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

Manuel Tiglio tiglio at icloud.com
Mon Jul 31 20:57:50 EEST 2017

> On Jul 31, 2017, at 1:39 AM, Nicolas George <george at nsup.org> wrote:
> Le duodi 12 thermidor, an CCXXV, Manuel Tiglio a écrit :
>> If you want to stream and build ladders you want to control the peak
>> bitrate, not just the average. That’s the point that I was trying to
>> convey. 
> Not exactly. Clients have a limited buffering ability, and what you need
> to control is the average bitrate over the duration of the buffer.

Apple HLS Authoring does not seem to agree with you 

https://developer.apple.com/library/content/documentation/General/Reference/HLSAuthoringSpec/Requirements.html#//apple_ref/doc/uid/TP40016596-CH2-SW1 <https://developer.apple.com/library/content/documentation/General/Reference/HLSAuthoringSpec/Requirements.html#//apple_ref/doc/uid/TP40016596-CH2-SW1>

1.23. For VOD content the peak bit rate SHOULD be no more than 200% of the average bit rate.

(actually before early this year it used to be 110%). 

> And two-pass encoding is not meant for that. Two-pass encoding is meant
> to optimize the bitrate globally, for when you want to fit a movie on
> exactly 2×700 mega-octets so you can burn it on a pair of CD-R.

Two pass encoding is used for CBR and constrained VBR. If you use it for something else, that is great but it is not my question. 

> Regards,
> -- 
>  Nicolas George
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