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

Manuel Tiglio tiglio at icloud.com
Mon Jul 31 21:41:09 EEST 2017

> On Jul 31, 2017, at 11:29 AM, Nicolas George <george at nsup.org> wrote:
> Le tridi 13 thermidor, an CCXXV, Manuel Tiglio a écrit :
>> 1.23. For VOD content the peak bit rate SHOULD be no more than 200% of the average bit rate.
> Without a proper definition of "peak bit rate", this sentence is
> meaningless.

Are you saying that Apple’s authoring requirements for HLS are meaningless?

> A bit rate is a quantity of information divided by a time.
> Since it is discrete, there is no way of doing calculus, and therefore
> there is no notion of instantaneous bit rate. Thus, without saying the
> period of time on which the bit rate is computed, it does not mean
> anything.

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. 

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).  

This is really really standard for anyone working on streaming. What is not standard is that ffmpeg has such large fluctuations between the peak and average bitrate

>> Two pass encoding is used for CBR and constrained VBR.
> No, not for CBR. For CBR, you use all bits allowed, 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. 


> Regards,
> -- 
>  Nicolas George
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