Convert avi to 3gp

Install ffmpeg:

$ sudo apt-get install ffmpeg

Connvert from avi to 3gp

$ ffmpeg -i movie.avi -s qcif -vcodec h263 -acodec mp3 -ac 1 -ar 8000 -r 25 -ab 32 -y movie.3gp

Convert from 3GP to AVI:

$ ffmpeg -i movie.3gp -f avi -vcodec xvid -acodec mp3 -ar 22050 movie.avi

Convert from MPG to 3GP:

$ ffmpeg -i movie.mpg -s qcif -vcodec h263 -acodec mp3 -ac 1 -ar 8000 -ab 32 -y movie.3gp

Convert from MPG to 3GP:

$ ffmpeg -i input -acodec amr_nb -ar 8000 -ac 1 -ab 12 -vcodec h263 -s qcif -r 10 -aspect 16:9 out.3gp