This is all before it gets to the DAC, that has its own issues. But if the signal being sent to it has problems, those issues magnify. The timing of the 0/1s is what makes a difference and whether they actually get there. There can be enough noise that the leading edge of a signal is not received in time, isnt latched in and is discarded. This is passed on to the DAC (or even happens as it enters the DAC chip). The DAC chip then has to interpolate which reduces quality. When the signal does get latched, timing discrepancies receiving the signal caused by problems such as Jitter are the issue. These cause noise, the less the better. This is why so much money is spent in digital systems on power supplies, isolation and shielding, to remove noise and keep the pulse timing as accurate as possible. Improvements in DACs are not all down to the DAC chip and basic circuit design.