Vegas, but not much info was given.
Given that any motion during the time that the "shutter" is open will be smeared on the final image, why is it that video often times appears so "sharp" even when the individual frames making up the video are quite blurred. I guess the answer must be that the sharpness is only a perceived thing due to the nature in which we process the information??? On a possibly related note, what is the use of "motion blur" icon in the video track in Vegas and how should be it be properly applied in 60i and 24p formats? I clicked on the help info for this in Vegas, but not much info was given.