CN1247664A - Adaptive device and method for echo canceller in communication system - Google Patents

Abstract

An apparatus and method for adapting an echo canceller in a communication system reduces the number of taps in the echo canceller filter in order to reduce the amount of processing resources consumed by the echo canceller. A near-end echo canceller (406) is disabled if the modem (402) is operating on the digital side of a digital-to-analog connection. A far-end echo canceller (408) is disabled if the modem (402) is operating on the analog side of the digital-to-analog connection or if the round-trip delay is below a predetermined threshold. If the widening of the far-end echo canceller covers the widening of the near-end echo canceller (406), the far-end echo canceller (408) is truncated.

Description

Adaptive device and method for echo canceller in communication system

1. Field of invention

This invention relates generally to communication systems, and more particularly to variable wavelength echo cancellers for modems.

2. Discussion on related technologies

In today's information age, the continued proliferation of personal computers used in homes, schools, and businesses seems apparently endless. The continued growth in the use of personal computers has driven the expansion of broadband use of computer networks that can provide an increasing number of on-line services, like the Internet. Although computer and communication technology has advanced greatly over time, it is still very common for users to use a modem to connect their personal computers to such computer networks through telephone lines.

Modems have historically been an add-on feature of personal computers, either as a peripheral device connected by a cable to the personal computer or as an internal device installed directly into an expansion slot in the personal computer. These modems typically include all the hardware and software components needed to provide modem functionality to a personal computer. In particular, modems typically include a microprocessor (and associated memory circuitry) for performing general software functions, input/output ports for data and control interfaces, signal processing for performing computationally intensive signal processing functions processor and many dedicated hardware sections for interfacing with the telephone network and for providing additional signal processing functions. These specialized hardware parts make modems quite expensive.

Today, there is a need for an inexpensive modem that can be built into a personal computer by the manufacturer or packaged with other external devices to provide the modem function for the personal computer. One approach is to implement primarily all modem processing functions in software (the remainder in dedicated hardware) and run the software as an application on a microprocessor in a personal computer. This type of modem, often referred to as a "software modem," is feasible today due to the large processing resources provided by modern microprocessors. Software modems can be relatively inexpensive by removing most of the dedicated hardware parts from the modem and utilizing the processing and memory resources of a personal computer.

One problem with software modems is that the modem software must share the processing resources of the personal computer with other application software, like word processors, spreadsheet programs or Internet browsers. This "feature" of software modems, while making software modems inexpensive, consumes processing resources that would otherwise be available to other application software. As a result, software modem introduction apparently affects the performance of other application software. Thus, one goal of a software modem is to provide full modem functionality using as few processing resources as possible.

In software modems, the echo canceller, which is used to compensate for signal reflections in the telephone network, represents a major part of the total amount of processing resources required by the modem. Typically two separate echo cancellers are used, one for near-end echo cancellation and one for far-end echo cancellation. These echo cancellers are typically designed and implemented to compensate for a worst-case (eg, maximum span) echo. Since the amount of processing resources required to implement an echo canceller in software is directly proportional to the number of taps in the echo canceller filter, and the number of taps is directly proportional to the span over which the echo canceller operates, it is designed And an echo canceller implemented to compensate for a worst-case echo necessarily consumes the maximum amount of processing resources. Thus, a need exists for a system, device, and method for reducing the number of taps in an echo canceler filter, thereby reducing the amount of processing resources consumed by the echo canceller.

graphic description

In the picture,

Figure 1 shows an analog to digital modem connection known in the art;

Figure 2 shows a digital-to-digital modem connection known in the art;

Figure 3 shows an exemplary relationship between near-end and far-end echoes;

Figure 4 shows an exemplary embodiment of a sign-driven echo canceller;

Figure 5 shows the relationship between the bulk-delay line, the near-end echo canceller and the far-end echo canceller;

Figure 6A shows an overview of an echo canceller filter with X filter taps;

Figure 6B shows a preferred embodiment of the near-end and far-end echo cancellers implemented using 3 sub-filters;

Figure 7 generally represents the relevant steps of the prior art V.34 start-up sequence;

FIG. 8A shows exemplary positioning of near-end and far-end echo canceller filters once initialized;

Figure 8B shows in more detail the positioning of the echo canceller filter once initialized;

Figure 9 shows exemplary positioning of the near-end and far-end echo canceller filters when the spread of the far-end filter overlaps with the spread of the near-end filter;

Figure 10 shows exemplary positioning of the near-end and far-end echo canceller filters when the spread of the far-end filter is truncated;

Figure 11 shows an exemplary embodiment comprising a number of steps taken during the start-up sequence in order for the echo canceller filter to adapt to reflections in the network.

A detailed description

As discussed above, there is a need for apparatus and methods that reduce the number of taps in an echo canceller filter in order to reduce the amount of processing resources consumed by the echo canceller. Embodiments of the present invention completely remove the near-end or far-end echo canceller under certain conditions, and reduce the number of taps in the near-end and far-end echo canceller filters under other conditions. As a result, these embodiments address the need to reduce the amount of processing resources consumed by an echo canceller.

