Re: [transcode-users] rc buffer underflow with ffmpeg

[achurch@xxxxxxxxxxx (Andrew Church)]

>Ah, after reading this, I wonder... perhaps the reason I
>get tons of buffer underflows when I do 2-pass encoding
>is because the ntsc_dvd profile uses -w 5000, and whereas
>that does not ever produce an average bitrate anywhere
>near that with single pass, maybe it really does with 2-pass,
>and mucks things up?

     Actually, the reason is that the current ffmpeg 2-pass rate control
code is completely broken. :P  IIRC, it completely ignores the max bitrate
when deciding how many bits to use during the second pass.  There's a patch
by MeanX somewhere in the FFmpeg list archives that does a better job at
keeping under the max bitrate in 2-pass mode, but I've found it easier to
just lower the max bitrate, which seems to affect the first pass enough to
keep scene peaks under 9800 kbps (my ffmpeg.cfg has vrc_maxrate=7500).

  --Andrew Church
    achurch@xxxxxxxxxxx
    http://achurch.org/