Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
2000-11-08
2004-10-12
Tieu, Binh (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S093010, C379S093060, C379S093080, C379S100170, C379S406120
Reexamination Certificate
active
06804348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to data communications, and more particularly to a circuit for matching a digital transmission on a subscriber line, where the matched transmission is used for echo-cancellation.
2. Description of the Related Art
FIG. 1
shows a communication system
10
which includes a subscriber line interface circuit
100
coupled to one end of a subscriber line
110
. The subscriber line
110
is a communication medium for carrying voice and/or data signals. One example of a subscriber line
110
is a conventional telephone line comprised of a twisted-pair of copper wires. The subscriber line
110
includes a tip line
112
and a ring line
114
. According to one configuration for which the present invention is suited, the subscriber line
110
is a two-way communication medium, whereby the tip and ring lines
112
and
114
together carry both signals being transmitted and signals being received. The transmission signals and receive signals overlap each other in both time and frequency domains on the communication medium. In effect, the communication medium carries a composite signal representing overlapping transmission and receive signals.
The line interface
100
includes a transmission path
122
and a receive path
124
, each of which having lines corresponding to the tip line
112
and the ring line
114
. The line interface
100
is coupled to the subscriber line
110
, typically by a coupling transformer
115
. A driver
120
drives transmission signals onto the subscriber line
110
. The driver is preferably a low-impedance driver. A pair of isolation resistors R
T
and R
L
are resistively matched as close as possible to an impedance Z
SL
of the subscriber line at the transformer
115
. The impedance Z
SL
is based on an impedance of the subscriber line Z
LOOP
as it is converted through the transformer
115
, as seen through the turn ratio of the coils of the transformer
115
. Secondarily, Z
SL
is also based on other attributes, such as capacitance, for example, of the transformer
115
and other potential circuit components of the line interface
100
, which are not shown in FIG.
1
.
Point A in
FIG. 1
represents a point between the output of the driver
120
and the isolation resistors, R
T
and R
L
. The driver
120
has a very low output impedance, approaching zero, so that transmission signals on the output of the driver
120
are not affected by signals being received. Thus, signals on the driver
120
side of isolation resistors R
T
and R
L
are largely, if not exclusively, transmission signals. On the subscriber line side of the terminal resistors R
T
and R
L
, several attributes of the communication system
10
transform the transmission signals. The attributes include an impedance of the subscriber line
110
and an impedance of the coupling transformer
115
At that point, the transmission signals are also combined with, and affecting, the signals being received. Point B in
FIG. 1
represents a point on the subscriber line interface circuit where transformed transmission signals are combined with receive signals.
As both receive and transmit signals are present on the subscriber line, and have overlapping spectral content, signals being received must be isolated from transmission signals at the receiving end, i.e. at the line interface circuit
100
. However, such a procedure is very complex, due to the difficulty of determining the signal being transmitted and its effect on the signal being received. This difficulty exists because the transmission signals are transformed from a known signal at the point where they are output from the driver
120
, to a transformed signal at the point where they reach a transformer coupled to the subscriber line
110
, influenced by a plurality of transforming attributes. Most of the transformation is related to the attributes of the communication system
10
described above.
Signals arriving at the line interface
100
have attenuated extensively, and thus make up a smaller relative portion of the combined signal present on the subscriber line
110
. Therefore, some line interfaces employ a device known as a hybrid circuit
130
to approximate the transformation of the transmission signals. The hybrid circuit
130
is configured to produce a simulated transformed transmission signal in order to remove any transformed transmission signals from the receive signals.
Operation of the line interface
100
shown in
FIG. 1
occurs as follows. The hybrid circuit
130
receives a pure transmission signal from the driver, and transforms it based on approximated characteristics of the subscriber line at the line interface
100
. A transformed transmission signal, representing a transmission signal that would occur at a point where it is combined with a receive signal, is passed to a subtractor
150
. A composite signal, having both transmission signals and receive signals, is coupled and filtered by a filter
140
, to remove aliasing or interfering frequencies. At the subtractor
150
, the signal provided by the hybrid circuit
130
is subtracted from the composite signal provided by the filter
140
, to theoretically yield only a receive signal. The recovered receive signal is then passed on for digital signal processing. The anti-aliasing filter
140
is normally provided separately from the hybrid circuit
130
to ensure a receive signal does not exhibit aliasing when the digitization process occurs in the DSP.
Conventional hybrid circuits, therefore, generally take a “known”signal being transmitted, and approximate how that signal will change in the presence of signal-transforming attributes or characteristics of the subscriber line. An approximated transmission signal is needed so that it may be removed from a receive signal. Accordingly, a hybrid circuit should model the transforming characteristics of the subscriber line on a transmission signal as accurately as possible. Conventional hybrid circuits are limited in how well they model those transforming characteristics of a given subscriber line.
SUMMARY OF THE INVENTION
The present invention is a circuit and method for modeling a plurality of attributes of a communication system that includes a line interface coupled to a subscriber line. The plurality of attributes collectively transform a transmission signal prior, and in some cases as it is being combined with a receive signal on the subscriber line. The attributes include impedances from various sources on the subscriber line and circuits of a line interface circuit. The present invention accurately models the plurality of attributes in order to substantially duplicate the transforming effects of the attributes on a representation of a transmission signal that has not yet been transformed, and provide a transformed transmission signal for removal from the receive signal.
In an embodiment of the invention, a circuit includes an input for providing a representation of an outbound transmission signal that is substantially free of time and frequency overlaps with inbound receive signals from the subscriber line, and wherein transmission signal bandwidth is asymmetrical to receive signal bandwidth. The circuit further includes a modeling section connected to the input, comprising resistive and a capacitive elements which are arranged for modeling a plurality of attributes of a subscriber line environment which transform transmission signals prior to overlapping with receive signals. The modeling section is further configured to substantially duplicate a transformation on the representation of the transmission signal. An output connected to the modeling section provides the transformed representation of the transmission signal.
In another embodiment of the invention, a method includes the steps of coupling an outbound transmission prior to its overlap with an inbound receive signal, to provide a representation of the transmission signal, modeling a plurality of attributes of the communication system, the attributes having a collective
Gray Cary Ware & Freidenrich LLP
Tieu Binh
Viadux, Inc.
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