Figure 1 shows a modem-to-modem connection as known in the art, in which modems 102 and 108 are connected to the telephone network via conventional analog local loops 103 and 107, respectively. 2 modems send and receive analog signals. The analog signal sent by modem 102 via analog local loop 103 is digitized at interface 104 for transmission via digital backbone network 110 and converted back to analog at interface 106 for transmission to modem 108 via analog local loop 107 . Similarly, an analog signal sent by modem 108 via analog local loop 107 is digitized at interface 106 for transmission via digital backbone network 110 and converted back to an analog signal at interface 104 for transmission to modem 102 via analog local loop 103 .

Figure 2 shows a modem-to-modem connection as known in the art, where modem 208 is connected to the telephone network via a conventional analog local loop 207 and modem 202 is connected to the telephone network via a digital connection 203. Modem 202 sends and receives digital signals, while modem 208 sends and receives analog signals. A digital signal sent by modem 202 via digital connection 203 is transmitted across digital backbone network 210 and converted back to an analog signal at interface 206 for transmission to modem 208 via analog local loop 207 . The analog signal sent by modem 208 via analog local loop 207 is digitized at interface 206 for transmission to modem 202 via digital backbone network 210 and digital connection 203 .

Communications over the telephone network are subject to various distortions in the analog and digital parts of the network. Of particular relevance to the present invention are signal reflections that echo a transmitted signal back to the transmitting modem. These signal reflections are introduced at many points within the network, particularly at the modem and at the hybrid interface at the telephone central office equipment, and arrive at the transmitting modem's receiver at different times depending on the distance between the transmitting modem and each reflection point. Reflections generated within the local loop are received more quickly with greater signal power than reflections generated at the far end of the network. For brevity, reflections that occur within the local loop are called "near-end reflections" or "near-end echoes," and reflections that occur at the far end of the network are called "far-end reflections" or "far-end echoes."

An exemplary relationship between the near-end and far-end echoes is shown in Figure 3. As previously discussed, the near-end echo generated within the local loop is stronger with greater signal power than the far-end echo generated at the far end of the network. Receive quickly. The peak of the near-end echo is substantially synchronous with the reception of the echo generated by the mixing from the transmitting modem, so the near-end echo appears after a fixed internal delay (called "sys_delay"). sys_delay is typically a constant value determined by placing a transmit modem in the local analog feedback and measuring the feedback delay. Likewise, the peak of the far-end echo is substantially synchronous with the reception of the echo produced by mixing from the remote modem, so the peak of the far-end echo is based on the round trip delay between the sending modem and the remote modem. Certainly. A round trip delay, which will be described in more detail below, is typically measured during the initial determination sequence.

The spread of each echo is relatively short, on the order of 10-20 microseconds, and the far-end echo is delayed by an amount B determined by the round-trip delay between the sending modem and the remote reflection point. If the round-trip delay is rather long as shown in Figure 3, there is essentially no reflected energy between the near-end echo and the far-end echo. However, if the round-trip delay is relatively short, the far-end echo will partially or completely overlap with the near-end echo.

As can be seen in Figures 1 and 2, the type of reflection that occurs in the network depends on the network topology. In an analog-to-analog connection as shown in Figure 1, each modem receives both near-end and far-end reflections. Near-end reflections occur at the hybrid interface of the sending modem and at the central office equipment on the local side of the network, while far-end reflections occur at the central office equipment on the remote side of the network and at the hybrid interface of the remote modem. However, in a digital-to-analog connection as shown in Figure 2, the modem on the digital side only receives far-end reflections and the modem on the analog side receives only near-end reflections.

For a particular modem, reflections of the modem's transmitted signal have the effect of distorting the signal received by the modem. In particular, the signal received by the modem includes a combination of the signal from the remote modem and the reflected signal sent from the modem itself. Unless the reflected signal is removed from the received signal or compensated by the modem, incorrect information will be extracted from the received signal, resulting in corrupted data.

Accordingly, the modem includes logic to cancel near-end and far-end reflections, typically in the form of a symbol-driven echo canceller. The symbol-driven echo canceller simulates signal reflections and produces an echo-canceled signal which, when subtracted from the signal received by the modem, substantially cancels the echo in the received signal. Since the near-end and far-end reflections are delayed in time and are related to the length of the signal path in the network, the overall echo channel response varies from network to network (or even connection to connection) and can be very long in some conditions ( on the order of 600 milliseconds). It is possible but impractical to simulate these channels with a single filter due to computational complexity. Therefore, a symbol-driven echo canceller typically includes two separate filters for constructing near-end and far-end echo channels, each capable of simulating the broadening of the respective echo channel. For simplicity, the filter used to construct the near-end echo channel is called a "near-end echo canceller", and the filter used to construct the far-end echo channel is called a "far-end echo canceller".

An exemplary embodiment of a sign-driven echo canceller is shown in FIG. 4 . In particular, the symbol-driven echo canceller includes a symbol modulator 402, an individual delay line 404, and two complex-valued adaptive transversal filters 406 and 408 for constructing near-end and far-end echo channels respectively. Each symbol representing a transmitted symbol from symbol modulator 402 is placed in bulk delay line 404 where it is held until needed by near-end echo canceller 406 and 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 the far-end echo canceller 408 are combined to form an echo-canceled signal 410 . The first and second delay times are determined based on the measured delays so that the echo cancellation signal 410 is provided to the receiver substantially synchronously with the corresponding near-end and far-end echoes received by the modem, as described in more detail below.

