Clear-Com EF-701M - Manual (Page 29)

E F - 7 0 1 M I N T E R F A C E 2 - 7
NULLING
“Sidetone” is the sound of the operator’s own voice in his or her headset or speaker. With
interfaces, it is necessary to “null” (minimize) the sidetone when external party lines are
connected over the secondary transmission media. Ideally, there should be no portion of
the talk signal in the listen signal. Every EF-701M sharing a channel in a system should
be nulled consecutively. Null is also referred to as “hybrid null.”
Null is affected by the local party-line system connected to the EF-701M, not the
secondary interface. The null circuit is effective on line lengths from 2 feet to 4,000 feet
(from 1.64 m to1220 m) with impedances in the range of 120–350 Ohms and can reduce
local audio in the received signal by more than 30 dB over the frequency range of 200
Hz– 8 kHz.
The EF-701M includes built-in nulling circuitry with a test-tone generator and is shipped
from the factory with an accessory earphone. Plug the earphone into the 1/8 in. (3.5 mm)
jack on the front panel. This will automatically activate the test tone (and the amber data
LED will blink slowly). The earphone monitors the output of the nulling circuit. The test
tone will be sent to any active stations and other EF-701M units on the channel, so
remove headsets, adjust volumes, and turn off speakers as appropriate.