Note: Descriptions are shown in the official language in which they were submitted.
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Apparatus and Method For Adapting an Echo Canceller
in a Communication System
Background
The invention relates generally to communication systems, and
more particularly to a variable-length echo canceller for a modem.
~ Discussion of Related Art
In today's information age, the number of personal computers
used in homes, schools, and businesses continues to proliferate with
apparently no end in sight. This increasing use of personal computers
has prompted the widespread deployment of computer networks such
as the Internet which provide an ever-increasing variety of on-line
services. While computer and communication technology has
advanced greatly over time, it is still very common for users to
connect their personal computers to such computer networks over
telephone lines using a modem.
Historically, the modem has been an add-on feature to the
personal computer, either as an external peripheral connected by a
cable to the personal computer or as an internal peripheral installed
directly into an expansion slot in the personal computer. These
modems typically included all of the hardware and software
components required to provide modem functionality for the personal
computer. Specifically, the modem typically included a
microprocessor (and associated memory circuits) for executing
general software functions, inputloutput ports for data and control
interfaces) a signal processor for executing computationally
intensive signal processing functions, and many special-purpose
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hardware components for intertacing with the telephone network and
for providing additional signal processing functions. These dedicated
hardware components made the modem relatively expensive.
Today, a need has arisen for a less expensive modem that can be
built into the personal computer by the manufacturer or packaged
together with other external peripherals to provide modem
functionality for the personal computer. One solution is to
implement predominantly all of the modem processing functions in
software (with the remaining functions implemented in specialized
hardware) and to run the software as an application on the
microprocessor in the personal computer. This type of modem, often
referred to as a "software modem," is feasible today due to the
amount of processing resources provided by modern microprocessors.
By eliminating most of the dedicated hardware components from the
1 S modem and utilizing the processing and memory resources of the
personal computer, the software modem is relatively inexpensive.
One issue with the software modem is that the modem software
must share the processing resources of the personal computer with
other application software such as a word processor, spreadsheet
program, or Internet browser. This "feature" of the software modem,
while making the software modem inexpensive) consumes processing
resources that otherwise would be available to the other application
software. As a result, the software modem may noticeably affect the
performance of the other application software. Thus, one goal of the
software modem is to provide full modem functionality using as
little processing resources as possible.
In a software modem, the echo cancellers which are used to
compensate for signal reflections in the telephone network represent
a substantial proportion of the total amount of processing resources
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required by the modem. Two independent echo cancellers are
typically employed) one for cancelling a near-end echo and one for
cancelling a far-end echo. These echo cancellers are typically
designed and implemented to compensate for a worst-case (i.e.,
maximum span) echo. Because the amount of processing resources
needed to implement an echo canceller in software is directly
proportional to the number of taps in the echo canceller filter, which
in turn is directly proportional to the span over which the echo
canceller works, an echo canceller that is designed and implemented
to compensate for the worst-case echo necessarily consumes a
maximum amount of processing resources. Thus, a need remains for a
system, device, and method for reducing the number of taps in the
echo canceiler filter to reduce the amount of processing resources
consumed by the echo canceller.