The relationship between bulk delay line, near-end echo canceler and far-end echo canceler is shown in Fig.5. The modulated symbols are inserted into the volume delay line at the insertion point and remain in the volume delay line for a predetermined number of symbol times. Conceptually the echo canceller filters can be thought of as being designed into the delay line such that each filter starts at an offset of one particular symbol from the insertion point and extends by a predetermined number of symbols, where by each filter The number of symbols in is determined by the number of taps in the filter. A particular symbol is inserted at the insertion point and is manipulated by the near-end echo canceller after a first delay time and again by the far-end echo canceller after a second delay time.

In a preferred embodiment, the sign-driven echo canceller is implemented in software. Bulk delay line 404 is implemented as a circuit-level buffer, 3 pointers are used to indicate the point where symbols are inserted (put_ptr) and symbols are read for use by the near-end echo canceller (ne_get) and the far-end echo canceller 2 points for (fe_get) processing. The near-end echo canceller and the far-end echo canceller each process one symbol per symbol time according to the get pointers for the respective echo cancellers. Every time a symbol is inserted in put_ptr, all 3 pointers are advanced by one symbol so that ne_get and fe_get maintain a fixed offset from put_ptr.

Figures 6A and 6B show details of a preferred embodiment of the near-end echo canceller and the far-end echo canceller. In the preferred embodiment, the receiver sampling rate is three times the transmitter symbol rate. Thus, for each modulated symbol S _n received from the bulk delay line, the echo canceller produces 3 echo canceller output signals, one for each of the 3 consecutive receiver samples _{R n (} 0 ), each of R _n (1) and R _n (2).

Since only one input symbol is available to generate the 3 echo canceller output signals, the echo canceller filter interpolates the 3 echo canceller output signals from a single input symbol. Specifically, the echo canceller filter is updated 3 times for each input symbol, where each update is based on the input symbol and three consecutive feedback samples e _n (0), e _n (1) and e _n (2) One of the three consecutive feedback samples is the 3 echo-cancelled receiver samples immediately preceding receiver samples _Rn (0), _Rn (1) and _Rn (2).

As shown in FIG. 6A , in principle, the echo canceller filter can act as a shift register with X filter taps shifted and updated 3 times for each input symbol S _n . For the first update, the symbol S _n is effectively placed at filter tap 0, and the filter is updated using feedback samples e _n (0). At that time, no symbols are effectively available to update filter taps 1 and 2 . Thus, the filter update only affects filter tap 0 and every 3rd filter tap from then on (corresponding to previously received symbols), while the remaining filter taps remain unchanged (indicated by a "0" value). For the second update, the symbol S _n is effectively shifted to filter tap 1, and the filter is updated using feedback samples e _n (1). At that time, no symbols are effectively available to update filter taps 0 and 2 . Therefore, filter updates only affect filter tap 1 and every 3rd filter tap from there. For the third update, the symbol S _n is effectively shifted to filter tap 2, and the filter is updated using feedback samples e _n (2). At that time, no symbols are effectively available to update filter taps 0 and 1 . Thus, the filter update point affects filter tap 2 and every 3rd filter tap from there. The filter update period then _uses feedback samples e _{n +} 1 (0), e _{n +1} (1) and e _n+1 (2), start again from the next symbol S _n+1 .

In the preferred embodiment, the echo canceller filter is not implemented as a shift register with X filter taps. Such an implementation would be a waste of processing resources since only 1/3 of the filter taps actually change during each filter update. Instead, as shown in Figure 6B, it is advisable to implement the echo canceller filter using 3 sub-filters. In this embodiment, each sub-filter is a complex-valued adaptive transversal filter supporting X/3 filter taps, the taps being updated using the least mean square algorithm using feedback 606 from the receiver. For each symbol 602 received from the bulk delay line, eg symbol S _n , each sub-filter is updated and produces an echo canceller output signal 604 . In particular, sub-filter 608 (corresponding to filter tap 0 and every 3rd filter tap from there) is updated using sign S _n and feedback samples e _n (0) and produces echo canceller output signal Y _n (0 ), sub-filter 610 (corresponding to filter tap 1 and every 3rd filter tap from there) is updated using sign S _n and feedback samples e _n (1) and produces echo canceller output signal Y _n (1) , sub-filter 612 (corresponding to filter tap 2 and every 3rd filter tap from there) is updated using sign S _n and feedback samples e _n (2) and produces echo canceller output signal Y _n (2). The echo canceller output signals Y _n (0), Y _n (1) and Y _n (2) from all echo cancelers are effectively sampled from the receivers R n (0), _{R n (} 1) and R _n (2), thereby substantially removing the reflected signal from the receiver samples _Rn (0), _Rn (1) and _Rn (2). The resulting echo canceller receiver samples are then used as feedback samples e _n+1 (0), e _n+1 (1) and e _n+1 (2), respectively, for the next input symbol S _n+1 Do the next filter update.

