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h.264 bitrates

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mike_a
h.264 bitrates
on Jun 27, 2007 at 7:12:33 am

Hello Compression Gurus,

Creative Cow is the best. But right now I'm having trouble encoding H.264 Quicktimes in Apple's Compressor. I start with the "H.264 LAN Intranet" preset and duplicate this to change to my own settings. I change the video settings to "Restrict Data rate to" to various rates (such as 1000kbps or 2000kbps), but the resulting files keep ending up at around 800Kbps. Even if I don't change anything on the preset, which has "restrict data rate to" 1392 Kbps. The files still come out at around 800Kbps. Not only does QT show this under "Show movie info", but I confirm this by dividing the file size by the duration. This issue is important because we have clients that need files cooked to various bitrates and the files need to be at those specific bitrates, not simply close by.

Other compressor settings I am using with this batch:
* Keyframes every 24 frames (changing this didn't solve the issue)
* Frame re-ordering on
* Dual-pass

Source file is a None-compressed QT 720x480 1.2 PAR

Here are the specs I can remember (I'm at home right now.)
PowerPC PowerMac G5 Dual 2.7
OS X 10.4.9
FCS 1 (FCP 5.1.4) with updated software
Quicktime 7.1.5 (maybe 7.1.6)

Thanks in advance for any insight you all may shed.

Mike A


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Danny2007
Re: h.264 bitrates
on Jun 27, 2007 at 8:59:27 am

Can you supply us with the rest of the encoding profile - eg frame size, what kind of content is it (talking heads or sports etc)

keyframes every 24 frames is way too frequent - should be more like every 240 frames..

It's a London thing.


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mike_a
Re: h.264 bitrates
on Jun 28, 2007 at 12:38:25 am

Danny2007 -

Thanks for your attention with this. I think I've figured it out.

I'm guessing that h.264 encoding from Compressor (and QTPro) is vbr (without saying anywhere that is it vbr). And the "restrict to" is simply the ceiling. I encoded different portions of the clips and some have 2Mbps bitrates---usually where the content would merit all 2000kbps. I guess I was test encoding a portion where Compressor (without telling me) had decided "i know you said don't use MORE than 2Mbps, but it this portion I don't even need that much so I'll only use what I think I need."

If you see I'm way off, let me know. If not, then Happy Cooking!

Mike A


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