i use rmvb as compromise for disk space, i know nothing about technical stuff, the observation is sorely base on my own eyes and ears. avi simply used too much space, by rmvb, the visual and audio quality reduced not even 1/5 but the space used is only 1/2 compare to avi, heck, sometimes u won't even know the file is rmvb unless u open 2 same video and compare. don't believe me? try compare a tv-rip avi and a dvd-rip rmvb with about half its size, u will know which 1 is better. if people say mkv is much better than rmvb  i agree, but avi? with proper source and bit rate configuration, i will choose rmvb over avi anytime, i hav enough dvds piling up in my room, i make my choice, avi just not worth it.