Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A SYSTEM AND METHOD FOR ECHO CANCELLATION
BACKGROUND OF THE INVENTION
The present invention relates to a system and method for implementing echo
cancellation in a communication network, and more particularly to a system for
echo
cancellation in a voice-over-data telephone network.
Voice over data communication utilizes existing data lines such as ISDN
(Integrated Services Digital Network) in order to transport digitized voice
signals along with
data on a common communication line. The analog voice signals are digitized
using coding
such as PCM (Pulse Code Modulation).
One of the problems associated with high-speed digital transmission of voice
signals is the generation of undesired echo. In general, a user's telephone
equipment is
connected via a line card and a twisted pair telephone line to a Public
Switching Telephone
Network (PSTN). The twisted pair circuit is normally a two-wire circuit, while
sections
between a PSTN is a four-wire circuit.
Because of mismatches in the line and network which are caused for example
by imperfect hybrids, a portion of a signal transmitted from one user is
received back by that
user after a delay of a predetermined time period. This delayed signal is
normally perceived
by the initiator or talker as an echo. The echo becomes more significant and
evident as signal
propagation time becomes longer.
Thus, the delay inherent with voice over data technology (such as digital
transport over a twisted pair loop) may increase the perceptibility of echo
and may lead to
unacceptable voice quality. Furthermore, the delays inherent with protocols
such as ATM
cell assembly and disassembly can create increase the perceptibility of echo.
Figure 1 shows a schematic diagram of a typical telephone network
configuration 10. A network echo path 12 is described from the perspective of
one of the
telephone users, referred to as "user A" 14.
As may be seen, the user A is connected via a twisted pair to line card A,
which, in turn, is connected to the telephone network. Similarly, a user B is
connected via a
twisted pair and a line card B to the network. The echo signal received at
user A (or B) may
be seen as the contribution of signals from various echo paths.
In the present context, the user A is referred to as the "near end talker," or
simply "near end" while the user B is referred to as the "far end talker" or
simply "far end."
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The near end echo is comprised of signals received from the near end line
card, the network,
and the far end line card.
Unfortunately, when voice over data communication is implemented, a further
echo is introduced. This echo is due to several reasons, including those
described below.
Voice over data systems typically attempt to transfer more information over a
twisted pair
wire than voice only systems. Therefore delay introduced by the data modem
will have an
increased effect on the quality of the voice over data signal, increasing the
perceptibility of
echo.
Various techniques have been implemented to cancel or reduce the echo signal
received at the near end. Such devices are known as echo canceller and attempt
to minimize
the echo signal. For example, U.S. Patent Number 5,859,907 describes an echo
canceller and
echo path estimation method which may be used with cellular telephone
networks.
When voice over data communication is used, such as voice over DSL, the
echo cancellation equipment needs to be adapted to provide echo cancellation
over the
twisted pair wire, since the superposition of voice with data in DSL systems
normally occurs
at the line card. Because the echo cancellation equipment is contained within
the network,
the equipment to be provisioned for each new voice over data connection.
Changes to the
equipment is labour intensive and, therefore, expensive. The type of equipment
available
may also limit the number of changes thereby limiting the number of voice over
data users.
Therefore, there is a need for a method for canceling the line echo without
burdening the network. Such a method would greatly enhance the deployment of
the voice
over data service, without the need to coordinate the availability of the
service on specific
subscriber lines with centralized network equipment.
Furthermore, with the number of telephone lines already in existence it is
important that the existing technology can be easily retrofit.
What is needed is to obviate or mitigate at least some of the above
disadvantages.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a line card for coupling
a
subscriber line to a switching network the line card which has an echo
canceling circuit for
reducing an echo signal received from the network and perceived at a user
instrument
coupled to the subscriber line. The present invention seeks to solve the
problem having to
provide new echo cancellation equipment in a switching network whenever any
new voice
over data services are added to the network.