Brief Description of the Drawing
in the Drawing,
FIG. 1 shows an analog-to-analog modem connection as is known
in the art;
FIG. 2 shows a digital-to-analog modem connection as is known
in the art;
FIG. 3 shows an exemplary relationship between the near-end
and far-end echoes;
FIG. 4 shows an exemplary embodiment of a symbol-driven echo
canceller;
FIG. 5 shows the relationship between the bulk-decay line, the
near-end echo canceller, and the far-end echo canceller;
FlG. 6A shows a conceptual view of an echo canceller filter
having X filter taps;
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FIG. 6B shows a preferred embodiment of a near-end or far-end
echo canceller implemented using three subfilters;
FIG. 7 shows generally the relevant steps of the prior art V.34
startup sequence;
FIG. 8A shows an exemplary positioning of the near-end and far-
end echo canceller filters upon initialization;
FIG. 8B shows positioning of an echo canceller filter upon
initialization in more detail;
FLG. 9 shows an exemplary positioning of the near-end and far-
end echo canceller filters in which the far-end filter span overlaps
the near-end filter span;
FIG. 10 shows an exemplary positioning of the near-end and far-
end echo canceller filters in which the far-end filter span is
truncated; and
FIG. 11 shows an exemplary embodiment including a number of
steps which are taken during the startup sequence in order to adapt
the echo canceller filters to the reflections in the network
Detailed Description
As discussed above) the need remains for an apparatus and
method for reducing the number of taps in the echo canceller filter in
order to reduce the amount of processing resources consumed by the
echo canceller. Embodiments of the present invention eliminate the
near-end or far-end echo canceller completely under certain
conditions, and also reduce the number of taps in the near-end and/or
far-end echo canceller filters under yet other conditions. As a
result, these embodiments satisfy the need for reducing the amount
of processing resources consumed by the echo cancellers.
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FIG. 1 shows a modem-to-modem connection as is known in the
art in which modems 102 and 108 are connected to the telephone
network over traditional analog local loops 103 and 107,
respectively. Both modems send and receive analog signals. The
5 analog signals sent by modem 102 over analog local loop 103 are
digitized at interface 104 for transport across the digital backbone
network 110 and converted back to analog signals at interface 106
for transport over analog local loop 107 to modem 108. Likewise, the
analog signals sent by modem 108 over analog local loop 107 are
digitized at interface 106 for transport across the digital backbone
network 110 and converted back to analog signals at interface 104
for transport over analog local loop 103 to modem 102.
FIG. 2 shows a modem-to-modem connection as is known in the
art in which modem 208 is connected to the telephone network over a
traditional analog local Poop 207 and modem 202 is connected to the
telephone network by way of a digital connection 203. Modem 202
sends and receives digital signals, while modem 208 sends and
receives analog signals. The digital signals sent by modem 202 over
digital connection 203 are transported across the digital backbone
network 210 and converted to analog signals at interface 206 for
transport over analog local loop 207 to modem 208. The analog
signals sent by modem 208 over analog local loop 207 are digitized at
interface 206 for transport across the digital backbone network 210
and the digital connection 203 to modem 202.
Communications over the telephone network are affected by
various types of distortions in both the analog and digital portions of
the network. Of particular relevance to the present invention are
signal reflections which echo the transmitted signal back to the
transmitting modem. These signal reflections are induced at various
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points within the network, specifically at the hybrid interfaces
within the modems and within the telephone central office
equipment, and arrive at the receiver of the transmitting modem at
varying times based on the distance between the transmitting modem
and each reflection point. Reflections generated within the local loop
are received sooner, and with greater signal power, than reflections
generated at the far end of the network. For convenience, reflections
generated within the local loop are referred to as "near-end
reflections" or "near-end echoes," and reflections generated at the
far end of the network are referred to as "far-end reflections" or
"far-end echoes."
An exemplary relationship between the near-end and far-end
echoes is shown in FIG. 3. As discussed previously, the near-end
echoes generated within the local loop are received sooner, and with
greater signal power, than the far-end echoes generated at the far
end of the network. The peak of the near-end echo is substantially
synchronized with the receipt of the echo produced from the hybrid in
the transmitting modem, and therefore the peak of the near-end echo
occurs after a fixed) internal delay (referred to as "sys delay"). The
sys delay is typically a constant value, and can be determined by
placing the transmitting modem in a local analog loopback and
measuring the loopback delay. Likewise, the peak of the far-end echo
is substantially synchronized with the receipt of the echo produced
from the hybrid in the remote modem, and therefore the peak of the
far-end echo is based on the round-trip delay between the
transmitting modem and the remote modem. The round-trip delay is
typically measured during an initial startup sequence) and is
described in more detail below.
