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andreac70344803
andreac70344803
Participant
May 27, 2019
Question

CBR is not constant in PREMIERE

  • May 27, 2019
  • 5 replies
  • 2136 views

Hi all.

My timeline is 24 fps and most of my clips are at this framerate. Some clips are instead at 60 fps.

Most of the source files have around 100megabit of bitrate.

When i export my project in h.264 (decoding software only) and set cbr of 100 megabit, the final output is around 80 mbps and i discovered with pot player that the bitrate is variable instead...

Is that because of different fps of the sources clip?

Seems that Premiere act like vbr instead.

I would like to have 100 megabit contanst no matter of the title transition o whatever else.

Any help is appreciated!

Thanks.

Andrea

This topic has been closed for replies.

5 replies

joel_1234
joel_1234
Known Participant
May 29, 2019

Have the exact same problem here. CBR is not constant when making a H264 MP4. When you play back the video in quicktime you can see the bitrate is variable. Surely it should be locked to what you set as the target rate?

E
excited_Genie16B8
Legend
May 29, 2019

It should be.  But do you actually need CBR?  As previously stated, it's the least desirable option for encoding.

R Neil Haugen
R Neil Haugen
Legend
May 29, 2019

Other than a client who has screwy ideas ... but as they're paying the bills, ya gotta do what they say ... I cannot think of a single reason to use CBR.

Neil

Everyone's mileage always varies ...
Stan Jones
Community Expert
Stan JonesCommunity Expert
Community Expert
May 27, 2019

Are you trying to create a lossless intermediate for further editing? Or a file for playback?

Ann Bens
Community Expert
Ann BensCommunity Expert
Community Expert
May 27, 2019

Set it to VBR 2 pass both to 100.

E
excited_Genie16B8
Legend
May 27, 2019

Frame rate won't have any impact on bitrate.

I have seen issues with Adobe's CBR setting going back a ways.  Of all the possible encoding methods, CBR is the normally the least desirable.  If you don't actually need it, I would recommend not using it.

R Neil Haugen
R Neil Haugen
Legend
May 27, 2019

CBR uses up a ton of wasted data on frames that don't NEED the extra data. I'm not sure, unless you have a deliverables spec sheet that absolutely requires CBR, why anyone would use it. It's not a quality thing at all.

Neil

Everyone's mileage always varies ...
    R
    rodneyb56060189
    Legend
    May 27, 2019

    I don't think the high compression codecs can be entirely accurate with the bitrate being exactly what it's set for .. depending on the 'motion' of the images... Like, if you shot a static scene with a static camera then you'll probably get real close to the so-called advertised bit rate available for the FPS you are shooting.  But as the image changes ( action, especially fast action ) the nature of the compression routine kinda slows up a bit cause it's trying to throw away pixels that don't change and keep pixels that are changing.

    I once had a book that showed what happens with stuff like mp4 (avc ? ) with block diagrams and it sorta illustrated how it actually tries to 'guess' which frames will change a lot even before it sees them. It's complicated.

    I don't think this particular article is written too great, but it's a good starting point. Notice the words 'lossy' vs. 'lossless' codec.

    Intra-frame coding - Wikipedia

    Your source footage ( wrapper with codec , like, maybe MOV DNxHD ) will be one type of animal that is getting changed (via export to H264 ) to another animal, and predicting that exact bitrate is probably impossible.  But 'constant' just means it basically stays the same as much as it can. Variable gives you a target with min and max I think … (kinda forget stuff ).

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