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 broadening. For simplicity, these filters are referred to as "fixed length" filters. In these modems, the amount of processing resources consumed by the echo canceller filter is not an issue because the echo canceller filter is usually implemented in hardware which provides a sufficient amount of processing resources for the echo canceller filter. Thus, even if the signal reflections experienced in the telephone network are spread over shorter intervals than a predetermined maximum spread, there is no good reason to remove or reduce the number of taps in the echo canceller in order to save processing resources.

In one embodiment of the invention, the echo canceller filter is designed and implemented so that the number of taps is variable and adapted to the actual broadening of the reflection. For simplicity, 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, so each variable length filter remains capable of compensating reflections with a predetermined maximum spread. However, in practice the number of taps used for the echo canceller is adapted according to the power level measured by the modem and the spread of the actual signal reflection. Since the spread of signal reflections encountered in modern telecommunication networks is an interval shorter than a predetermined maximum spread, it is usually possible to remove at least some echo canceler filter taps thereby reducing the amount of processing resources consumed by the echo canceller filter.

In another embodiment of the invention, either or both echo cancellers can be disabled completely so that the disabled echo canceller consumes no processing resources at all (except perhaps some overhead for software conditional execution). Referring to FIG. 2, modem 202 only receives far-end echoes, and modem 208 only receives near-end echoes. Therefore, in a preferred embodiment, the near-end echo canceller of a modem like modem 202 is completely disabled, and the near-end echo canceller of a far-end modem like modem 208 is completely disabled. Further, another embodiment does not enable the far-end echo canceller at all if the delay between the near-end echo and the far-end echo is zero or so small that any far-end echo can be satisfactorily handled by the near-end echo canceller alone .

Programming of the echo canceller is typically performed during a start-up sequence, during which the modem characterizes the communication link and exchanges operating parameters with the remote modem. In a preferred embodiment, the modem supports a start-up sequence, like the ITU V.34 start-up sequence, which includes, inter alia, phase 2 for measuring round-trip delay and for exchanging certain operating parameters and for training echo cancellation stage 3 of the device.

Figure 7 shows the general steps of the prior art V.34 start-up sequence that are relevant to this discussion. Beginning in step 702 and following at least a first start-up phase (not shown), the modem executes a phase 2 sequence including measuring 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 a modem is operating on the digital side of the digital-to-analog connection (eg, modem 202 in FIG. 2). After measuring the RTDE, the modem sets up its echo canceller for worst case echo in step 706 and executes a phase 3 sequence including training the echo canceller in step 710 . The modem then proceeds to the remaining stages of the start-up sequence (not shown), finally terminating at step 799.

In a preferred embodiment of the invention, a number of echo canceller adjustments are made during the start-up sequence in order to adapt the echo canceller filter to reflections in the network.

In one embodiment, the modem does not enable its near-end echo canceller at all if it is operating on the digital side of the digital-to-analog connection. As discussed earlier, modems operating on the digital side of a digital-to-analog connection do not require a near-end echo canceller, since only far-end reflections occur. By removing the near-end echo canceller entirely, the amount of processing resources required to drive the echo canceller by symbols can be significantly reduced.

In another embodiment, the modem does not enable its far-end echo canceller after Phase 2 if the remote modem indicates that the remote modem is operating on the digital side of the digital-to-analog connection. As discussed earlier, modems operating on the analog side of a digital-to-analog connection do not require a far-end echo canceller, since only near-end reflections occur. By removing the far-end echo canceller entirely, the amount of processing resources required to drive the echo canceller by the symbols can be significantly reduced.

In yet another embodiment, after Phase 2, the modem completely disables its Far-end echo canceller. When the round-trip delay is very small, there is a significant overlap between the near-end echo and the far-end echo. In this case, the near-end echo canceller alone is sufficient to compensate both near-end and far-end echo. By removing the far-end echo canceller entirely, the amount of processing resources required to drive the echo canceller by the symbols can be significantly reduced.

。一般性地在图8A和更详细地在图8B中示出的滤波器的这种定位为了更有效地模拟回音信道，把每个回音响应的峰值放置在每个滤波器内的一个合适的点上。为简便起见，用于滤波器的符号边界被称为滤波器的“中心”，在中心的任何一侧的抽头被称为滤波器的“尾”。After some adjustments after stage 2, the remaining near-end and far-end echo canceller filters are initialized (step 706 in Figure 7). The step of building an echo canceller for a worst-case echo involves inserting the remaining near-end and far-end echo canceller filters in the bulk delay line at suitable offsets from the insertion point. Each echo canceller filter is initialized to its maximum spread (48 symbols for the near-end filter and 35 symbols for the far-end filter in the preferred embodiment) and inserted into the bulk delay line so that each The peak of each echo response falls approximately at a predetermined point within the spread of each filter (in the preferred embodiment, 2/5 of the filter spread for the near-end filter, 1/3 of the filter spread for the far-end filter filter widening). In a preferred embodiment, positioning the echo canceller filter includes converting the sys_delay and RTDE times to an integer number of symbols for determining the first and second symbol boundaries, respectively, before placing the near-end filter at the first symbol boundary 2/5 points of , place the far-end filter 1/3 point on the second symbol boundary. Since both the 2/5 point of the near-end filter and the 1/3 point of the far-end filter have an integer number of symbols, the number is rounded down to the nearest integer number of symbols. Thus, in a preferred embodiment, the near-end echo echo canceller is positioned to the volume delay line so that the ne_get pointer is 19 symbols ahead of the first symbol boundary while the far-end echo canceller is positioned to the bulk delay line so that the fe_get pointer is 11 symbols ahead of the second symbol boundary

. 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 a suitable point within each filter in order to simulate the echo channel more effectively. superior. For brevity, the symbol boundary used for the filter is referred to as the "center" of the filter, and the taps on either side of the center are referred to as the "tail" of the filter.

