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Apple & x264 cap data rates

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Chris Blair
Apple & x264 cap data rates
on Jul 12, 2010 at 8:57:25 pm

While outputting a commercial for a TV station that requested a specific data rate, I came across something I don't understand and can't find much of anything about online.

If I use Quicktime Pro or Handbrake to encode a 720x486 uncompressed AVI file to H264 (same pixel size), both cap the data rate at 32768kb/sec (no matter what data rate that I input). If I use Carbon Coder and export to H264 using it's native codec (MainConcept), it allows me to plug-in literally any data rate I want and reflects it in the resulting file.

A TV station asked for an H264 file at 35000kb/sec. Most ask for around 25000kbs so that's why we've never discovered this issue.

If I plug in 35000 in the data rate field of Quicktime Pro (Windows), it shows 35000 in the settings summary window, but as soon as you click back into the main settings tab, the number drops to 32768. You can change to anything below that and it shows and encodes correctly. But no matter what number above 32768 you input, the resulting file always ends up at the 32768kb/sec data rate. Roughly the same thing happens in Handbrake, although I believe its settings still show whatever you input into the data rate field.

I know that it's capping the data rate because no matter what number you plug in above 32768, the file size stays the same on each encode. Conversely, using Carbon Coder, the resulting file sizes correctly reflect the various data rates that are input above 32768.

Anybody know why Apple's H264 and x264 both seem to do this??

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


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Craig Seeman
Re: Apple & x264 cap data rates
on Jul 12, 2010 at 9:32:36 pm

No problem on this end.
Opened a 1080p EX XDCAM MOV file in Quicktime Pro 7.6.6 on OS 10.6.4.
Set it to 50000kbps, H.264 1920x1080. Resultant encode shows 53.26mbps so it overshot by a little.



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Chris Blair
Re: Apple & x264 cap data rates
on Jul 12, 2010 at 10:43:15 pm

Interesting...

Maybe it's a Windows Quicktime Pro thing. We've seen it on multiple PCs with latest version of Quicktime Pro. But doesn't explain the issue on Handbrake (Windows version) since it's using x264 and not the Apple codec (again...that's what it specifies in the codec options in Windows version of Handbrake).

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


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Phillip Grey
Re: Apple & x264 cap data rates
on Oct 25, 2011 at 4:09:53 pm

Having the exact same issue with Quicktime Pro 7.7 for Windows, running Windows 7. At work, we use Quicktime Pro (not sure which version #) for Mac, and we can set the data rate to whatever we fancy (we go with 70,000).

Any help appreciated.


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