Clear-Com TEL-14 - Manual (Page 19)

T E L - 1 4 T W O - C H A N N E L T E L E P H O N E I N T E R F A C E
2 - 5
connected to a different telephone line. Once this nulling procedure is
performed, the TEL-14 will remember the telephone line characteristics when it
is removed from the frame or loses power. It is necessary to null (minimize) the
sidetone (the sound of the operator's own voice in his or her headset) when an
external telephone line is placed in an intercom environment. Ideally, there
should be no portion of the talk signal in the listen signal.
TEL-14'S SIDETONE NULLING PROCEDURE
Before calling the TEL-14 to automatically null it, connect the external
telephone line and any other devices that affect the line's impedance. These
include other telephone extensions connected in parallel on the same line. The
phone line must be active (turned on at the central telephone office) during the
nulling process, and all local parallel phone sets must be plugged in but on-hook
so the line is in its normal operating state.
1. Enable the "auto answer" function using the front panel button.
2. Call the TEL-14 interface from a standard telephone set or cellular phone. Do
not call the TEL-14 from another TEL-14 in an intercom.
3. Press and hold the "auto answer" button for 5 seconds. The green "auto
answer on" LED will turn off and then turn on again after 5 seconds. When
this happens, release the "auto answer" button.
4. Maintain silence on the telephone line. The TEL-14 will listen for one second
of silence before beginning. The "ring/of- hook" LED will then flicker yellow
and a series of tones will appear on the telephone line. These tones will be
heard on the telephone line, but not on the intercom. Maintain silence during
this process.
5. At the conclusion of the tones, the automatic nulling process is complete.
After this initial automatic nulling adjustment has been made, the TEL-14 will
remain adapted to this telephone line. There will be no white noise or tones at
the beginning of each call, but instead the TEL-14 will use voices to dynamically
adapt the interface to account for any residual nulling which may be needed.
Typically, the TEL-14 will have adapted within 5 seconds of the start of the call.
Before it has adapted, voices may temporarily sound hollow. This effect is
normal, but may be minimized by reducing the send and/or receive volume
settings. Throughout the call the TEL-14 will continue to perform continuous,
automatic echo cancellation and adaptation to acoustic changes.
The above initial automatic nulling process should not need to be repeated,
unless the telephone line connection is changed to a different telephone number
or if there is reason to believe the telephone line impedance has changed
significantly--for example, if echoing or feedback occurs at a lower volume
setting than before.
If an almost complete null cannot be obtained, it is likely something is wrong
either with the wiring in the telephone line, or with one or more of the other
devices attached to the telephone line. Refer to the “Option Switch” section of
this manual for ways to use the TEL-14 options to improve its ability to adapt to
poor conditions. Note that the null circuits in the TEL-14 are designed in
accordance with North American standards, and require the signal levels and