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The span of each echo is relatively short, on the order of 10 to
20 milliseconds, and the far-end echo is delayed by an amount B
which is based on the round-trip delay between the transmitting
modem and the remote reflection points. If the round-trip delay is
S relatively long, as shown in FIG. 3) then there will be substantially no
reflective energy between the near-end echo and the far-end echo.
However, if the round-trip delay is relatively short, then the far-end
echo may overlap the near-end echo either partially or completely.
As can be seen in FIGs. 1 and 2, the types of reflections that
occur in the network depends on the topology of the network. in an
analog-to-analog connection as shown in FIG. 1, each modem receives
both near-end and far-end reflections. The near-end reflections are
produced at the transmitting modem's hybrid interface and at the
central office equipment on the local side of the network, while the
far-end reflections are produced at the central office equipment on
the remote side of the network and at the remote modem's hybrid
interface. In a digital-to-analog connection as shown in F1G. 2,
however, the digital side modem receives only far-end reflections)
while the analog side modem receives only near-end reflections.
With respect to a particular modem, reflections of the modem's
transmitted signals have the effect of distorting the signals received
by the modem. Specifically, the signals received by the modem
include signals from a remote modem combined with the reflected
signals from the modem's own transmissions. Unless the reflected
signals are removed from the received signal or are otherwise
compensated for by the modem, incorrect information may be
extracted from the received signals resulting in corrupted data.
Accordingly, the modem includes logic for cancelling the near-
end and far-end reflections, typically in the form of a symbol-driven
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echo canceller. The symbol-driven echo canceller emulates the signal
reflections and produces an echo cancelling signal which, when
subtracted from the signals received by the modem, substantially
eliminates the echoes from the received signals. Because the near-
s end and far-end reflections are delayed in time and are dependent
upon the length of the signal paths in the network, the total echo
channel response varies from network to network (and even
connection to connection) and can be very long (on the order of 600
milliseconds) in certain situations. Emulation of such channels using
a single filter is possible but unrealistic due to the computational
complexity. Therefore, the symbol-driven echo canceller typically
includes two separate filters for constructing the near-end and far-
end echo channels, respectively, each capable of emulating the span
of its respective echo channel. For convenience, the filter for
constructing the near-end echo channel is referred to as the "near-
end echo canceller" and the filter for constructing the far-end echo
channel is referred to as the "far-end echo canceller."
An exemplary embodiment of a symbol-driven echo canceiler is
shown in FIG. 4. Specifically, the symbol-driven echo canceller
includes a symbol modulator 402, a bulk-delay line 404, and two
complex-valued adaptive transversal filters 406 and 408 for
constructing the near-end and far-end echo channels, respectively.
Each symbol from the symbol modulator 402, representing a
transmitted symbol, is placed in the bulk-delay line 404 where it is
held until needed by the near-end echo canceller 406 and the far-end
echo canceller 408. Each symbol is delayed by a first delay time
before being processed by the near-end echo canceller 406 and by a
second delay time before being processed by the far-end echo
canceller 408. The outputs of the near-end echo canceller 406 and
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the far-end echo canceller 408 are combined to form echo cancelling
signal 410. The first and second delay times are determined
according to measured delays such that the echo cancelling signal
410 is provided to the receiver substantially in synchronization with
the corresponding near-end and far-end echoes received by the
modem, and are described in more detail below.
The relationship between the bulk-delay line, the near-end echo
canceller) and the far-end echo canceller is shown in FIG. 5.
Modulated symbols are inserted into the bulk-delay line at the
insertion point and remain in the bulk-delay fine for a predetermined
number of symbol times. The echo canceller filters can be
conceptualized as being projected onto the bulk-delay line such that
each filter begins at a particular symbol offset from the insertion
point and spans a predetermined number of symbols) where the
number of symbols spanned by each filter is determined by the
number of taps in the filter. A particular symbol is inserted at the
insertion point) is .operated upon by the near-end echo canceller after
the first delay time, and is operated upon by the far-end echo
canceller again after the second delay time.
