[x264-devel] Re: Is constant bitrate supported by x264?

[Måns Rullgård]

Piotr Narewski said:
> Hello,
>
> I try to prepare some h264 video files for streaming.
> The problem is that (because of network limit) I cannot exceed the bandwith
> limit.
> Unfortunatelly, I am unable to generate a real CBR stream.
>
> For example, when I use encoding parameters like in attached BAT file (CBR
> 1.5Mbps), the output bitrate looks like in file bitrate.log (each line
> represents one second - you have also bitrate in Mbps).
> Average bitrate of the output file (when I calculate file size and video
> length) is what I wanted, but there are fragments where the bitrate is much
> too big to stream it with reguested bitrate.
> And thats the problem.
>
> I was playing with encoder's parameters a little bit and found out that when
> I add such an option:
> --vbv-bufsize 1
> ... then decoder keeps the limit of video bitrate much, much better.
> But I dont think this is the right way to produce CBR stream - VBV is too
> low and different decoders behave strange.
>
> Could you help me please?
> Is there any way to generate CBR stream, for streaming?

The generated streams should be suitable for constant bitrate streaming.  It
doesn't matter if the instantaneous bitrate is higher or lower than average,
as long as there is no VBV over/underflow.  A larger VBV buffer will allow
a greater variation in bitrate, as you observed, which can be good for image
quality.  The various profiles and levels define limits for the buffer size,
so encoders know what the decoders will be able to accept.

-- 
Måns Rullgård
mru at inprovide.com

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