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APPENDIX D
Echo Return Loss
CELP
C
O
D
E
C
D-14
Since we are using 4-wire telephones with internal gain sec-
tions, we could just as easily have added the +1 dB of gain in
the telephone, and left the CELP gain sections alone.
This concept of a Loss Plan can be applied to networks which vary from
the very simple, as in the above example, to the most complex networks.
GAIN/LOSS
GAIN/LOSS
4-WIRE ANALOG
Hybrids are devices for interfacing between 4-wire and 2-wire analog
circuits. They are four port devices. The fourth port is connected to an in-
ternal matching network whose impedance would ideally match the
impedance of the 2-wire line. In practice, this match is never perfect.
For a perfect matched hybrid with perfect lines, we could expect none of
the receive voice energy coming in to the 4-wire receive port to be reflected
into the 4-wire transmit port. The receive energy will appear at the 2-wire
circuit with a 3 dB loss in power (i.e., ideal hybrid insertion loss is 3 dB).
Hybrids normally have attenuators in the 4-wire circuits for greater level
control. Active hybrids also have gain sections in the 4-wire circuits. Use
this gain with caution.
The effect of the imperfect matches which exist in real hybrids are:
•
The hybrid insertion loss varies and must be measured.
•
Some of the voice energy received at the 4-wire input port is reflected
back to the 4-wire transmit port and cause an echo.
The difference of receive voice power to reflected transmit voice power,
expressed in dB, is known as the echo Return Loss (ERL). ERL is always
expressed in terms of a positive value. If the loss is calculated to be a
negative value, then a gain exists and steps need to be taken to obtain a
positive ERL. The more positive or greater the ERL the better the rejection
of the hybrid.
In the case of the CELP, we are interested in the ERL as measured at the
CODEC. It is clear that the ERL at the CODEC will be affected by gain
and loss selections in both the transmit and receive paths between the
CODEC and the external hybrid circuit. The formula used to calculate the
NOTE
HYBRID CIRCUIT
GAIN/LOSS
GAIN/LOSS
H
Y
B
R
I
D
(2-WIRE)
036R480DG
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