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Sony XDCAM PMW-F3 CineAlta
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Old May 19th, 2017, 08:34 PM   #16
Vortex Media
 
Join Date: Jan 2009
Location: New England, USA
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Re: Shooting format for Cable TV using F3

I don't shoot anything that isn't RAW or XAVC-I so the data sizes are definitely double for me.

But isn't he reason you are getting similar file sizes for 30P and 60P long-GOP is because they are both being captured with a 50Mbps data rate? It's the same amount of data being captured every second regardless of how many frames it is being divided into. So obviously the trade-off is that 60P is having twice as much compression being applied to every frame as 30P. So even if 30P and 60P might result in files of similar sizes, I'd still rather shoot 30P with half the amount of compression being applied.
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Old May 19th, 2017, 10:14 PM   #17
Inner Circle
 
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Re: Shooting format for Cable TV using F3

Yes you are correct with the average data rate but with Long GOP only the differences between i frames are recorded so there will be less movement difference between the 60P frames than 30P frames so more data is needed to be recorded for the differences for the 30P than the 60P per frame GOP making up for the fact there are twice as many i frames for 60P. Essentially using the data rate for i frames or difference data. The encoders are VBR too so peak data rate for 60P may also be higher. For a nominal 50Mbps XAVC-S the peaks are normally around 60Mbps for 60P in my files. So in practice there is not a lot of difference for lower resolution HD images. When the resolution goes up to UHD there is though. 30P at 100Mbps is about the same quality as 60P at 150Mbps. So potentially 50% more data for 60P at these higher resolutions a saving over RAW or I frame only encoders.. We are dealing with 4 times the image data moving from HD to UHD with double the data rate for 30P and 3 times for 60P. I believe to get similar quality from an I frame only encoder one needs over 400Mbps for 30P and even more for 60P of course.
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