Coming from Tesoros I have picked up a NOX 600.  I am use to setting threshold to a barely audible hum.  Then setting discrimination. The quieter the threshold hum, the better discrimination works. Or so it seems.   So what exactly then does setting threshold do on the NOX? According to the manual; it is used to notch out an item, like a nickel or penny. Can someone please explain the differences.  And also explain that if I set the threshold on the NOX to a barely noticed hum wha