[x264-devel] Re: Multipass encoding with CBR

[Josef Zlomek]

> > 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.

If I understand the meaning of vbv_buffer_size correctly, the vbv_buffer_size
is just for 1 second in 1500 kbit/s. I have of course tried to set it to
smallest possible value (10 * bitrate / FPS) but that did not help.

Josef

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