AFAIK GTIA is fully digital.
GTIA is not fully digital. The delay line (pin 17) is analog. But that's just the easy part that an ADC should solve.
Do you mean because of the higher frequencies?
I thought gtia output digital luma (4 bits on 4 pins) and digital chroma (phase shifted square wave on 1 pin).
Yes, I mean the higher frequencies on the chroma output. While it is not analog in the sense of the voltage level, it's a VCO (voltage controlled oscillator) output. Of course, the VCO output can be implemented digitally if the device works with a clock at a frequency high enough. But considering that the exact frequencies need to be adjusted at runtime depending on the calibration input, the working frequency required might be too high for a low cost device.
Edited by ijor, Thu Dec 28, 2017 6:52 AM.