directshowsource returns whatever directshow's filter graph wants to output. it's hard to predict because directshow is different on every computer. but you can be fairly sure it's outputting yv12. it's easily checked in virtualdub (load the avs, go "file > information"). you can also find tateu's quicktime import filter around here. not sure what colourspace it outputs, but if you're downscaling it shouldn't make a big difference either way. i find tateu's plugin more stable than relying on directshow. the catch is audio doesn't work properly on mp4 files (though it does on pretty much everything else). you can export audio with quicktime player or ffmpeg anyway...