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BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described by way of example
only, with reference to the accompanying drawings in which:
Figure 1 is a schematic diagram of a typical network connection showing a
connection path between a pair of subscribers;
Figure 2 is a schematic diagram of a line card for far end network echo
cancellation;
Figure 3 is a schematic diagram of a line card for near end network echo
cancellation; and
Figure 4 is a schematic diagram of a line card with near end network echo
cancellation and balance filtering.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
For convenience in the following description, like numerals refer to like
structures in the drawings.
Refernng to Figure l, a schematic diagram of a telecommunications network
is shown generally by numeral 10: The network 10 connects at least a pair of
subscribers 14
and 15. The network 10 is generally comprised of line card 22 connecting a
digital switching
network 17 (which may comprise more than one network) of a central office (CO)
exchange
to a plurality of subscriber lines 18. These subscriber lines are typically
twisted pair Tip and
Ring copper lines that connect subscriber instruments (such as a telephone,
fax, data modem,
and the like) located at the subscriber location and thus remote to the CO. A
line card 22 may
comprise a plurality of POTS subscriber line interface circuits (SLIC) and
CODECS. The
SLIC connects a balanced two-wire transmission path (the path to and from the
subscriber
telephone handset) with an unbalanced four-wire transmission path (the path to
and from the
telephone central station). SLICs perform various functions, including battery
feed,
overvoltage protection, ringing, signaling, hybrid and timing. The line card
may also include,
for voice over data transmissions, an xDSL transceiver or integrated POTS/xDSL
circuit such
as for example described in the applicants co-pending U.S. Patent Application
No.
09/614,927.
For convenience of explanation in the following embodiment, a forward
communication path 18 is shown schematically from a first user A 14 to a
second user B 15
(Figure 1). Furthermore, an echo path 20 is defined between user A 14 and user
B 15. From
the point of view of user A, the echo signal is perceived as a contribution of
at least four echo
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signals each due to segments of the echo path 20 between user A and user B,
each segment is
labeled e1, e2, e3, and e4 in the drawing of Figure 1.
The present invention is based on the recognition that the line card 22 is the
first circuit card to which a subscriber's twisted pair telephone line is
coupled. It is the first
point of access for either digital or analog communication over the twisted
pair. Therefore,
by implementing echo cancellation at the line card makes it possible to
account for transport
delays across the twisted pair loop to the subscriber set, without burdening
the rest of the
network with a requirement to perform echo cancellation.
Accordingly, referring to Figure 2, a Line Card A for reducing the echo
returned to user A from path segments e2, e3 or e4, according to an embodiment
of the
invention is shown generally by numeral 22. This form of echo cancellation is
referred to as
"far end" echo cancellation, wherein the term "far end" refers to the path
direction with the
greater physical distance from user A.
Conversely, referring to Figure 3, a schematic block diagram of A Line Card
1 S B 24 for reducing the echo returned to user A 14 from path segment 4 only
is shown
generally by numeral 25. This form of echo cancellation is referred to a "near
end" echo
cancellation, wherein the term "near end" refers to the path direction with
the lesser physical
distance to user A.
If the communication path 18 were to be defined from user B to user A, the
terms applied to the type of echo cancellation performed by each line card 22
and 24 would
be reversed.
Furthermore, when a data modem transports the voice signal across the twisted
pair loop 18, network echo cancellation on the line card enhances the quality
of the voice
connection. Similar to the situation described above, implementation of echo
cancellation on
the line card eliminates the need to burden the rest of the network with
knowledge about the
delays introduced by the transportation of the voice signal.
In addition, having the echo cancellation performed on the line card makes the
technology easy to retrofit since all that is required to upgrade the system
is the replacement
of the line card.
Referring back to Figure 2, a block diagram of a line card for "far end"
network echo is indicated generally by numeral 22. The line card 22 is coupled
at one end to
the subscriber line 18 and at the other end to the network 17 and includes an
echo
cancellation circuit 42 which is coupled at one end to the network 17, an
analog interface
circuit 43 for connecting at one end to a twisted pair subscriber line 18, and
a digital interface
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transceiver 45 for processing analog signals coupled from the analog interface
and passing it
via the echo cancellation circuit 42 to the network 17. Both the analog
interface circuits 43
and digital interface circuits 45 are well known in the art.
