JP2002232329A - Echo canceller - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To significantly improve performance with a small amount of calculation for the problem of frequency resolution in an echo canceller. SOLUTION: In addition to echo canceller 15, LPF1
8 and an FIR filter 19 are added. LPF1
8 extracts the low-frequency component of the transmission signal, shapes the waveform by the FIR 19, and adds it to the echo canceller output. Similarly to the main echo canceller 15, the FIR of the low-frequency echo canceller is also updated by an LMS update or the like, using the differential signal during the training period as an error signal. By shaping the waveform by FIR and adding the gain and phase together, the transfer function of the entire echo canceller can be better approximated. Because it only covers a very limited band,
The gain and phase can be adjusted with a small number of FIR taps, and high-frequency components that cannot be removed by the IIR can be removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an echo canceller used in a full-duplex communication device such as a modem and a speakerphone.

[0002]

2. Description of the Related Art When performing full-duplex communication on a two-wire general public line, a transmission signal goes around a reception path in a hybrid circuit (two-wire four-wire conversion circuit) and is mixed into the reception signal as an echo. This echo is only noise for the receiver and has a great influence on the reception performance. Therefore, the band of the transmission signal and the band of the reception signal are divided, and the HPF (Hig
h Pass Filter) or LPF (Low)
A band-separated communication method that extracts only a received signal component using a pass filter has been used for a long time. After that, as higher communication speeds are required, echo cancellation technology has been developed to use a wider band as a signal, and it is possible to remove only the echo component from the received signal using an adaptive filter. Now you can.

[0003] However, the signal called "Zan difference" which cannot be completely removed by the echo canceller remains in the received signal as it is, and the receiving performance is degraded as noise. In recent years, while the performance of general public telephone lines has increased and attenuation and noise have been reduced, there has been an increasing number of cases in which this slight error becomes a bottleneck in performance due to the increase in communication speed.

[0004] The following can be cited as the cause of the residual difference. (1) Complete training cannot be performed due to noise or the like. (2) The accuracy of the filter coefficient and the operation accuracy are not sufficient. (3) The filter length is not sufficient, and the tail portion of the impulse response cannot be covered. (4) The filter length is not sufficient and the frequency resolution is insufficient.

[0005] By lengthening the training time of the echo canceller, it is possible to reduce the influence of noise and the like in (1). Regarding (2), in order to keep the accuracy of the filter coefficient while suppressing the amount of calculation, a method having 32 bits as the coefficient and using only the upper 16 bits when performing the filter calculation is widely used.

The echo canceller described in Japanese Patent Application Laid-Open No. 03-088433 relates to the problem (3), and makes use of the characteristic that the tail part is gradually attenuated to create a pseudo tail part so that the filter length is reduced. Trying to get the same effect as the long one. However, at present, when the DSP has been developed, a filter length about the tail portion of the impulse response can be easily realized, and the problem (3) does not often become a serious problem.

[0007]

On the other hand, as for (4), as shown in FIG. 5, the transfer function cannot be sufficiently approximated near the band edge where the characteristics change rapidly, and the error is regarded as noise. The problem is that they cannot be introduced or cancel the echo. The change in the frequency characteristics near the band edge is very steep. Even if the filter length is doubled and the frequency resolution is doubled, it is difficult to sufficiently cope with it, and the performance is improved in accordance with the increase in the amount of calculation. Cannot be obtained. Conversely, too long filter lengths
Training can be adversely affected.

The digital transmission device described in Japanese Patent Laid-Open Publication No. 03-280627 attempts to solve this problem by inserting an HPF into a reception path and cutting low-frequency components. However, in this method, the low frequency component of the received signal itself is also cut, and the S / N of the band cannot be changed.

The present invention provides a means for greatly improving the performance with a small amount of calculation for the above-described problem of the frequency resolution in the echo canceller.

[0010]

According to the present invention, there is provided an echo canceller for removing an echo generated when a transmission signal wraps around a reception path, using the transmission signal and the received echo signal. In addition to an adaptive filter for adaptively estimating a pseudo echo signal, a subband echo canceller for estimating only a pseudo echo signal with a limited band is provided.

The invention of claim 2 is characterized in that, in the invention of claim 1, the sub-band echo canceller comprises means for limiting a band of a transmission signal and means for shaping the waveform of the band.

According to a third aspect of the present invention, in the second aspect of the present invention, an IIR filter is used as the band limiting means.

According to a fourth aspect of the present invention, in the second aspect, the waveform shaping means performs gain adjustment and phase adjustment.

According to a fifth aspect of the present invention, in the second aspect, an FIR filter is used as the waveform shaping means.

According to a sixth aspect of the present invention, in the fifth aspect, the coefficients of the FIR filter are adaptively changed according to the echo path.

According to a seventh aspect of the present invention, in the second aspect, a band is limited to a low frequency component.

The invention of claim 8 is characterized in that, in the invention of claim 7, downsampling is performed before band limitation.

