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

Nicolas George george at nsup.org
Mon Jul 31 21:55:36 EEST 2017

Le tridi 13 thermidor, an CCXXV, Manuel Tiglio a écrit :
> Are you saying that Apple’s authoring requirements for HLS are meaningless?

Unless there are extra definitions that you neglected to quote, yes.
Unless you intend to burn me at the stake for bad-mouthing Apple, of
course, in that case I deny everything.

> 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.

> 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.


  Nicolas George
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