A noisy signal creeping past a single threshold flips the output on, off, on, off — useless chatter. Give the switch two thresholds, a high to turn on and a lower one to turn off, and the noise can’t reach back across the gap. One clean edge instead of a hundred. Slide the noise up and watch hysteresis hold the line.
A plain comparator switches at one level T; a Schmitt trigger switches HIGH only above VH and LOW only below Vʟ (VH>Vʟ), the gap between them a dead-band noise cannot rattle. The instrument feeds the SAME noisy triangle to both and counts output transitions. A fail-loud self-check throws unless a clean rising ramp gives the Schmitt EXACTLY ONE transition (at VH) AND, on a noisy mid-level signal, the plain comparator flips strictly more than the Schmitt — hysteresis as manufactured noise-immunity.
The noise here is uniform and added to a fixed triangle — illustrative; real hysteresis width is set by feedback resistors and traded against how large an input step the switch will still respond to. The one-edge-on-a-ramp and plain>schmitt facts are exact.