After initializing the remaining filters and before training the filters, a determination is made whether the far-end echo echo canceller can be removed or truncated. This adjustment is only possible if both the near-end and far-end echo canceller filters are preserved (ie, neither is previously removed) and the widening of the far-end echo canceller overlaps the near-end echo canceller filter.

If the entire spread of the far-end echo canceller is within the spread of the near-end echo canceller filter, the far-end echo echo canceller is removed. This can happen, for example, when the RTDE is small. In the preferred embodiment where the near-end echo canceller filter has a stretch of 48 symbols and the far-end echo canceller has a stretch of 35 symbols, if the start of the far-end echo canceller (i.e. fe_get pointer) is less than or This happens when equal to 13 symbols after the start of the near-end echo canceller filter (i.e. the ne_get pointer). By completely removing the far-end echo echo canceller, the amount of processing resources required to drive the echo canceller by symbols can be significantly reduced.

Even if the entire spread of the far-end echo canceller is not within the spread of the near-end echo canceller filter, if its spread partially overlaps with the spread of the near-end echo canceller filter (i.e. at least one symbol is within the spread of the 2 filters within the stretch), the echo canceller for the far-end echo can also be truncated. In the preferred embodiment where the near-end echo canceller filter has a stretch of 48 symbols and the far-end echo canceller has a stretch of 35 symbols, if the start of the far-end echo canceller (i.e. fe_get pointer) is less than or This happens when equal to 48 symbols after the start of the near-end echo canceller filter (i.e. the ne_get pointer). An example of overlapping filter widening is shown in FIG. 9 . In this case, the far-end echo canceller is shortened by moving the fe_get pointer to the sign after the end of the near-end echo canceller filter as shown in FIG. 10 . Truncation of far-end echo The echo canceller reduces filter spread and thus reduces the number of filter taps. By truncating the far-end echo canceller, the amount of processing resources required to symbol drive the echo canceller is reduced by an amount proportional to the number of filter taps removed from the far-end echo canceller.

In the example shown in Figure 10, the truncated far-end echo echo canceller continues to have 2 tails. Care should be taken, however, that if the far-end echo echo canceller overlaps the near-end echo canceller filters such that the center of the far-end echo echo canceller is within the spread of the near-end filter, then the far-end echo echo canceller is truncated The tail shown on the right-hand side of the filter will be removed and some or all taps on the left-hand side will be removed (eg, if RTDE is 0, all far-end filter taps may be removed). Thus, in a preferred embodiment, if the RTDE is below a predetermined threshold (described above), the step of removing the far-end echo echo canceller is not performed, and instead the step of truncating the far-end echo echo canceller is used if the RTDE In fact below a predetermined threshold the echo canceller will have the effect of completely removing the far-end echo.

After some echo canceller filter adjustments are made prior to training the echo canceller filters, the remaining echo canceller filters are trained as part of stage 3 of the start-up sequence (step 710 in Figure 7). The step of training the echo canceller filters allows each filter to adapt to its respective echo channel to produce a suitable echo canceller output signal.

After training the remaining echo canceller filters, a final adjustment is made to remove any unnecessary filter taps from each remaining filter. In a preferred embodiment, the filter taps are checked in groups of 3 (where each group of 3 filter taps corresponds to a symbol), with each group starting at the end of each tail and leading in towards the center. A set of 3 echo canceller filter taps is considered unnecessary if the echo power level of the corresponding symbol is below a predetermined threshold, otherwise it is considered necessary. In a preferred embodiment, the predetermined threshold for the near-end echo canceller filter is 1.0e-5, and the predetermined threshold for the far-end echo canceller is 1.0e-3. where the magnitude M of the filter tap n is a complex coefficient of the form:

M _n =R _n +jI _n

The echo power level P of the 3 taps is determined according to the following formula:

P＝ ⁰ Rn ² +I _n ²

For n=1 to 3. The clipping of a particular tail stops once a necessary set of 3 filter taps is found or if the whole tail is removed. By reducing the number of filter taps in the remaining filter, the amount of processing resources required to sign drive the echo canceller is reduced by an amount proportional to the number of filter taps removed.

It should be noted that this last adjustment may be used as a default mechanism for removing the echo canceller in certain circumstances. This is especially important for modems operating on the analog side of a digital-to-analog connection, because when the connection rate is less than 56Kbit/s, the information exchanged during phase 2 is not sufficient to confirm that the connection is a digital-to-analog connection. Thus, a modem can initially have its far-end echo canceller enabled, and subsequently remove its far-end echo canceller when it finds all the near-end echo canceller filters unnecessary due to the lack of far-end echo.

