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-   -   Why only 50/25 overcrank? (http://www.dvinfo.net/forum/sony-xdcam-pmw-f3-cinealta/495732-why-only-50-25-overcrank.html)

Ola Christoffersson May 10th, 2011 03:19 PM

Why only 50/25 overcrank?
I cannot find a way to over crank more than 50 fps when shooting 25p. On my EX1r I can overcrank 60/25. Any clues? Can't think of a reason for it being different with the F3 from the EX-series...

Matthieu Robert May 10th, 2011 04:16 PM

Re: Why only 50/25 overcrank?
you need to go to the setup, and switch from PAL to NTCS system.
With the EX1/3, you can go from 1 to 60 in PAL mode, I don't know why it's different for the F3.
Maybe it will be corrected in a future firmware.

James Houk May 15th, 2011 10:09 AM

Re: Why only 50/25 overcrank?
Once in the NTSC mode, you can choose to either use the base 720 60p setting, or to use a 720 24p or 30p setting and turn on the S&Q and overcrank.

Either way, presuming your delivery format is 25p, you'll need to conform the results in post. Fortunately, conforming can be done with Cinema Tools without any encoding or transcoding.

The significance of which of the above frame rates to use comes down to audio and bit rates. Using the 720 60p setting means audio will be recorded, but the 35Mbps VBR is not overcranked.

This is to say, the camera uses 35Mbps VBR by default, regardless of frame rate. So, when not using S&Q, 1 second of recorded material is:

720 60p: 4.375MB / 60 = 73KB per frame
720 24p: 4.375MB / 24 = 182KB per frame

The math is likely inexact, I'm dreadful at this. But the key point is, overcranking from NTSC 720 24p to 60p retains the highest possible data rate per frame.

James Houk May 15th, 2011 10:21 AM

Re: Why only 50/25 overcrank?
Of course, the first flaw in the math I propose is the the XDCAM EX codec is Long GOP. So single frames don't really exist, that data is shared across the group of pictures. But the ultimate principle holds true: by overcranking 24p to 60p via Slow and Quick, the camera will use lighter compression, and produce more data to work with in post than if you chose to use the 720 60p setting.

Alister Chapman May 16th, 2011 11:00 AM

Re: Why only 50/25 overcrank?
It's a good point James, but just to add that we should not forget that 1280x720 contains 1/4 of the data of 1920x1080 depending on the picture detail etc, but anyway 1280x720 requires significantly less data to achieve the same compression ratio compared to 1080P.

So, when you shoot 720P25 or 720P30 the video is considerably less compressed than at 1080P.

Now when you increase the frame rate with a long GoP codec, lets say we double it to 50fps then the amount of data needed to record the scene does not double, in fact it's a long way from it, because the amount of motion remains the same, it's just spread over more frames. This means that when you overcrank the compression ratio and artefacts are very significantly reduced compared to shooting 1080P.

Another side note is that perceived image sharpness is a combination of both resolution and contrast. This is measured using the MTF or modulation Transfer. When you drop the resolution of the F3 down from 1080P to 720P as the contrast in the image does not change (it's coming from the same sensor/optical system) the MTF does not drop as much as you would perhaps assume, so the perceived sharpness of the image does not decrease as much as one might expect. Yes, of course there is a noticeable drop in sharpness, but it's not as bad as many people expect and the lower compression means a very clean image, so overall the overcrank footage looks remarkably good.

James Houk May 16th, 2011 06:54 PM

Re: Why only 50/25 overcrank?
Thanks Alister. You're much more precise than I am about these things.

And it's an excellent point about the quality of the 720p image on the EX & F3 cameras. I've always been impressed with it, but I'm definitely looking forward to playing with uncompressed 1080 60p output from the F3.

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