[FFmpeg-user] 2 pass CBR or VBR not really fixing the bitrate?

Manuel Tiglio tiglio at icloud.com
Mon Jul 31 05:00:04 EEST 2017


> On Jul 29, 2017, at 9:00 PM, Eric Wilde <ewilde at gntrains.com> wrote:
> At 12:37 AM 7/30/2017 +0200, Moritz Barsnick wrote:
>> Are you talking audio? Yes, libmp3lame hits the target bitrate (be it
>> ABR or CBR) pretty much spot on. But Manuel obviously asked about
>> video:
> Yes, you're right.  I completely missed that, since CBR makes no sense
> to me, for video.  As you said, the goal of two pass encoding is to hit
> an average bitrate -- which ffmpeg does quite well.
> I'm not sure why you'd try to force the codec to have no variations
> from the average bit rate because that's not how it works.  Lower bit
> counts on frames that don't need a lot of bits (e.g. all black) make
> up for higher bit counts on frames that do need a lot of bits (e.g.
> motion).  The first pass basically encodes the frames with no limits
> and writes their bit counts to the pass file.  The second pass can
> then alter how much more it compresses high bit count frames (than the
> first pass did) so that it hits the target bit rate.  The average is
> all that matters.
> But, I'm stating the obvious.  And, I'm probably still missing the
> point.  So, I'll clam up and let wiser souls jump in.

If you want to stream and build ladders you want to control the peak bitrate, not just the average. That’s the point that I was trying to convey. 

>                                   Eric
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