Thus, in the exemplary embodiment shown in FIG. 11, a number of steps are taken during the start-up sequence in order for the echo canceller filter to adapt to reflections in the network. Beginning in step 1102 and after at least a first start-up phase (not shown), the modem disables its near-end echo canceller in step 1104 if the modem is operating on the digital side of the digital-to-analog connection. A phase 2 sequence including measuring the round trip delay (RTDE) and exchanging certain operating parameters is performed in step 1106 . After executing the Phase 2 sequence, in step 1108, if the modem is operating on the analog side of the digital-to-analog connection as determined by the information exchanged during Phase 2, the modem does not enable its far-end echo canceller. Optionally, the modem may disable the far-end echo canceller in step 1110 if the RTDE is less than a predetermined threshold, although this step is not used in a preferred embodiment. In preparation phase 3, the modem then builds up the remaining echo cancellers for the worst case embodiment in step 1112 . After establishing the echo canceller, the modem removes the far-end echo canceller in step 1114 if the spread of the far-end echo echo canceller is entirely within the spread of the near-end echo canceller filter, if the far-end echo echo canceller The widening of the near-end echo canceller filter is only partially covered, then the far-end echo canceller is truncated in step 1116. If any of the echo canceller filters have been removed prior to step 1112, steps 1114 and 1116 are not available. The modem then executes a Phase 3 sequence including training filter taps in step 1118 . After training the echo canceller, the modem adapts the echo canceller in step 1120 by removing some unnecessary filter taps. The modem then proceeds with the remaining stages of the start-up sequence (not shown), finally terminating in step 1199. It will be apparent to the skilled artisan that the techniques for removing, truncating and adapting the echo canceller can be applied alone or mixed with each other.

The techniques for removing, truncating and/or adapting the echo canceller can be embodied in hardware, firmware or software. An exemplary implementation of a variable-length echo canceller, like a software modem or other modem, includes functions for disabling the echo canceller, truncating the far-end echo, and/or enabling the echo canceller by removing unnecessary filter taps. Adaptive logic or computer readable program code means. Again, it will be obvious to the skilled person that the logic or computer readable program code means for removing, truncating and/or adapting the echo canceller may be used alone or mixed with each other.

Since 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 those skilled in the art that the same techniques are generally applicable to echo cancellers. For example, it is well known that filters are often not perfect, and that filters that are too long can actually amplify noise. Therefore, techniques for removing, truncating and/or adapting the echo canceller are useful to adapt each filter to an appropriate length so that the filter does not generate additional noise.

Finally, it will be apparent to those skilled in the art that by removing or reducing the number of echo canceller filter taps when echoes are present, some reduction in consumed processing resources can be achieved at the expense of echo canceller performance. As stated above, if the echo power level at the symbol corresponding to the filter tap is below a predetermined threshold, a set of 3 echo canceller filter taps are deemed unnecessary and removed. Thus, possibly by removing certain filter taps, the echo signal energy canceled by these taps is left in the received signal as noise to be overcome by the receiver afterwards. Thus, there is usually a trade-off between filter length and filter performance, with the predetermined threshold being chosen so that the performance of the echo canceller remains 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 as illustrative and not restrictive.

Claims (39)

1, a kind of device that is used for through communication system work, this device comprises:

Have the filter tap of first pre-determined maximum number that is used to simulate the first echo channel that receives in first time of delay and have the near-end echo Canceller of the first predetermined maximum broadening;

The far-end echo neutralizer that has the filter tap of second pre-determined maximum number that is used to simulate the second echo channel that receives in second time of delay and have the variable-length of the second predetermined maximum broadening;

The logic whether broadening that is used for determining the far-end echo neutralizer overlaps with the broadening of near-end echo Canceller; With

Overlap if be used for the broadening of far-end echo neutralizer and the broadening of near-end echo Canceller, be used for the logic of brachymemma far-end echo neutralizer.

Whether 2, the device of claim 1, the broadening that wherein is used for determining the far-end echo neutralizer comprise with the logic that the broadening of near-end echo Canceller overlaps:

Whether the broadening that is used for determining the far-end echo neutralizer whole logic in the broadening of near-end echo Canceller.

3, the device of claim 2, the logic that wherein is used for brachymemma far-end echo neutralizer comprises:

Whole in the broadening of near-end echo Canceller the time when the broadening of far-end echo neutralizer, be used to remove the logic of far-end echo neutralizer.

Whether 4, the device of claim 1, the broadening that wherein is used for determining the far-end echo neutralizer comprise with the logic that the broadening of near-end echo Canceller overlaps:

Be used to determine whether the logic of at least one symbol in the broadening of near-end echo Canceller and far-end echo neutralizer.

5, the device of claim 4, the logic that wherein is used for brachymemma far-end echo neutralizer comprises:

Be used to shorten the logic of far-end echo neutralizer, if thereby at least one symbol not only had been positioned at the near-end echo Canceller but also had been positioned at the broadening of far-end echo neutralizer, the sign-on that the broadening of far-end echo neutralizer last symbol in the broadening of near-end echo Canceller is closelyed follow later.

