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Features Description 2-69
2.9.1. Near Talker Echo
The Series II provides near end talker echo simulation for station A and station B.
Station A (B) near talker echo is illustrated in Figure 2-30 and is the reflection of
the modem A (B) transmit signal that occurs at modem A (B). This reflection is
caused by the impedance mismatch between the modem's hybrid balance
impedance and the input impedance (Zmag) of the Series II station A (B) 2-wire
interface.
Figure 2-30. Near Talker Echo without Loop
The simulation is generated with the near A (B) echo attenuator and polarity
control circuit. This capability allows the user to program the attenuation and
polarity of the signal that is feedback from the transmit port to the receive port to
the receive port of the Series II 2 to 4 wire hybrid. The net result of the signal
feedback is that the input impedance of the Series II 2-wire interface is modified
from its nominal magnitude of 600 ohms. The following table indicates the
attenuation level and polarity setting for the near end echo parameters that
generate a specific nominal magnitude of input impedance. The echo polarity
should be set to non-inverting to create impedance values less than 600 ohms, and
should be set to inverting for values greater than 600 ohms. The attenuation is
calculated as follows:
Attenuation = - 20 log (abs((Zmag - 600)/(Zmag + 600)))
where:
abs = absolute value
Zmag = desired impedance magnitude