In a preferred embodiment, the symbol-driven echo canceller is
implemented in software. The bulk-delay line is implemented as a
circular buffer, and three pointers are used to indicate the point
where a symbol is inserted (put_ptr) and the two points where
symbols are read for processing by the near-end echo canceller
(ne~et) and the far-end echo canceller (feet). The near-end echo
canceller and the far-end echo canceller each process one symbol per
symbol time according to the get pointer for the respective echo
canceller. Each time a symbol is inserted at the put_ptr, all three
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pointers are advanced by one symbol such that the ne_get and feet
pointers remain at fixed offsets from the put_ptr.
FICs. 6A and 6B show details of a preferred embodiment of a
near-end or far-end echo canceller. In the preferred embodiment) the
receiver sampling rate is three times the transmitter symbol rate.
Therefore, for each modulated symbol S~ received from the bulk-delay
line) the echo canceller produces three echo canceller output signals)
one for each of three consecutive receiver samples R~(0), R~(1)) and
R"(2) corresponding to symbol S~.
Because only one input symbol is available for generating the
three echo canceller output signals, the echo canceller filter
interpolates the three echo canceller output signals from the single
input symbol. Specifically, the echo cancelfer filter is updated three
times for each input symbol, where each update is based on the input
symbol and one of three consecutive feedback samples e~(0), e~(1 ),
and e~(2) which are the three echo cancelled receiver samples
immediately preceding receiver samples R~(0), R~(1 ), and R~(2).
As shown in FIG. 6A, the echo cancelier filter can be
conceptualized as a shift register having X filter taps which is
shifted and updated three times for each input symbol S~. For the
first update, symbol S~ is effectively placed in filter tap 0, and the
filter is updated using feedback sample e~(0). At that time, there is
effectively no symbol available for updating filter taps 1 and 2.
Therefore) the filter update affects only filter tap 0 and every third
filter tap therefrom (corresponding to previously received symbols)
while the rest of the filter taps remain unchanged (indicated by the
value "0"). For the second update, symbol S~ is effectively shifted to
filter tap 1, and the filter is updated using feedback sample e"(1 ). At
that time, there is effectively no symbol available for updating filter
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taps 0 and 2. Therefore, the filter update affects only filter tap 1
and every third filter tap therefrom. For the third update, symbol S"
is effectively shifted to filter tap 2) and the filter is updated using
feedback sample a"(2). At that time, there is effectively no symbol
S available for updating filter taps 0 and 1. Therefore, the filter
update affects only filter tap 2 and every third filter tap therefrom.
The filter update cycle then begins again with the next symbol S"+,
using feedback samples a"+, {0)) a"+, (1 ), and e~+, {2) which are the echo
cancelled receiver samples R"(0), R"(1 )) and R"(2), respectively.
In the preferred embodiment, the echo cancelfer filter is not
implemented as a shift register having X filter taps. Such an
implementation would be wasteful of processing resources because
only one-third of the filter taps actually change during each filter
update. Instead, it is preferable to implement the echo canceller
filter using three subfilters, as shown in FIG. 6B. In this
embodiment, each subfilter is a complex-valued adaptive transversal
filter supporting X/3 filter taps which is updated using the Least
Mean Squared algorithm using feedback 606 from the receiver. For
each symbol 602 received from the bulk-delay line, for example,
symbol S", each subfilter is updated and produces one echo canceller
output signal 604. Specifically) subfilter 608 {corresponding to
filter tap 0 and every third filter tap therefrom) is updated using
symbol S" and feedback sample a"(0) and produces echo cancelier
output signal Y"(0), subfilter 610 {corresponding to filter tap 1 and
every third filter tap therefrom) is updated using symbol S" and
feedback sample a"{1 ) and produces echo canceller output signal Yn{1 ),
and subfilter 612 {corresponding to filter tap 2 and every third filter
tap therefrom) is updated using symbol S" and feedback sample a"(2)
and produces echo canceller output signal Y"{2). The echo canceller
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output signals Y"(0)) Y"(1 )) and Y"(2) from both echo cancellers are
effectively subtracted from the receiver samples R~(0)) R~(1), and
R~(2), respectively, thereby substantially removing the reflected
signals from the receiver samples R~(0), R"(1 ), and R~(2). The
resulting echo cancelled receiver samples are then used as feedback
samples e~+, (~), en+t (1 )~ and e"+, (2), respectively, for the next filter
update using the next input symbol S~+,.
