2-pass encoding with avconv

Question

I don't understand why the video quality with multi-pass encoding is worst than one pass?

When I do this, it is correct:

avconv -i video.mpg -s 640x360 -threads auto \
       -vcodec libxvid -b 800k -r 24 -g 300 -bf 2 \
       -acodec libmp3lame -ac 2 -ab 128k -ar 48000 \
       video.avi

This is worst:

avconv -i video.mpg -s 640x360 -pass 1 -threads auto \
       -vcodec libxvid -b 800k -r 24 -f avi -an -y /dev/null
avconv -i video.mpg -s 640x360 -pass 2 -threads auto \
      -vcodec libxvid -b 800k -r 24 -g 300 -bf 2 \
      -acodec libmp3lame -ac 2 -ab 128k -ar 48000 \
      video.avi

What am I doing wrong?

Solution

If you're getting worse quality, it's probably because you're using the -b flag incorrectly. It used to refer to just video bitrate, but with the recent ffmpeg/avconv syntax overhaul, it now refers to overall bitrate. So the first pass is telling avconv that it has 800kbit/s to play with for the video, but the second pass is telling it that it has (800-128)=672kbit/s to work with.

What the first pass does is create a sort of roadmap for the second pass to take advantage of - this place is an action scene, it needs a higher bitrate; this is a slow conversation, it needs less - that sort of thing. Because it's working with incorrect information, the roadmap is incorrect.

Use -b:v instead (and -b:a instead of -ab), like so:

avconv -y -i video.mpg -s 640x360 -pass 1 -threads auto -c:v libxvid -b:v 672k -r 24 -f avi -an /dev/null
avconv -i video.mpg -s 640x360 -pass 2 -threads auto -c:v libxvid -b:v 672k -r 24 -g 300 -bf 2 -c:a libmp3lame -ac 2 -b:a 128k -ar 48000 video.avi

Also, a lot of those options are possibly unnecessary, as avconv will use the same options the input has. Unless you're resizing the video, omit the -s 640x360 bit. Likewise, unless you need to change the framerate, omit the -r 24, and unless you really need to change the sampling rate, omit the -ar 48000. Obviously, if you know that you need them, leave them in.

Comments

[johny why]

problem with 2-pass, i'm getting: $ avconv -vn -y -i "movie.mp4" -pass 1 /dev/null ....Unable to find a suitable output format for '/dev/null'