Hmm. I'm getting out of my depth at this point. I've not dealt in detail with many colour modulators but I recall the Sinclair Spectrum had YUV as opposed to RGB on its expansion port. The computer obviously uses RGB internally (although I don't see why one couldn't design a machine to do YUV instead!) and it has to be converted to YUV at some stage in the modulation process. My guess (no evidence to back this up) is that the more expensive modulators - the ones one is likely to be able to buy for one's own projects - accept composite, but many mass-producing manufacturers got away with a cheaper modulator by putting the conversion in the (custom) chip in the video circuitry. Philip.