| The knock system circuit is a perfect example of an amplifying circuit with a signal caring cable in a high RF environment where you do not want a condition known as "ground loop". Here is a link to a PDF file which explains how and what happens when there is a ground loop problem. Start at page 20 which precedes into the ground loop explanation. [ http://www.sem.org/PDF/Szary_GroundLoopProblems.pdf ] In this type of application (the knock circuit) grounding the shielded cable at both ends will cause a ground loop. In a ground loop circuit RF noise will be amplified causing unwanted signals in the line. Floating the cable at one end keeps the unwanted RF and higher currents out of the signal carrying line. As strange as it may sound it's true. I've dealt with this sort of problem as a tester in an EMI (electromagnetic interference testing) lab.
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