Hello for all, my name is Felipe and I am a retro hobbist.
I'm developing this converter for coco3, MSX and now I doing it for Apple IIc/e using the composite video. I don't have an Apple II+ to test.
https://www.youtube.com/watch?v=dw5HMWgIjnw
You can find more information at:
https://sites.google.com/site/tandycocoloco/rgb2vga
It uses an Altera DE0-Nano and a daughterboard I developed.
Felipe.
I have some documents from a PAL-Card. It converts NTSC to PAL by converting it first into 4bits RGB then mapping the 4bits to 8bis and after that converting the 8bit RGB to PAL.
Complete Documentation
Sadly all the files are in german, but you should be able to read the circuit plan
BTW: The Card needs some signals from the Apple II Board (its not a direct NTSC-> RGB converter). But because of that you get white text from the card when the Textmode is active.