6, a kind of device that comprises computer-usable medium has been embedded in the computer-readable program code means that is used to make the adaptation of far-end echo neutralizer at medium, and this computer-readable program code means comprises:

Be used to have first computer-readable program code means of being scheduled to the near-end echo Canceller of maximum broadening;

Be used to have second computer-readable program code means of being scheduled to the variable-length far-end echo neutralizer of maximum broadening;

The computer-readable program code means whether broadening that is used for determining the far-end echo neutralizer overlaps with the broadening of near-end echo Canceller; With

If the broadening of the broadening of far-end echo neutralizer and near-end echo Canceller overlaps, be used for the computer-readable program code means of brachymemma far-end echo neutralizer.

Whether 7, the device of claim 6, the broadening that wherein is used for determining the far-end echo neutralizer comprise with the computer-readable program code means that the broadening of near-end echo Canceller overlaps:

Whether the broadening that is used for determining the far-end echo neutralizer whole computer-readable program code means in the broadening of near-end echo Canceller.

8, the device of claim 7, the computer-readable program code means that wherein is used for brachymemma far-end echo neutralizer comprises:

If the broadening of far-end echo neutralizer is whole in the broadening of near-end echo Canceller, be used to remove the computer-readable program code means of far-end echo neutralizer.

Whether 9, the device of claim 6, the broadening that wherein is used for determining the far-end echo neutralizer comprise with the computer-readable program code means that the broadening of near-end echo Canceller overlaps:

Be used to determine whether that at least one symbol is positioned at the computer-readable program code means of the broadening of near-end echo Canceller and far-end echo neutralizer.

10, the device of claim 9, the computer-readable program code means that wherein is used for brachymemma far-end echo neutralizer comprises:

If at least one symbol is positioned at the broadening of near-end echo Canceller and far-end echo neutralizer, be used to shorten the far-end echo neutralizer so that the computer-readable program code means of the sign-on that the broadening of far-end echo neutralizer last symbol in the broadening of near-end echo Canceller is closelyed follow later.

11, in the device of a far-end echo neutralizer that comprises near-end echo Canceller and variable-length, the near-end echo Canceller has the filter tap of first pre-determined maximum number that is used to simulate the first echo channel that receives in first time of delay and has the first predetermined maximum broadening; The far-end echo neutralizer of variable-length has the filter tap of second pre-determined maximum number that is used to simulate the second echo channel that receives in second time of delay and has the second predetermined maximum broadening, and a kind of method that is used to the far-end echo neutralizer is adapted to comprises step:

Whether the broadening of determining the far-end echo neutralizer overlaps with the broadening of near-end echo Canceller; With

If the broadening of the broadening of far-end echo neutralizer and near-end echo Canceller overlaps, brachymemma far-end echo neutralizer.

Whether 12, the method for claim 11, the broadening of wherein determining the far-end echo neutralizer comprise with the method that the broadening of near-end echo Canceller overlaps:

Whether the broadening of determining the far-end echo neutralizer is whole in the broadening of near-end echo Canceller.

13, the method for claim 12, wherein the method for brachymemma far-end echo neutralizer comprises:

If the broadening of far-end echo neutralizer is whole in the broadening of near-end echo Canceller, remove the far-end echo neutralizer.

14, whether the method for claim 11, the broadening of wherein determining the far-end echo neutralizer comprise step with the method that the broadening of near-end echo Canceller overlaps:

Determine whether that at least one symbol is positioned at the broadening of near-end echo Canceller and far-end echo neutralizer.

15, the method for claim 14, wherein the method for brachymemma far-end echo neutralizer comprises step:

If at least one symbol is positioned at the broadening of near-end echo Canceller and far-end echo neutralizer, be used to shorten the far-end echo neutralizer so that the sign-on that the broadening of far-end echo neutralizer last symbol in the broadening of near-end echo Canceller is closelyed follow later.

16, a kind of device that is used for through communication system work, this device comprises:

Variable-length echo neutralizer with pre-determined maximum number filter tap;

Logic according to the echo specifit training variable-length echo neutralizer of communication system; With

Be used for removing the logic of unnecessary filter tap from the variable-length echo neutralizer of training.

17, the device of claim 16, the logic that wherein is used to remove unnecessary filter tap comprises:

Be used to calculate logic corresponding to the echo power level of a predetermined symbol time;

Be used for determining that described echo power level is whether less than the logic of a predetermined threshold; With

If the echo power level less than a predetermined threshold, is used to remove the logic corresponding to the filter tap of predetermined symbol time.

18, the device of claim 17, the logic that wherein is used to calculate the echo power level comprises:

Be used to calculate corresponding to the amplitude of each filter tap of predetermined symbol time square and logic.

19, the device of claim 17, the logic that wherein is used to remove filter tap comprises:

Be used for the broadening of echo neutralizer is shortened the logic of a symbol.

