Clear-Com CCI-22 - Manual (Page 20)

Vitec Group Communications
CCI-22 Dual Channel Party-Line Interface Instruction Manual
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ADJUSTMENTS
POWER LED
The green “PL Power” LEDs indicate whether the interface channels
are getting +30 VDC power from the external party line to which they
are connected.
LEVEL CONTROLS
The “Send” level controls affect the level of the audio signals from the
matrix to the external party line.
The “Recv” level controls affect the level of the audio from the party
line to the matrix. The level controls have a range of ±13 dB; they are
normally set to the midrange position.
AUDIO REJECTION NULL ADJUSTMENT
Sidetone is the sound of the operator’s voice in his headset. In
interfaces, it is necessary to null (minimize) the sidetone as much
as possible when an external party line is placed in the matrix
environment. Ideally, there should be no portion of the talk signal
in the listen signal.
The CCI-22 features sophisticated built-in nulling circuitry, including a
test-tone generator and an accessory earphone. The earphone plugs
into a phone jack on the front panel. When the earphone is plugged in,
it automatically switches on a test tone and monitors the output of the
null circuit.
Separate “R” (resistance), “L” (inductance), and “C” (capacitance)
controls compensate for each component of the line’s impedance,
providing the best null possible.
The null circuit is effective on line lengths between zero and 4000 ft
(1200m) with impedances in the range of 120 to 350 ohms, and can
reduce local audio in the received signal by more than 30 dB over the
frequency range of 200 Hz to 8 kHz.
To null one channel of the CCI-22:
1. Connect the external party-line devices to the CCI-22 channel. Make
sure that any connected devices do not have their microphones on.
2. Plug the accessory earphone into the front-panel jack labeled “Test.”
This will disconnect the interface from the matrix and enable a test
oscillator. The oscillator produces a square wave with both low and
high harmonics, allowing testing of all frequencies. The test tone
pulses approximately every 0.5 seconds.
3. While listening to the test tone in the earphone, adjust the “R”
control until the tone is at a minimum.
4. Repeat Step 3 with the “L” and “C” controls. Because these controls
interact, steps 3 and 4 will have to be repeated several times the