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This is a type of gate detector that looks at a varying input signal and outputs either a “high” (typically 0, 10, or even 15 volts) signal or a “low” signal (typically 0 volts). When the input goes above one reference threshold – say, 4 volts – the output goes high. When the input then goes back below a second, different threshold – say, 1 volt – then the output goes back low.

Having separate high and low thresholds allows you to derive pulse waves of user-defined widths from a steady input signal such as a sawtooth wave. A Schmitt Trigger is also good for converting the output of an envelope follower into a useful note-on gate: the higher threshold prevents false triggering caused by low-level audio, while the lower threshold waits until a sound dies away to near silence before signaling that the note is finished. If the two thresholds were the same, an input signal that was very close to that threshold might cause the output to randomly flip between high and low states.

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