[FFmpeg-user] 2 pass CBR or VBR not really fixing the bitrate?

Manuel Tiglio tiglio at icloud.com
Mon Jul 31 22:07:15 EEST 2017

>> When doing streaming you typically send packets of 6-10 secs, so in
>> that interval the peak (i.e. maximum value) bitrate does not have to
>> exceed 10% (110% constrained VBR) or 100% (200% constrained VBR) of
>> the average bitrate in that interval. 
>> But the exact value of that length of time is irrelevant. 
> Quite the opposite, it is very relevant. Unless you are using
> intra-refresh, I-frames will be significantly bigger than P- and
> B-frames. Therefore, whether the window used for averaging is larger or
> narrower than the interval between I-frames is very important.

Correct. I tried first fixing the distance between keyframes to 2 seconds and then 2-pass VBV and the other way around and also not fixing the distance between keyframes. 

I think I see your point now. To answer your question, the average is done through the entire video, so in a much longer timescale than the distance between keyframes. By looking at the data (I can send you some plots) I’d say that decreasing the averaging time would not change much, but I can try. Any recommendations on that? 

>> The fact that it is a discrete series is also irrelevant, you can
>> compute discrete bitrates in the same way that you compute finite
>> differences (for example).  
> Exactly: only an average over a period.
>> Can you type a typical example of what you mean for that? CBR is a
>> 100% constrained VBR (i.e. ideally no fluctuations in the bitrate from
>> its average), essentially. 
> Except it does not mean anything practical.

What do you mean? 


> Regards,
> -- 
>  Nicolas George
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