[x264-devel] Re: Multipass encoding with CBR

[Måns Rullgård]

Josef Zlomek said:
> Hello,
>
> I want to create a h264 stream for streaming by multipass encoding.
> Because the video will be streamed, the bitrate can't be more than
> few percent greater than requested value.
>
> I am using 3-pass encoding (1st pass creates log, 2nd pass reads and write it
> and 3rd pass reads it).
> Although I have set following parameters controlling bitrate, the output
> bitrate is much higher in some small parts of the stream than the requested
> bitrate (wanted 1500, reached 2650 on one 1 second interval).
>
> The params (FPS is 25):
>   params->rc.b_cbr = TRUE;
>   params->rc.i_bitrate = 1500;
>   params->rc.i_vbv_max_bitrate = 1500;
>   params->rc.i_vbv_buffer_size = 1500;
>   params->rc.f_rate_tolerance = 0.05;
>
>
> Is it possible to make x264 create a h264 stream that uses multipass
> encoding and that does not overflow the wanted bitrate (by more than
> few percent) ?

The instantaneous bitrate may very well be much larger than the average for
a short period, provided the vbv buffer doesn't underflow.  The numbers you
mention are well within the limits.  If you need less variation, try
reducing the vbv_buffer_size.

-- 
Måns Rullgård
mru at inprovide.com

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