_,,,^..^,,,_ said:

that will be reduced to 30-35 pixel with your method

30-35? 843.75KHz is 90. That's 67.5 TVL. Where are you getting 30-35?

You don't just divide 120 by 4. Capture cards oversample chroma at 3.375MHz. That's 360 lines.

Even if there's only 120 lines of "effective" resolution within the 360, a low pass filter would bring 360 closer to 120 before bringing 120 closer to 0 (or 30 as you claim).

You have to start with the actual resolution (oversampled) rather than the effective resolution when doing these calculations.

Imagine what would happen if you started with 120 vertical alternating white and black lines and then stretched (oversampled) it to 360 and then did this.