TMPGenc 2.59 seems to ask the system for RGB input if you open an AVISynth script with it. Why in the world would it do that? It has to convert back to YUY2 anyway for MPEG encoding, and those 2 conversions cost you both time and quality.

This is easily provable by the fact that, if you disable all YUV codecs on your Control Panel -> Multimedia -> Devices, and then load up an AVISynth script which produces YUY2 output instead of RGB, TMPGenc will give you an "unsupported" message. Add "ConvertToRGB" to the end of the AVISynth script and it works again.

What's the point of this? Is it a bug in TMPGenc? Or what?