In a typical modem) the near-end and far-end echo canceller
filters have a fixed number of filter taps sufficient to compensate
for reflections having a predetermined maximum span. For
convenience, these filters are referred to as "fixed-length" filters.
in these modems, the amount of processing resources consumed by
the echo canceller filters is not an issue, since the echo canceller
filters are typically implemented using hardware which dedicates a
sufficient amount of processing resources to the echo canceller
filters. Therefore, even if the span of signal reflections encountered
in the telephone network are of shorter duration than the
predetermined maximum span, there is no compelling reason to
eliminate or reduce the number of taps in the echo canceller filters
in order to save processing resources.
fn an embodiment of the present invention, the echo canceller
filters are designed and implemented so that the number of taps is
variable and adaptable to the actual span of the reflections. For
convenience, these filters are referred to as "variable-length"
filters. Each variable-length filter is designed and implemented to
support the same number of taps as the corresponding fixed-length
filter) and therefore each variable-length filter remains capable of
compensating for reflections having a predetermined maximum span.
However, the number of taps actually used for echo cancellation is
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adapted based on the power level and span of the actual signal
reflections as measured by the modem. Because the span of signal
reflections encountered in modern telephone networks is typically of
shorter duration than the predetermined maximum span, it is
generally possible to eliminate at least some of the echo canceller
filter taps and to thereby reduce the amount of processing resources
consumed by the echo canceller filters.
In another embodiment of the present invention, either one or
both of the echo cancellers can be disabled completely such that the
eliminated echo canceller(s) consumes no processing resources (other
than perhaps some overhead for conditional execution of software).
Referring to FIG. 2, modem 202 receives only a far-end echo, while
modem 208 receives only a near-end echo. Therefore) in a preferred
embodiment, the near-end echo canceller of a modem such as modem
202 is disabled completely, while the far-end echo canceller of a
modem such as modem 208 is disabled completely. Furthermore, an
alternate embodiment completely disables the far-end echo canceller
if the delay between the near-end echo and the far-end echo is either
zero or is so small that any far-end echo can be satisfactorily
handled by the near-end echo canceller alone.
Programming of the echo cancellers is typically performed
during a startup sequence in which the modem characterizes the
communication link and exchanges operating parameters with the
remote modem. In a preferred embodiment, the modem supports a
startup sequence such as the ITU V.34 startup sequence which
includes, among other things, a Phase 2 for measuring round-trip
delay and for exchanging certain operating parameters, and a Phase 3
for training the echo canceliers.
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FIG. 7 shows generally the steps of the prior art V.34 startup
sequence that are relevant to the present discussion. After beginning
in step 702 and going through at least a first startup phase (not
shown)) the modem performs the Phase 2 sequence which includes
measuring a round-trip delay (RTDE) and exchanging certain operating
parameters in step 704. Of particular relevance to the present
invention is the exchange of information indicating whether or not
one of the modems is operating on the digital side of a digital-to-
analog connection (for example, modem 202 in FIG..2). Subsequent to
measuring the RTDE, the modem sets up its echo cancellers for a
worst-case echo in step 706 and performs the Phase 3 sequence
which includes training the echo cancellers in step 710. The modem
then continues on with remaining phases of the startup sequence (not
shown), ultimately terminating in step 799.
In a preferred embodiment of the present invention, a number of
echo canceller adjustments are made during the startup sequence in
order to adapt the echo canceller filters to the reflections in the
network.
