White’s Electronics TDI - Manual (Page 20)

0 PulseScan TDI Owner’s Guide
You must operate with a threshold which is as smooth as possible, so you can
hear the clarity of a true signal, and not be confused by false signals, chatter,
or blanking out of the threshold hum. Keeping the detector ground balanced is
important, but one of the best ways to keep a smooth operation is to apply the
appropriate amount of gain.
The threshold will let you know if the gain is set too high. If it is erratic, jumping
around, falsing, noisy, gasping - turn down the gain.