There is at least one patent they were granted for a "Grayscale Video Conversion System". There are some details in the patent about the method and a rough schematic, but they do not have to divulge all the design details in order to be issued one. At the time the patent was relevant, it may have discouraged copying, but poor sales was probably a bigger deterrent given it is not very easy to find. I would not be worried about infringement at this point....if whoever owns the IP is known, they might be willing to release the design, Micron may have sold-off the IP at some point (when Micron was dropped from the name) so it may be harder to figure out now.