Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02289275 1999-11-10
A SYSTEM AND METHOD FOR ECHO CANCELLATION
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.
BACKGROUND OF THE INVENTION
The convergence of voice and data networks is now a reality. Every day more
and
more companies are recognizing the value of transporting voice over data
packet
networks in order to reduce telephone charges and also as a prerequisite to
the
implementation of advanced multimedia applications.
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).
1 S One of the problems associated with high-speed digital transmission of
voice
signals is the generation of 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 caused, for example by
imperfect hybrids, a portion of a signal transmitted from one user is received
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 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.
CA 02289275 1999-11-10
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". 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 fiu ther
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 defects on the wire will have an
increased
effect on the quality of the voice over data signal, increasing the
perceptibility of echo.
Further, voice over data systems typically include the higher frequency
spectrum of a
signal for transporting information. Therefore, the crosstalk on the twisted
pair wire
becomes more significant, 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
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CA 02289275 1999-11-10
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.
It is an object of the present invention to obviate or mitigate at least some
of the
above disadvantages.
SU1VE1~IARY OF THE INVENTION
The present invention seeks to solve the problem of providing new echo
cancellation equipment in a switching network whenever new voice over data
services
are added to the network.
According to the present invention, there is provided a line card for coupling
a
subscriber line to a switching network the line card comprising:
an echo canceling circuit for reducing an echo signal received from said
network
and perceived at a user instrument coupled to said subscriber line.
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.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
For convenience in the following description, like numerals refer to like
structures
in the drawings.
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Referring to figure 1, 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 be comprised of 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 such like) located at the
subscriber location
remote, to the CO. A line card 22 may be comprised of 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 Canadian Patent Application No.
2,277,534.
For ease of explanation in the following embodiment, a forward communication
path 18 is shown schematically in figure 1, from a first user A 14 to a second
user B 15.
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
signals each due to segments of the echo path 20 between user A and user B,
each
segment is labeled el, 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 refernng 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
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CA 02289275 1999-11-10
end" echo cancellation. That is, the term "far end" refers to the path
direction with the
greater physical distance from user A.
Conversely, refernng to figure 3 a schematic block diagram of A Line Card 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. That is, the teen "near end" refers to the path direction with
the lesser
physical distance to user A.
If the communication path 18 was 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 couples 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 couples at one end to the network 17, an analog
interface
circuit 43 for connecting at one end to a twisted pair subscriber line 18, a
digital interface
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, 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
subtracts 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.
CA 02289275 1999-11-10
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 modeling the network delay.
The model 44
uses the signal transmitted to the network as its input and predicts the echo
to be returned.
These type of echo cancellation circuits are well known in the art.
S The echo Canceller may also include a fax or modem detection circuit for
disabling
echo cancellation which may be used for communications other than voice; a
speech
activity detection for detecting the presence of a voice signal to activate
the echo
cancellor; and a center clippers or some other non-linear processors for
removing residual
echo.
In operation then, he 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 echo cancellor 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.
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.
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CA 02289275 1999-11-10
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 1 ms, the balance
function
handles a signal that is large compared with the delay.
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 is required to 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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