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H.264 bitrate inconsistent

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Raven Plenty
H.264 bitrate inconsistent
on Jan 13, 2009 at 6:10:10 pm

I am exporting H.264 QuickTime movies from within FCP. I have chosen to limit bitrate to 400 kbps, but I'm not getting that as a result. I have two test videos. One (about 60 sec clip) ends up as 3.5MB and QT shows it as being 489 kbps. The other (about 2:25 clip) ends up as 24MB and QT shows it as 1373 kbps.

Why am I getting a 1373 kbps movie when I asked for a 400 kbps movie?


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Daniel Low
Re: H.264 bitrate inconsistent
on Jan 14, 2009 at 12:12:55 am

A few things

You can't expect tight rate control out of FCP/Quicktime Export, it's simply not designed for that, you need an application like compressor or Episode to do that for you.

Even compressor or Episode cannot hit a datarate spot on and keep it there, no encoder can.

None of the above can perform the impossible; if you set a datarate of 400Kb/s for a 1920x1080 25fps clip there is no way it could get anywhere near as low as that.

Make sure you choose the right kind of rate control: So for example for 400Kb/s....

1 pass CBR is going to hit a rough average of 400Kb/s over say 30 secs of material, if it can. (see above)

2 pass CBR is going to be a more accurate version of the above.

VBR is going to take 400Kb/s as an average and will go to whatever peak value you have set and may drop down well below if it needs to.







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Raven Plenty
Re: H.264 bitrate inconsistent
on Jan 14, 2009 at 4:45:17 pm

I'm scaling them down to 480x360. I wasn't as surprised by the one video ending up as 489 kbps as I was by the one that went to 1373 kbps. That's more than 3 times the size I specified. Is that kind of variation really "normal" (when doing QuickTime exports)?

On the other hand, I have done a Compressor export and it was around 470 kbps, so you're right about it being more accurate in that regard. Thanks!



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Daniel Low
Re: H.264 bitrate inconsistent
on Jan 14, 2009 at 8:09:09 pm

"That's more than 3 times the size I specified. Is that kind of variation really "normal" (when doing QuickTime exports)? "

Like I said, it really depends on what kind of 'rate control' you used, but the figures you quote are not out of the ordinary.

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Please post back saying what solved your problem. It could help others, and saying 'thanks' is free!


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