A streamlined format that contains only the Xbox DVD Filesystem (XDVDFS) partition where the actual game data resides.
Standard "Redump" ISOs are 1:1 replicas of an original Xbox disc, including a video partition (which displays a "this is an Xbox disc" message on DVD players) and significant padding data.
extract-xiso -i game_fixed.xiso
Several community-developed tools can handle the conversion process depending on your operating system and technical comfort level.
