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Old March 3rd, 2011, 02:33 PM   #1
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720 50p is half the quality (based on bitrate) than 720 25p?!?

I'm getting a little confused. If I am shooting 720 50p in HQ (35Mbit/s) the overall picture quality should be worse than when I am shooting 720 25p?? Is that correct? Because no matter how much frames I record/second there is always a maximum bitrate of 35 Mbit per second available. So if I put 50 frames within this second every frame can use 1/50 of this bitrate, if I only have only 25fps - the bitrate per frame is doubled and because of that the quality is increased. Is that correct, I've been thinking about this the whole day now ;).

If it's true, it makes no sense to shoot everything in 50p?!
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Old March 3rd, 2011, 03:20 PM   #2
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

"If it's true, it makes no sense to shoot everything in 50p?!"

It depends how important temporal resolution is to you amongst other things. Fast action sports for example.
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Old March 3rd, 2011, 03:24 PM   #3
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

Exactly. I'm doing a lot of action sports, and I was wondering if I could get higher quality pictures if I would only shoot those action shots in 50p and everything else in 25p (interviews, cut-aways and so on)
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Old March 4th, 2011, 01:13 AM   #4
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

If your cards can support 50p @ 35mps then there should be no quality loss. Think about shooting a still at 1/125 sec and another one at 1/250 sec would the second picture be half the quality? As I say, if your camera/card can support 35mbs then use it.
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Old March 4th, 2011, 02:21 AM   #5
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

The best quality of the picture when frozzen, would probably be 25 fps. I have ha a lot of problems when running 25 fps - the footage is simply to stuttering. It would be okay to use for fixed camera positions but not for anything that is really moving like animals running, birds flying or sports. I have choosen to use 720-50P for all my work until 1080-50P is available. If you have any worries about the quality when making fixed syncs then use the ND filter to blur the background to save place for important videoinformation. I believe that 50p is the best for your eye to watch - and it can be converted to interlaced and look great.
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Old March 4th, 2011, 03:03 AM   #6
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

Dear Stefan,

You are right though. I had the same puzzle one day and put a post on tne nanoflash forum.

http://www.dvinfo.net/forum/converge...ame-rates.html

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Old March 4th, 2011, 03:49 AM   #7
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

The difference won't be anything like as big as you suppose - not in a long GOP codec anyway, 50p may be doubling the frame rate cf 25p - but it's not doubling the amount of information to be coded. The second set of 25 frames each second will be very similar to the first set, and a good coder can easily exploit this.

Another way of looking at it is that the I frames will still occur at the same time interval, and they are the ones that take up most of the bit rate. The extra frames will then only need coding as difference frames, and the differences between frames 1/50 second apart will obviously be less than between frames 1/25 second apart.
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Old March 4th, 2011, 04:36 PM   #8
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

Thanks a lot, that made things clearer. As Bo I am also shooting everything in 720 50p because I shoot a lot of sports, and so it's nice to have the possibility to create a nice slow-motion in post. I am using a Sony EX3 and I am quite sattisfied with my results. At the time I am trying get the most out of the camera and I try to understand some fundamental aspects of my production workflow I haven't thought about yet.

David: I-Frames occur at a specific frame-interval (for example every 25 frames) and not at a specific time interval (for example every second), aren't they? So, when I go from 25fps to 50fps, I am not changing the GOP length. Let's say there is one I-Frame each 25 frames. So if I am shooting 25p, there is one frame per second that has a lot of informatione to be coded. If I am shooting 50p there are two such frames a second. But the available bitrate hasn't changed, so theoretically there is more information to be coded. I know that the MPEG codec does more compressing than just creating I-, B- and P-Frames. So, because of that it is like you said.

So, higher framerates in fact mean some loss of quality, but they are not that obvious.

Thanks guys, I have a better point of view now.
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Old March 4th, 2011, 04:40 PM   #9
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

Quote:
Originally Posted by Stefan Koler View Post
If I am shooting 720 50p in HQ (35Mbit/s) the overall picture quality should be worse than when I am shooting 720 25p?
If you were shooting chromakey, scenic General Views or Talking Heads,then perhaps your bandwidth budget would be better spent with 25p. If you were shooting fast paced action, wanted the full-on Standard Definition video look to cut with big broadcast SD cameras, or wanted a liquid and transparent motion look to your 720p footage, then you go with the latter.

Quote:
Originally Posted by Stefan Koler View Post
If it's true, it makes no sense to shoot everything in 50p?!
We have the choice to shoot 720p25/24/30 for general stuff, 720p50/60 for the 'ultravideo' HD on steriods look. It's a choice.

A colleague was infatuated with 720p50 - he loved it: it was SD made 'real'. It was everything he bought his EX1 for. However, his clients did not see the difference. It was the 'same old "same old"' in their eyes.

Shooting 50/60p means we can shoot in the same way we used to shoot DV interlaced. Shooting 25/24p means shooting with a care for zooms, pans, etc. 720p25 does have more temporal quality than 720p50/60. I still shoot at 720p25 for the lack of spatial/temporal compression artefacts.

Your mileage (and footage) may vary. :-)
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Old March 4th, 2011, 05:31 PM   #10
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Re: 720 50p is half the quality (based on bitrate) than 720 25p?!?

Quote:
Originally Posted by Stefan Koler View Post
David: I-Frames occur at a specific frame-interval (for example every 25 frames) and not at a specific time interval (for example every second), aren't they?

So, when I go from 25fps to 50fps, I am not changing the GOP length.
I don't think that's true. I-frames can occur whenever you like, and in software coding it may be a good idea to force an I-frame on the frame of a shot change - even if it's not due according to the nominal GOP length.

Same holds for editing MPEG2 without decompressing the entire stream. You have to force an I-frame at the edit and have a non-standard GOP length around the edit.

In terms of camera video, I'd expect an I-frame to be about every 1/2 second - regardless of frame rate. So with 25p you'll get about 12 difference frames in between, with 50p you'll get 25 difference frames between I-frames. In each case you only get two I-frames (accounting for the lions share of the bit rate) every second.
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