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APPENDIX D
GENERAL
D-2
The CELP module can be installed in TMS-3000, Univ. MM+ V4 or
MINIMUX Plus equipment manufactured by General DataComm. The
CELP module is used to provide an end to end (MUX to MUX) voice cir-
cuit. VF (Voice Frequency) signals into the CELP module are converted
to digital data and transmitted across an aggregate trunk to a remote
MUX where it is converted back into an VF signal.
The CELP is less forgiving of VF signal overload than the ADPCM mod-
ules. When upgrading from ADPCM to CELP what may have appeared to
work acceptably for ADPCM may not work well for CELP. Therefore,
care must be taken during the installation to insure that the alignment of
the levels of CELP circuits is done correctly. This appendix explains how
to properly align these levels.
Each piece of equipment in a circuit has a nominal interface impedance.
This impedance is due to the electrical characteristics of the components
and wires in the circuits. Varying designs and combinations of equip-
ment being connected to a CELP module may present different actual
impedances. The differences in the load presented by the
equipment to the CELP vary from installation to installation.
measuring the VF signal levels, they should be measured using actual
loads, especially where hybrids are involved. The use of test equipment to
provide the termination should be avoided when possible. When measur-
ing levels using the terminal equipment as the load, it is also very
important that you use the meter in BRIDGE mode. Failure to do so causes
the readings to be inaccurate.
To properly install a circuit it is necessary that the 'LOSS PLAN' be in
place. The Loss Plan will have to be designed using later sections of this
appendix. A Loss Plan is a means of planning and defining the expected
VF signal levels at all points in a network for future reference. The Loss
Plan need not be overly complicated; however, it is important that it be
complete. When, in the future, adjustments need to be made for any rea-
son, then the Loss Plan must be consulted and levels set
When installing the circuit, the installer will refer to the Loss Plan. This
will allow for proper adjustments at the various adjustment points. Much
of the data used to design this plan should be provided in the
documentation of the equipment being connected to the CELP. In the event
that this data is not available, it is necessary to perform tests, specified
later in this appendix, to obtain the necessary data.
The reason it is so important that the levels being input to the CELP are
not excessive is as follows: the CELP uses a standard CODEC in order to
produce a digital representation of the input VF signal. This digital
signal is then run through an algorithm (a mathematical process) which
produces a low speed message for transmission to the remote CELP. The
process used to produce this digital signal takes many of the CODEC's 8
bit samples and creates one sample which is decoded to recreate the
original VF signal. The CELP algorithm depends on the constant change
of the samples in order to process the digital signal. When the CODEC's
VF signal input is excessive (overloaded), the digital signal which is
output from the CODEC reaches a maximum numeric value and ceases to
change. In contrast the ADPCM algorithm used is more forgiving. It
takes each of the CODEC's digital samples and creates a smaller sample
terminal
When
accordingly.
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