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Confused about 'Keyframes' -- less=lower quality?

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Kevin Camin
Confused about 'Keyframes' -- less=lower quality?
on Mar 12, 2011 at 3:23:57 pm

I'm doing some tests with Adobe Media Converter CS5.

I'm compressing an uncompressed Quicktime file into a Quicktime h.264.

I understand what 'keyframe' means in the context of compression--how often a full frame occurs between delta frames. As a test I set the keyframes to 'every 1 frame', which in my mind means every frame will be whole and would create the best quality and largest file size. But it looked terrible. I increased it to 'every 2 frames' just to make sure there isn't some weird bug with choosing '1', but it still looked terrible. I then bumped it up to 'every 100 frames' and it looked good. '200 frames' looked better.

It seems counter-intuitive that having less keyframes would make a better compression. Can someone explain?

Also, when keyframes is set to 'automatic' what is occuring?

Thanks so much!

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Jeff Greenberg
Re: Confused about 'Keyframes' -- less=lower quality?
on Mar 12, 2011 at 3:45:32 pm

Full frames require more data.
More I frames/keyframes means MORE DATA IS BEING SPENT on those frames (meaning LESS on the delta frames)

A data rate is a budget of how much data gets spread across a second.

LESS keyframes mean more data can go into the delta frames = higher quality.

Automatic? Software is using a given data rate to build a 'best' or quality rate...who knows what the algorhythim is.


Jeff G

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Rich Rubasch
Re: Confused about 'Keyframes' -- less=lower quality?
on Mar 13, 2011 at 1:40:03 am

And it also matters if you set a bitrate rather than choose a quality slider. My guess is if you specified more keyframes with a quality slider at high it would make a larger file than less keyframes at the same setting.

But when you set bitrate and keep that the same for two encodes, the one with less keyframes (or let's say optimum number for the footage) would look better because the in-between frames would get more bits.

Rich Rubasch
Tilt Media Inc.
Video Production, Post, Studio Sound Stage

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