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

[Erik Slagter <erik@xxxxxxxxxxxxx>]

On Tue, 2005-03-01 at 20:34 -0800, Phil Ehrens wrote:

> > >vrc_maxrate=9800
> > 
> >      Obvious solution: lower vrc_maxrate.  Even the default only puts it
> > at 9000 kbps, and raising it farther really won't gain you anything
> > noticeable.  If you continue having problems, lower it even further.  See
> > also http://www.transcoding.org/cgi-bin/transcode?FFmpeg_Vs._Mpeg2enc

I've never seen a reason to use a value different than the default, so 
just leave it alone.

> > ># vrc_buf_size=1792 # suggestion online
> > >vrc_buf_size=1835 # DVD setting in man page
> > 
> >      1792 is correct, leave it alone.  (The manpage you read probably used
> > kbit=10^3 bits; this setting uses kbit=2^10 bits.)

Although most dvd players actually have a larger vbv buffer, you cannot
rely
on it, so leave it alone as well.

> 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? Erik, how did you decide on -w 5000 as
> the default anyway?

The default of 5000 is pure random. The user should really
set this one itself.

Anyway, whatever avg bitrate you specify, it should _never_
lead to buffer under- or overruns. If you specify a very high
avg bitrate, you do risk having lots of ugly frames though
(where the bandwidth is simply used up).