[ChrisW6ATV]

Do you mean a digital-TV-to-analog-NTSC converter? Those tuners are extremely complex overall, but devices like that are made from "building blocks" also known as integrated circuits and reference designs. I am sure that any number of AK members (maybe even I) could design one if we had the tools (design notes/specifications from the specialized chip manufacturers) and, especially, the time required to accomplish such a product. The cost would be absolutely prohibitive, by the way. The development/evaluation kits and license fees for the hardware and software you would need probably cost thousands of dollars, and we have not even begun to think about the value of the time to be put into the project-probably hundreds of hours, minimum.

Oh, if you meant any D/A converter, you could build a basic one with a few TTL ICs and some resistors. It would convert "digital signals" (say, 8-bit binary signals) into an analog voltage (say, zero to five volts) corresponding to the number of positive bits in the digital signal.

If you mean something in between these two extremes, ultimately the best way to do it is still to use existing D/A converter chips such as those made by Burr-Brown and customize the interface/control circuitry to accomplish your idea. Again, there are probably some AK members who could do a project like this, but don't count on it being inexpensive unless you can line up a few hundred or more customers in advance. Is this a closer answer to your question?