APPENDIX D
Simple 4-Wire
Connection
This circuit connects two 4-wire elements end to end. There are no
hybrids in this circuit. The elements may be 4-wire telephone
instruments or 4-wire electronic PBXs.
TMS-3000
CELP
TMS-3000
CELP
TIE
TRUNK
TIE
TRUNK
4-WIRE
4-WIRE
PBX
OR
PHONE
PBX
OR
PHONE
036R480DA
NOTE
It is preferable, when designing circuits, to choose a 4-wire
interface without a hybrid rather than a 2-wire interface
with a hybrid whenever possible due to the better electrical
characteristics (as pertains to echo and loss) of the 4-wire
interface.
A checklist should be used when designing and installing a voice circuit.
Listed here are some basic guidelines and cautions to use during the
alignment procedure.
•
Adjust levels with the actual loads. Use dB meter in “Bridge’’ or
“High Impedance’’ mode.
•
Start adjusting levels at talker end, finishing at listener end.
•
Adjust the levels in the reverse direction (using the same method).
•
Use the published TLP levels (from the loss plan) for setting levels.
•
When designing circuits choose 4-wire interfaces rather than 2-wire
interfaces, where possible.
•
There may be some question as to the accuracy of the loss plan. In
such cases use an 1004 Hz test tone from PBX. The installer should
assure that speech levels are below –15 dB relative to the TLP level,
e.g., if interface is –16 TLP then set voice level to –31 dBm.
•
Write down all VF signal levels in the Loss Plan which have previ-
ously not been identified. Subsequent readjustments will be made af-
ter referring to this Loss Plan.
•
Beware of too much gain in either the transmit or receive voice paths.
When the transmit level is too hot, distortion is introduced; when the
receive level is too hot the voice appears excessively loud.
•
Check effects of possible multiple echo paths via PBXs. This could
happen when calls are tandem via other trunks in the PBX.
•
Setting the CELP for 0 Tx TLP/0 Rx TLP is recommended.
D-7
