Many years ago I placed a paper which is still in the files forum on Pronto HEX format. I just updated my web site (www.the-gordons.net) to have three monographs packed into an archive all of which relate to IR and the Philips pronto. I suggest you look them over. One of them is the original paper, one discusses IR to x10 conversion, and the newest is called IR 101 and is a primer on IR theory and practical use.

If you can not find them on the web site, you probably would not be able to understand them (;-).