A codifier/decodifier (CODEC) filter circuit (250) connected in a subscriber
line interface circuit (202) includes a transmit section (264, 262, 260)
for converting differential voltage audio transmit signals representing voice transmissions
from the subscriber instrument (202) into encoded digital data for transmission
to the digital switching network. A receive section (254, 252) coupled between
the digital switching network and subscriber instrument (202) within CODEC
(250) for converting encoded digital data representing voice signals switched
through the digital switching network to differential voltage audio receive signals
for transmission to the subscriber instrument (202). The subscriber loop
and subscriber instrument (202) reflect the digital voltage audio signals
to the transmit section (264, 262, 260). An impedance section (258, 256)
within CODEC (250) connects between the transmit section (264, 262, 260)
and the receive section (254, 252) and is disposed to provide an audio band
feedback signal between the transmit section (264, 262, 260) and the receive
section (254, 252) for synthesizing a source impedance for the subscriber
line that matches the subscriber loop impedance. Impedance section (258, 256)
includes an analog impedance scaling network (246) coupled between the transmit
section (264, 262, 260) and receive section (254, 252). The impedance
section (258, 256) also includes a programmable digital filter (258)
coupled to the transmit section (264, 262, 260) having a transfer function
equal to:
where R2 is the second subscriber loop impedance, C2 is
the subscriber loop capacitance, R1 is the first subscriber loop impedance,
T is the sampling rate of the analog-to-digital converter and z is the frequency
of the signal. Furthermore, a summer circuit (256) provides feedback between
the programmable digital filter (258) and the receive section (254, 252)
by summing the single-ended audio receive signals from the digital switching network
with the audio band feedback signal output by the programmable digital filter (258).
