White’s Electronics TDI - Manual (Page 11)

PulseScan TDI Owner’s Guide 
THRESHOLD -
What it does
This control adjusts the threshold of the unit. This is the faint background hum
which is present during the normal operation of the Pulsescan TDI. This hum,
which is always present, is increased when the knob is turned clockwise.
How to use it
In most detectors, including the Pulsescan TDI, the threshold is adjusted to a
faintly audible level by turning the knob clockwise. Unique to the Pulsescan TDI,
it is best to adjust the threshold past the chatter point, so that the sound is rea-
sonably continuous. Then adjust the headphone volume to a comfortable level.