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The "skinny" on keyframes

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Chris Blair
The "skinny" on keyframes
on May 14, 2009 at 10:37:12 pm

We're outputting a bunch of 30 minutes shows for a client. We've settled on sizes, frame rates, data rates etc. But now their web developer is telling us while the videos load and play great (mp4 via progressive download in Flash Player), but that they don't playback as smooth as they should. We're not sure what they mean when they say "they play great"...then say they "don't play smoothly" and we've askd for a link to the test site to see, but we haven't seen them yet.

They're asking us to encode with more keyframes (shorter time between them). Everything I've read on Adobe and StreamingMedia.com's sites has led me to believe that you want your keyframe rate as far apart as possible. 150 is the number that Jan Ozer and other compression gurus's mention using for mp4 and flash.

Is there a consensus on this?? We typically set it fairly high and have never had anyone complain about playback issues prior to this, or tell us anything as specific as changing a keyframe setting.

Thanks

Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com


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Daniel Low
Re: The "skinny" on keyframes
on May 14, 2009 at 11:02:33 pm

Typically for web video you'd set the KF rate to be 10x the frame rate (so for 30fps, every 300 frames)

For Flash you may need to have this set much lower.

Send them a clip at with a KF set to every 30 frames and see what they think. "Don't play smoothly" is pretty vague.

Too many forced keyframes usually hurts picture quality, but the customer is always right.


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Chris Blair
Re: The "skinny" on keyframes
on May 15, 2009 at 2:18:56 am

That's exactly what we were going to try. Good to know I was thinking correctly. And we said the same thing on the playback comments. We did tests with the videos using flash player from our website and they played great. So not exactly sure what they're seeing. Hopefully they'll send us a link.

Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com


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