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Different file sizes, not variable bit rates!

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Rick Sebeck
Different file sizes, not variable bit rates!
on May 6, 2009 at 12:03:43 pm

I am taking one large movie file and splitting it into six 1920x480 files that will be displayed back together using 6 mac minis and Renewed Visions Pro Video player. I rendered out my large movie then split it into six parts in after effects. They are all the same duration, frame size, frame rate, ect. For some reason, no matter what codec I use for compression, all six files are coming out at different file sizes. The only codec that didn't result in different sizes was 8bit blackmagic codec. So I then took the six movie files (again al the same durration, frame size, and now at this point - file size) into compressor. And still -- all my h.264 files, photo-jpeg files, ect are coming out at different bitrates. I even have the limit bit rate checked and set the same on each.

Any ideas as to why I am getting variable bit rates?


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Daniel Low
Re: Different file sizes, not variable bit rates!
on May 6, 2009 at 1:56:56 pm

[Rick Sebeck] "Any ideas as to why I am getting variable bit rates?"

I'm assuming that it's because the content in each of these clips is different. Just because you've set a datarate, that rate is only an average, the codec can choose to use more or less per second depending on what it need to describe the picture; less information in the picture needs less data, more information (and movement) requires more data..

The only way to force a fixed rate is to use an i-frame only codec like DV, or one of the uncompressed Blackmagic type codecs.

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Terry Mikkelsen
Re: Different file sizes, not variable bit rates!
on May 6, 2009 at 3:53:37 pm

Why do they need to be the same file size?

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