In one embodiment, the modem completely disables its near-end
echo canceiler if it is operating on the digital side of a digital-to-
analog connection. As discussed previously, a modem operating on
the digital side of a digital-to-analog connection does not require a
near-end echo canceller, since only far-end reflections are present.
By completely eliminating the near-end echo canceller) the amount of
processing resources required by the symbol-driven echo canceller is
substantially reduced.
In another embodiment) the modem completely disables its far-
end echo canceller following Phase 2 if the remote modem indicates
that the remote modem is operating on the digital side of a digital-
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to-analog connection. As discussed previously, a modem operating on
the analog side of a digital-to-analog connection does not require a
far-end echo canceller, since only near-end reflections are present.
By completely eliminating the far-end echo canceller, the amount of
processing resources required by the symbol-driven echo canceller is
substantially reduced.
In yet another embodiment, the modem completely disables its
far-end echo canceller following Phase 2 if the RTDE is zero or is
less than a predetermined value such that the span of the far-end
echo canceller is entirely within the span of the near-end echo
cancelier. When the round-trip delay is very small, there is
significant overlap between the near-end echo and the far-end echo.
In this case) utilization of the near-end echo canceller alone is
sufficient to compensate for both near-end and far-end echoes. By
completely eliminating the far-end echo canceller, the amount of
processing resources required by the symbol-driven echo canceller is
substantially reduced.
After any adjustments are made following Phase 2, the
remaining near-end and far-end echo canceller fitters are initialized
(step 706 in FIG. 7). The step of setting up the echo cancelier for a
worst-case echo involves projecting the remaining near-end and far-
end echo canceller filters onto the bulk-delay line at appropriate
offsets from the insertion point. Each echo canceller filter is
initialized for its maximum span (48 symbols for the near-end filter
and 35 symbols for the far-end filter in the preferred embodiment)
and is projected onto the bulk-delay line such that the peak of each
echo response falls roughly at a predetermined point within the span
of the respective filter (2/5 of the filter span for the near-end filter
and 1/3 of the filter span for the far end filter in the preferred
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embodiment). In a preferred embodiment, positioning the echo
cancelier -filters involves converting the sys delay and RTDE times
into an integral number of symbols for determining a first symbol
boundary and a second symbol boundary) respectively, and then
placing the 2/5 point of the near-end filter at the first symbol
boundary and placing the 113 point of the far-end filter at the second
symbol boundary. Since neither the 215 point of the near-end filter
nor the i/3 point of the far-end filter is an integral number of
symbols, the numbers are rounded down to the nearest integral
number of symbols. Thus, in a preferred embodiment, the near-end
echo cancelier filter is positioned onto the bulk-delay line such that
the ne_get pointer precedes the first symbol boundary by 19 symbols
(~0.4*48~), while the far-end echo canceller filter is positioned onto
the bulk-delay line such that the fe get pointer precedes the second
symbol boundary by 11 symbols (~0.333*35~). This positioning of the
filters, shown generally in FIG. 8A and in more detail in FIG. 8B,
places the peak of each echo response at an appropriate point within
each filter in order to more effectively emulate the echo channels.
For convenience) the symbol boundary for a filter is referred to as the
"center" of the filter, and the taps on either side of the center are
referred to as a "tail" of the filter.
After initializing the remaining fiiter(s), and before training
the filter(s)) a determination is made whether or not the far-end echo
canceller filter can be eliminated or truncated. This adjustment is
only possible if both the near-end and far-end echo cancelier filters
remain (i.e., neither was previously eliminated) and span of the far-
end echo canceller filter overlaps the span of the near-end echo
canceller filter.
*rB
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The far-end echo canceller filter is eliminated if the entire
span of the far-end echo canceller filter is within the span of the
near-end echo canceller filter. This can happen) for example, when
the RTDE is very small. In the preferred embodiment where the near-
s end echo canceller filter span is 48 symbols and the far-end echo
canceller filter span is 35 symbols, this situation occurs if the start
of the far-end echo cancelier filter (i.e., the fe_get pointer) is less
than or equal to 13 symbols behind the start of the near-end echo
canceller filter (i.e., the ne_get pointer). By completely eliminating
the far-end echo canceller, the amount of processing resources
required by the symbol-driven echo cancelfer is substantially
reduced.
