Reply To: RF & Composite support

[SpeccyFan]

I’ve recently ordered myself OSSC, intending, apart from all RGB-capable machines that I have, to plug composite and s-video machines like C64 and Atari 65XE. Then I realized there’s no support for those 😛

I thought about making some kind of composite/s-video to RGB decoder using older analog TV circuits and processors like TDA3566, TDA8362+TDA4665, etc. I still considering making such decoder as the “last resort”.

I don’t know yet all the technical details of what’s going inside the FPGA of OSSC and how full it is at the moment, what kind of sample rates are used, etc., but I have this theoretical idea that maybe, color demodulation of PAL/SECAM/NTSC may be offloaded onto FPGA, so those inputs could be implemented without extra hardware at the cost of extra logic inside FPGA.

From what I’ve figured out (I may be wrong here), in order for that to work, those conditions need are needed:
1. Composite video/S-video luma is plugged into green channel of RGB input, so ADC can extract sync from it as if it were sync-on-green. I suspect that I should be atleast able to see monochrome image from composite in RGsB mode – that “trick” seems to work with CRT television that I have. ADC most likely doesn’t care if it received RGB or composite, we just need to extract signal value;
2. ADC on the board can be set up to pass the entire line of data (to extract composite colorburst portion) into FPGA with sample rate high enough to accurately sample colorburst to properly sync to it – seems possible, judging by its (TVP7002) datasheet;
3. There’s enough space in FPGA to put a PAL/SECAM/NTSC color demodulator in there and how much will it take – I don’t have any idea at all. And, as far as I’m aware, nobody wrote such demodulator at this point.

I might tinker with that idea in my spare time once I receive my OSSC, not sure what would come out of it, if anything.