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Old March 6th, 2007, 10:44 AM   #1
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GOP data rate & frame rate HD201etc?

Simple question but tough to locate the answer. Is there a quality gain (such as a more frequent GOP or key frame) with a lower frame rate? If the recording datarate is 19mb/sec regardless of the frame rate it seems there must be a gain in frame quality (say recording motion) at the lower frame rate. Perhaps the lower frame rate enables you to get more light into the camera thus improving the quality of the film?

I should just go & turn the thing on & see for myself :-).
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Old March 6th, 2007, 02:54 PM   #2
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The illumination is based on an electronic shutter, so it isn't really ganged to frame rate like with a film camera.
For example, you would ideally use a 1/50th shutter speed (180) for 25P, and 1/100 (180) for 50P, meaning 1 stop less exposure. However, there is nothing stopping you from using a 1/50th shutter speed at 50P to maintain the exact same exposure level.

As for your question of quality. There is some logic to your conclusion that higher frame rates at the same bitrate will yield lower 'quality' images - at least with regard to compression. However, it is a little more complicated than just frame rate.
I have observed macro blocks on still frame grabs of 720p60 m2t. However, the much higher temporal frame rate essentially means that
a) you won't notice during playback
b) there are less "differences" between each frame, so the 12-frame GOP in 60P mode can conceivably be as efficient as 6-frame GOP in 30P mode.

As soon as I have time I will put this theory to the test.
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Old March 7th, 2007, 02:20 AM   #3
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Quote:
Originally Posted by Tim Dashwood View Post
The illumination is based on an electronic shutter, so it isn't really ganged to frame rate like with a film camera.
For example, you would ideally use a 1/50th shutter speed (180) for 25P, and 1/100 (180) for 50P, meaning 1 stop less exposure. However, there is nothing stopping you from using a 1/50th shutter speed at 50P to maintain the exact same exposure level.

As for your question of quality. There is some logic to your conclusion that higher frame rates at the same bitrate will yield lower 'quality' images - at least with regard to compression. However, it is a little more complicated than just frame rate.
I have observed macro blocks on still frame grabs of 720p60 m2t. However, the much higher temporal frame rate essentially means that
a) you won't notice during playback
b) there are less "differences" between each frame, so the 12-frame GOP in 60P mode can conceivably be as efficient as 6-frame GOP in 30P mode.

As soon as I have time I will put this theory to the test.
So your saying the camera obviously must use a shorter GOP (gap between key frames) at 24fps then at 60fps? & as your watching 60fps you see the key frames at more or less the same frequency?

But if you want to pause the film & use a frame for an image its going to look better or compress better when you use a lower frame rate?

regards Phil
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