Even if the entire span of the far-end echo cancelier filter is
not within the span of the near-end echo canceller filter, the far-end
echo canceller filter may be truncated if its span partially overlaps
the span of the near-end echo canceller filter (i.e., at least one
symbol is within the span of both fitters). In the preferred
embodiment where the near-end echo canceller filter span is 48
symbols and the far-end echo canceller filter span is 35 symbols,
this situation occurs if the start of the far-end echo cancelier filter
(i.e.) the fe_get pointer) is less than 48 symbols behind the start of
the near-end echo canceller filter (i.e., the ne get pointer). An
example of overlapping filter spans is shown in FIG. 9. In this case,
the far-end echo canceller filter is shortened by moving the fe_get
pointer to the symbol following the end of the near-end echo
canceller filter as shown in FIG. 10. Truncating the far-end echo
canceller filter reduces the span of the filter and thus reduces the
number of filter taps. By truncating the far-end echo canceller
filter, the amount of processing resources required by the symbol-
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driven echo canceller is reduced by an amount proportional to the
number of filter taps eliminated from the far-end echo canceller
filter.
In the example shown in FIG. 10, the truncated far-end echo
canceller filter continues to have two tails. It is important to note,
however, that if the far-end echo canceller filter overlaps the near-
end echo canceller filter such that the center of the far-end filter is
within the span of the near-end filter, then truncating the far-end
filter will eliminate the tail shown on the right-hand side of the
filter and may eliminate some or all of the taps on the left-hand side
of the filter (all of the far-end filter taps may be eliminated, for
example, if the RTDE is zero). Thus, in a preferred embodiment) the
step of eliminating the far-end echo canceller filter if the RTDE is
below a predetermined threshold (described above) is not employed)
and instead this step of truncating the far-end echo cancelier filter
is used and will have the effect of completely eliminating the far-
end echo canceller filter if the RTDE is indeed below the
predetermined threshold.
After making any echo canceller filter adjustments prior to
training the echo canceller filters) the remaining echo canceiler
filters) are trained as part of Phase 3 of the startup sequence (step
710 in FIG. 7). This step of training the echo canceller filters allows
each filter to adapt to its respective echo channel and to thereby
produce an appropriate echo canceller output signal.
After training the remaining echo canceller filter(s), a final
adjustment is made to eliminate any unessential filter taps from
each remaining filter. In a preferred embodiment, the filter taps are
examined in groups of three (where each group of three filter taps
corresponds to one symbol) beginning at the end of each tail and
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working in toward the center. A group of three echo canceller filter
taps is deemed to be unessential if the echo power level for the
corresponding symbol is below a predetermined threshold, and is
otherwise deemed to be essential. In the preferred embodiment) the
predetermined threshold for the near-end echo canceller is l.Oe-5,
and the predetermined threshold for the far-end echo canceller is
l.Oe-3. Where the magnitude M of a filter tap n is a complex
coefficient of the form:
M"=R"+Jh,
the echo power level P for three taps is determined according to the
formula:
P-gR~2+hz
for n = 1 to 3. Paring of a particular tail is stopped once an essential
group of three filter taps is found or if the entire tail is eliminated.
By reducing the number of filter taps in the remaining filter(s), the
amount of processing resources required by the symbol-driven echo
canceller is reduced by an amount proportional to the number of filter
taps eliminated.
It is important to note that this final adjustment may be used
as the default mechanism for eliminating an echo canceller under
certain circumstances. This is particularly important for a modem
operating on the analog side of a digital-to-analog connection
because the information exchanged during Phase 2 may be
insufficient to identify the connection as a digital-to-analog
connection when the connection rate is less than 56 kilobitslsecond.
