Translating/reverse engineering firmware commands
This is how it really works, they can"t give you details as it is running licenced intellectual property from someone else. At least you would need an NDA with them to prevent you from publishing non-public information. Also they might not even get back to you as you are a single hobbyist without NDA so it is a waste of their time talking to and supporting someone that buys one chip. It would be different if you were a company making a product that sells even 1000 chips per year. I don't know the details and it was very long ago, but I have worked with NJU DSPs.