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CBR vs VBR in master archive exports

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Juan Juig
CBR vs VBR in master archive exports
on May 12, 2020 at 3:49:13 pm

Hello,

on the film acady that I went to they always told us to export the final master archive files in DNxHD or DNxHR codecs. I always did so, but lately started wondering about different characteristics of the exports, especially bitrate.

To my understanding all DNxHD and DNxHR codecs are strict CBR, while the ProRes files are VBR.

Since I work on Windows machines most of the times, DNxHD and DNxHR are definitely better options for me, but I read some articles about CBR vs. VBR and many claim that VBR is always better, since it distributes bitrate among frames unevenly. Now, am I right to assume, that this mostly applies to smaller files, like h264 files with low bitrate? I tend to export those VBR 2-pass.

However, when it comes to master archive files I assume CBR is a completely viable option, since DNxHD and DNxHR only offer this sort of encoding? So far I have always exported my files in either DNxHD 185x or DNxHR 444.

My ultimate question is: could CBR encoding theoretically result in unacceptable quality loss and visual "pumping" (visible changes in quality over time) of Picture in scenes, where a lot of information is needed to encode (action scenes with a lot of details for example) or is that in practice unnoticable and neglectable with files that have high birate?

Let's take DNxHD 185x as an example. Is 185Mbps a bitrate high enough that what I described above is something not to worry about? With VBR it could for example encode something more demanding at 300Mbps and something less demanding at 100Mbps. Is VBR more acceptable with master archive files or is CBR even better, since there is more constantcy?

I'm assuming, since DNxHD and DNxHR which are widely used for archiving, my question might be nitpicking, but I would really appreciate a definite and reassuring answer.

Thank you


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Tero Ahlfors
Re: CBR vs VBR in master archive exports
on May 12, 2020 at 4:10:49 pm

VBR will not raise the maximum bit rate. It will lower it when it sees information it can compress.


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Juan Juig
Re: CBR vs VBR in master archive exports
on May 13, 2020 at 11:33:54 am

Hi, thnak you for your reply.

"Although ProRes is a VBR codec, the variability is usually small. The
actual data rate is usually close to the target data rate. For a given
ProRes format, there is also a maximum number of bits per frame that is
never exceeded. This maximum is approximately 10 percent more than
the target number of bits per frame."

This is what I found in connection with ProRes codecs in Apple's ProRes codec guide. Even though the target bitrate stays the same, the maximum bitrate does seem to increase where that is necessary.

I would really appreciate answers to my questions that I posted originally.


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Marc Wielage
Re: CBR vs VBR in master archive exports
on May 14, 2020 at 1:57:36 am

[Juan Juig] "Since I work on Windows machines most of the times, DNxHD and DNxHR are definitely better options for me, but I read some articles about CBR vs. VBR and many claim that VBR is always better, since it distributes bitrate among frames unevenly. Now, am I right to assume, that this mostly applies to smaller files, like h264 files with low bitrate? I tend to export those VBR 2-pass."
The advantage as I see it in choosing Constant Bit Rate for ProRes (which is an option, at least on Mac OS), is that you can then use a utility like CineXTools to repair just part of the file rather than re-render the entire thing:

https://cinedeck.com/cinedeck.com/cinextools-2/

I don't think QC companies will reject files for being CBR or VBR -- they'll go either way.


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Juan Juig
Re: CBR vs VBR in master archive exports
on May 14, 2020 at 8:20:58 pm

Hey Marc, thank you for your reply!

So far I had no problems with files that I have exported in terms of quality check when sending them to TV companies for example.

Would you say that the bitrate of DNxHD 185x is high enough for an archive master file? I'm thinking of also creating DNxHR 444 from now on.

Cheers


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Marc Wielage
Re: CBR vs VBR in master archive exports
on May 14, 2020 at 9:59:17 pm

[Juan Juig] "Would you say that the bitrate of DNxHD 185x is high enough for an archive master file? I'm thinking of also creating DNxHR 444 from now on."
No, I think DNxHR 444 is a much better choice.


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