Therefore, the modem may initially leave its far-end echo canceller
enabled) but subsequently eliminate its far-end echo canceller when
all of the filter taps are found to be unessential due to the absence of
a far-end echo.
*rB
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Thus, in an exemplary embodiment as shown in FIG. 11, a number
of steps are taken during the startup sequence in order to adapt the
echo canceller filters to the reflections in the network. After
beginning in step 1102 and going through at least a first startup
5 phase (not shown)) the modem disables its near-end echo canceller in
step 1104 if the modem is operating on the digital side of a digital-
to-analog connection. The Phase 2 sequence, which includes
measuring a round-trip delay (RTDE) and exchanging certain operating
parameters, is performed in step 1106. Subsequent to performing the
10 Phase 2 sequence, the modem disables its far-end echo canceller in
step 1108 if the modem is operating on the analog side of a digital-
to-analog connection as determined by the information exchanged
during Phase 2. Optionally, the modem may also disable its far-end
echo canceller in step 1110 if the RTDE is less than a predetermined
15 threshold, although this step is not used in the preferred embodiment.
In preparation for Phase 3) the modem then sets up the remaining
echo canceller(s) for a worst-case echo in step 1112. After setting
up the echo canceller(s), the modem eliminates the far-end echo
canceller in step 1114 if the span of the far-end echo canceller is
20 entirely within the span of the near-end echo canceller, and truncates
the far-end echo canceller in step 1116 if the span of the far-end
echo canceller only partially overlaps the span of the near-end echo
canceller. Steps 1114 and 1116 are not applicable if either of the
echo canceller filters had been eliminated prior to step 1112. The
modem then performs the Phase 3 sequence which includes training
the echo canceller(s) in step 1118. After training the echo
canceller(s), the modem adapts the echo canceller(s) in step 1120 by
eliminating any unessential filter taps. The modem then continues on
with remaining phases of the startup sequence (not shown),
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ultimately terminating in step 1199. It will be apparent to a skilled
artisan that the techniques for eliminating, truncating, and adapting
the echo cancellers may be applied individually or in combination
with each other.
The techniques for eliminating, truncating) andlor adapting the
echo cancellers may be embodied in hardware, firmware, or software.
An exemplary implementation of a variable-length echo canceller
such as a software modem or other modem includes logic or computer
readable program code means for disabling an echo canceller,
truncating a far-end echo canceller, andlor adapting an echo
canceller by eliminating any unessential filter taps. Again ) it will be
apparent to a skilled artisan that the logic or computer readable
program code means for eliminating, truncating, andlor adapting the
echo cancellers may be applied individually or in combination with
each other.
While the techniques of the present invention have been
described in relation to a software modem for the purpose of
reducing the amount of processing resources required by the software
modem, it will be apparent to a skilled artisan that the same
techniques are applicable to echo cancellers generally. For instance,
it is well known that filters are typically less than perfect, and that
a fitter which is too long can actually amplify noise. Therefore, the
techniques for eliminating, truncating, andlor adapting the echo
canceller filters are useful for adapting each filter to an appropriate
length so that the filter does not generate excessive noise.
Finally, it will be apparent to a skilled artisan that, by
eliminating or reducing the number of echo canceller filter taps when
an echo is present, any reduction in the consumption of processing
resources may be realized at a cost in echo canceller performance.
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As described above, a group of three echo canceller filter taps is
deemed unessential and is eliminated if the echo power level at the
symbol corresponding to the filter taps is below a predetermined
threshold. Therefore, it is possible that, by eliminating certain filter
taps, echo signal energy which would nave been cancelled by those
taps is left in the received signal as noise which must then be
overcome by the receiver. Thus, there is generally a tradeoff
between the filter length and the filter performance, and the
predetermined threshold is selected so that the echo canceller
performance remains at all times within acceptable operating limits.
The present invention may be embodied in other specific forms
without departing from the essence or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative and not restrictive.