20, a kind of device that comprises computer-usable medium has been embedded in the computer-readable program code means that is used to make the adaptation of far-end echo neutralizer at medium, and this computer-readable program code means comprises:

Be used to have the computer-readable program code means of variable-length echo neutralizer of the filter tap of predetermined maximum number;

Computer-readable program code means according to the echo specifit training variable-length echo neutralizer of communication system; With

Be used for removing the computer-readable program code means of unnecessary filter tap from the variable-length echo neutralizer of training.

21, the device of claim 20, the computer-readable program code means that wherein is used to remove unnecessary filter tap comprises:

Be used to calculate computer-readable program code means corresponding to the echo power level of a predetermined symbol time;

Be used for determining that described echo power level is whether less than the computer-readable program code means of a predetermined threshold; With

If the echo power level less than a predetermined threshold, is used to remove the computer-readable program code means corresponding to the filter tap of predetermined symbol time.

22, the device of claim 21, the computer-readable program code means that wherein is used to calculate the echo power level comprises:

Be used to calculate corresponding to the amplitude of each filter tap of predetermined symbol time square and computer-readable program code means.

23, the device of claim 21, the computer-readable program code means that wherein is used to remove filter tap comprises:

Be used for the broadening of echo neutralizer is shortened the computer-readable program code means of a symbol.

24, in the device of the variable-length echo neutralizer that comprises the filter tap with pre-determined maximum number, a kind of method that is used to echo neutralizer is adapted to comprises step:

Echo specifit training variable-length echo neutralizer according to communication system; With

From the variable-length echo neutralizer of training, remove unnecessary filter tap.

25, the method for claim 24, the step of wherein removing unnecessary filter tap comprises step:

Calculating is corresponding to the echo power level of a predetermined symbol time;

Determine that whether described echo power level is less than a predetermined threshold; With

If the echo power level less than a predetermined threshold, is removed the filter tap corresponding to predetermined symbol time.

26, the method for claim 25, the step of wherein calculating the echo power level comprises step:

Calculating corresponding to the amplitude of each filter tap of predetermined symbol time square and.

27, the method for claim 25, the step of wherein removing filter tap comprises step:

Be used for the broadening of echo neutralizer is shortened the logic of a symbol.

28, a kind of device through communication system work, this device comprises:

The echo neutralizer of the echo in can the filtering communication system;

Be used for determining whether echo neutralizer is unnecessary logic; With

If it is unnecessary having determined echo neutralizer, be used for not enabling the logic of echo neutralizer.

29, the device of claim 28 comprises:

If echo neutralizer is a far-end echo neutralizer with device when being confirmed as being operated in the analog side that digital-to-analog is connected, be used for not enabling the logic of echo neutralizer.

30, the device of claim 28 comprises:

If echo neutralizer is a near-end echo Canceller with device when being confirmed as being operated in the digital side that digital-to-analog is connected, be used for not enabling the logic of echo neutralizer.

31, the device of claim 28 comprises:

Be used for definite logic that postpones through the round trip of communication system; With

If echo neutralizer is a far-end echo neutralizer and round trip when postponing to be confirmed as less than a predetermined thresholding, be used for not enabling the logic of echo neutralizer.

32, a kind of device that comprises computer-usable medium has embedded the computer-readable program code means that is used to make the adaptation of far-end echo neutralizer at medium, and this computer-readable program code means comprises:

The computer-readable program code means that is used for the echo neutralizer of echo that can the filtering communication system;

Be used for determining whether echo neutralizer is unnecessary computer-readable program code means; With

If it is unnecessary having determined echo neutralizer, be used for not enabling the computer-readable program code means of echo neutralizer.

33, the device of claim 32 comprises:

If echo neutralizer is a far-end echo neutralizer with device when being confirmed as being operated in the analog side that digital-to-analog is connected, be used for not enabling the computer-readable program code means of echo neutralizer.

34, the device of claim 32 comprises:

If echo neutralizer is a near-end echo Canceller with device when being confirmed as being operated in the digital side that digital-to-analog is connected, be used for not enabling the computer-readable program code means of echo neutralizer.

35, the device of claim 32 comprises:

Be used for definite computer-readable program code means that postpones through the round trip of communication system; With

If echo neutralizer is a far-end echo neutralizer and round trip when postponing to be confirmed as less than a predetermined thresholding, be used for not enabling the computer-readable program code means of echo neutralizer.

36, in the device of the echo neutralizer with the echo in can the filtering communication system, a kind of logic that is used to remove echo neutralizer comprises step:

Determine whether echo neutralizer is unnecessary; With

If it is unnecessary having determined echo neutralizer, do not enable echo neutralizer.

37, the method for claim 36 comprises:

If echo neutralizer is a far-end echo neutralizer with device when being confirmed as being operated in the analog side that digital-to-analog is connected, do not enable echo neutralizer.

38, the method for claim 36 comprises:

If echo neutralizer is a near-end echo Canceller with device when being confirmed as being operated in the digital side that digital-to-analog is connected, do not enable echo neutralizer.

39, the method for claim 36 comprises:

Determine to postpone through the round trip of communication system; With

If echo neutralizer is a far-end echo neutralizer and round trip when postponing to be confirmed as less than a predetermined thresholding, do not enable echo neutralizer.

Images