Downsampling a video with avconv / ffmpeg

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http://unix.stackexchange.com – I know really little about encoding and using avconv / ffmpeg. I am trying to downsample a video as follows: avconv -i blah_in.avi -s 640x360 -pass 1 blah.avi avconv -i blah_in.avi -s 640x360 -pass 2 blah.avi I am aware that this is very simplistic, yet I can't figure out what the problem is. Here is the error message from pass 1: avconv version 0.8.3-4:0.8.3-0ubuntu0.12.04.1, Copyright (c) 2000-2012 the Libav developers built on Jun 12 2012 16:37:58 with gcc 4.6.3 [avi @ 0x9fdd240] non-interleaved AVI Input #0, avi, from 'blah_in.avi': Metadata: encoder : MEncoder svn r345 (HowTos)