What is the shader, if any, that is most close to composite signal that the old Spectrum/C64 (and Megadrive/NES if i remember well) output? From some tests i think gtuv50 could be close but just asking. I have an Amiga 500 that can output composite too via modulator and i see that if there is any dithering it results in a 3rd color (cannot see dithered pixels) but the image is sharp too with minimal color bleed and colors are a bit desaturated compared to RGB.
There’s a whole directory of NTSC modulation/demodulation shaders. I’d check there first.
Looks like ntsc-gaussian-svideo is doing the job perfectly (256 and 320). The one without svideo has much flickering like an RF cable.
There is a “Presets” dir with a subdir “Tv out”. These shaders have with and without scanlines versions and the look is IMO more accurate and with more options than the ones under “NTSC” dir.