The echo cancellation circuit 42 comprises transmit and receive paths 47 and
48, respectively, and a network delay model circuit 44 coupled to receive an
input signal
from the transmit path 47 and outputs a signal 46 to a subtractor 49. The
subtractor is
operative to subtract the signal received along the receive path 48 from the
output 46 of the
delay model circuit 44 to produce a receive signal to the digital interface
45.
The network delay model circuit 44 uses a digital circuit such as a FIR
filter,
IR filter, or other adaptive filter and is based on a model of the network
delay. The model 44
uses the signal transmitted to the network as its input and predicts the echo
to be returned.
These types of echo cancellation circuits are also well known in the art.
The echo canceller may also include a fax or modem detection circuit for
disabling echo cancellation when the subscriber loop is used for
communications other than
voice; a speech activity detection for detecting the presence of a voice
signal to activate the
echo canceller; and a center clipper or some other non-linear processor for
removing residual
echo.
In operation, the output 46 of the network delay model 44 is subtracted from
the signal received 48 from the network. Matching the magnitude and phase of
the echo
signal returned with the predicted echo 46 allows the output 46 of the network
delay model
44 to cancel the echo returned from the network and provide a voice
transmission that is
virtually free of echo.
Referring to Figure 3, a block diagram of a line card having a near end
network echo cancellation circuit 52 is shown generally by numeral 24. The
near end
Network Echo Canceller 52 on the Line Card operates in a similar way to the
far end echo
canceller 42 in that it also uses a digital circuit such as a FIR filter, IR
filter or other adaptive
filter as its model 53. This model 53, however, uses the signal received 54
from the network
17 as its input and predicts the echo 56 to be returned.
In the near end network echo canceller 52, the output of the network delay
model is subtracted from the signal transmitted 57 to the network 17. Matching
the
magnitude and phase of the actual echo signal with the predicted echo allows
the output of
the network delay model 53 to cancel the echo returned from the twisted pair
58 and provide
a voice transmission that is virtually free of echo.
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The near end echo cancellation aids in implementing echo cancellation on the
line card. It is this function that allows the delays inherent in voice over
data technology to
be accounted for, thereby improving the quality of voice transmissions for DSL
communication.
The near end network echo cancellation 24 as described with reference to
figure 3 may also include a balance circuit which is provided in typical voice
CODECS.
Referring to Figure 4, a line card having near end echo cancellation and a
balance function circuit is shown generally by numeral 60. The line card 60
includes a
balance function circuit 62 for coupling to the subscriber line 58 and an echo
cancellation
circuit 52 for coupling the balance function circuit to the network 17. The
echo cancellation
circuit 52 provides some echo cancellation for voice communication. However,
it may only
cancel delays in the order of microseconds and is a compromise for a range of
actual loops.
Since the duration of the echo is typically in the order of a thousand times
greater (1 ms), the
balance function handles a signal that is large compared with the delay.
1 S In comparison, the network echo canceller cancels delays in the order of
milliseconds. Therefore, the duration of the echo is relatively small compared
to the delay.
The echo canceller 52 provides cancellation that enhances the performance of
the balance
function 62 and can take into account components of the system that are
between the balance
function 62, and the network echo canceller 52.
Furthermore, it may be required that Line Card A 22 perform both far and near
end echo cancellation. It is possible to implement both types of echo
cancellation, as
previously described, on one line card, further integrating and simplifying
the architecture of
the system. Both the delay model circuit and the balance function circuits are
well known in
the art.
Although the invention has been described with reference to certain specific
embodiments, various modifications thereof will be apparent to those skilled
in the art
without departing from the spirit and scope of the invention as outlined in
the claims
appended hereto.
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