1. The high level of the previous output is not the standard level or the waveform is not ideal. Two continuous inverters are used to shape the waveform and convert it into a high-level output of standard voltage.

2. The input terminals of other elements in the circuit need the reverse signal of this output terminal, so the output of the first-stage inverter is provided to other input terminals, and then the output signal of the first-stage inverter is restored to the original signal through the second-stage inverter, which plays the role of shaping and balancing the signal delay of the original signal.

I hope this can give you some help!

I'm talking about receiving digital signals in series, and then converting the digital signals into 7-bit parallel outputs through registers for digital-to-analog conversion.

The shaping principle of NOT-gate for distortion level signal is basically the same as that of Schmidt trigger, the only difference is that it has no return voltage.