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Old November 16th, 2010, 05:35 PM   #1
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Getting my head around format settings

Hi,

It seems that the most popular setting is 50/60fps 720p.

I just don't understand why 50/60fps is the same bitrate (same amount of footage on an SD card) as 25/30fps.

Shouldn't you get half as much 50/60fps stuff per GB or is 25/30fps twice the quality per frame.

Thanks

Joe
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Old November 18th, 2010, 09:31 PM   #2
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Format sizes

First, there are Progressive and interlaced formats.

720p is about 921k pixles per frame.

1080p (30p and 24p) = 2,073,000 pixels per frame
So 1080p is a little over twice the number of pixles per frame. (2.25)

Now 1080 60i is still 30 FRAMES per second, but each frame is made up of 2 FIELDS
This is the key. 60i is actually still 30 frames per second.

So 720 60p has about the same data rate (a little less to be accurate) than 1080 formats.


Now for the MPG2 compression. JVC uses 35 mbits to compress the data. This is a constant rate.
The data stream from 1080 60i is the same as 1080 30p. 1080 24p is of course a little less.
So 1080 24p is slightly less compressed that 1080 30p or 60i

720 60p has about the same data rate (a little less to be accurate) than 1080 formats.

So when you capture these different formats, there is slightly more compression of the video in 1080 60i 30p than 1080 24p and slightly less compression on 720p formats
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Old November 18th, 2010, 11:48 PM   #3
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Hi Andy,

Thanks for taking the time to respond.

That makes perfect sense in regards to 1080 vs 720.

But why is my 720p50 footage the same size as 720p25 footage. Is 720p50 compressing each frame twice as much as 720p25 to get double the number of frames?

I captured some tests. 720p25 datarate is showing in Quicktime as 35.50 mbits/s. 720p50 is 37.76 mbits. Is there some magic way of getting twice as many frames with only tiny increase in datarate.

Thanks Again.

Joe
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Old November 19th, 2010, 12:57 AM   #4
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The camera is compressing the 50p footage to fit in approx 35mbit and also compresses the 25p into approx 35mbit. It uses more compression on the 50p footage than the 25p footage.

So in theory the 25p footage should be a littler "cleaner" regarding compression artifacts.
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Old November 20th, 2010, 05:49 PM   #5
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and less chances of macroblocking with 24p/25p! If I had the HM100 I would be shooting 720p 24/25fps (depending upon which country I was in) at the 35mbs for the cleanest video to start with. Assuming my editing software was up to date to handle the XDCAM-EX format.
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Old November 23rd, 2010, 05:12 PM   #6
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Just Googled macroblocking, ugly!

Any opinion on the benefits of 50fps over 25fps producing smoother motion on fast pans and movement?

Thanks for all the help.
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Old November 25th, 2010, 02:42 PM   #7
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What is your delivery intention?
What is your subject matter?

50i and 60i is more forgiving for camera panning and movement than 25p/24p. But with a little skill it's a non issue.

I shoot 24p (in USA) for everything, even sports. You can't pay me to shoot 60i. So I am just a little opinionated. :)
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