A ninth aspect of the present invention is characterized in that, in the invention of the seventh aspect, the transmission signal whose band is limited is down-sampled before entering the waveform shaping means.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a method of echo cancellation used in a full-duplex communication device such as a modem, a speakerphone, and the like. The same applies to applications such as speakerphones.

When performing full-duplex communication on a two-wire general public line, a transmission signal goes around a reception path by a hybrid circuit (two-wire four-wire conversion circuit) and is mixed into the received signal as an echo. This echo is only noise for the receiver and has a great influence on the reception performance. Therefore, the band of the transmission signal and the band of the reception signal are divided, and the HPF (Hig
h Pass Filter) or LPF (Low)
A band-separated communication method that extracts only a received signal component using a pass filter has been used for a long time. After that, as higher communication speeds are required, echo cancellation technology has been developed to use a wider band as a signal, and it is possible to remove only the echo component from the received signal using an adaptive filter. Now you can.

However, the signal called "Zan difference" which cannot be completely removed by the echo canceller remains in the received signal as it is, and deteriorates the receiving performance as noise. In recent years, while the performance of general public telephone lines has increased and attenuation and noise have been reduced, there has been an increasing number of cases in which this slight error becomes a bottleneck in performance due to the increase in communication speed.

The cause of the residual difference is, as described above,
The following can be given: (1) Complete training cannot be performed due to noise or the like. (2) The accuracy of the filter coefficient and the operation accuracy are not sufficient. (3) The filter length is not sufficient, and the tail portion of the impulse response cannot be covered. (4) The filter length is not sufficient and the frequency resolution is insufficient.

By lengthening the training time of the echo canceller, it is possible to reduce the influence of the noise (1). Regarding (2), in order to keep the accuracy of the filter coefficient while suppressing the amount of calculation, the coefficient is 32 bits.
A method of using only the upper 16 bits when performing a filter operation is widely used. JP 03
“Echo canceller” described in JP-A-088433
Has attempted to obtain an effect similar to the case where the filter length is long by making a pseudo tail portion by using the characteristic that the tail portion is gradually attenuated with respect to the problem (3). However, at present, when the DSP has been developed, a filter length about the tail portion of the impulse response can be easily realized, and the problem (3) does not often become a serious problem.

On the other hand, the problem (4) is that the transfer function cannot be sufficiently approximated in the vicinity of the band edge where the characteristic changes abruptly, and the error is introduced as noise or the echo cannot be canceled. Has become. The change in the frequency characteristics near the band edge is very steep. Even if the filter length is doubled and the frequency resolution is doubled, it is difficult to sufficiently cope with it, and the performance is improved in accordance with the increase in calculation amount. Cannot be obtained. Conversely, too long a filter length may adversely affect training. The "digital transmission apparatus" described in Japanese Patent Application Laid-Open No. 03-280627 attempts to address this problem by inserting an HPF into a reception path and cutting low-frequency components. However, in this method, the low frequency component of the received signal itself is also cut, and the S / N of the band cannot be changed.

The present invention provides means for greatly improving the performance with a small amount of calculation for the above-mentioned problem of the frequency resolution in the echo canceller. In particular, the present method exhibits an excellent effect of compensating for the low band side of the band edge.

FIG. 1 is a diagram showing an example of the configuration of the present invention. A signal transmitted from a transmitting unit 11 is converted into an analog signal through a D / A circuit 12 and is converted into a 4-wire 2-wire signal by a hybrid circuit 16. Later, it is sent to a general public line. The transmission signal is subjected to 2-wire 4-wire conversion by the hybrid circuit 17 of the central office, A / D-converted, and sent to the other party. In these two hybrid circuits 16 and 17, a signal wraparound from the transmission path to the reception path occurs, and A
/ D circuit 14. This echo is called a near-end echo. Further, in the two hybrids on the partner device side, an echo similarly occurs and returns as a far-end echo. However, since the same treatment is possible in the present invention, the description is omitted here and the near-end echo is omitted. Only edge echo shall be handled.

In a general echo canceller, only the echo canceller 15 is used. In order to adaptively obtain the transfer function of the echo path, an echo canceller is configured using FIR. The filter coefficient is obtained by using the difference signal at the time of half-duplex training as an error signal, and using LMS (Le
As a result, an update is performed so as to reduce the difference by using an as-mean-square method. The number of taps of the filter should be 1 according to the length of the impulse response of the echo.
Those equivalent to 0 ms to 15 ms are often used.

In the present invention, the echo canceller 15
In addition to Low Pass Filter (hereinafter LP)
F) 18 and FIR Filter 19 are added. The LPF extracts the low-frequency component of the transmission signal, shapes the waveform with an FIR filter, and adds the waveform to the output of the echo canceller. Similarly to the main echo canceller, the FIR of the low-frequency echo canceller is also updated by an LMS update or the like using the differential signal during the training period as an error signal.

When the LPF is constructed using the IIR, for example, a filter having a spectrum as shown in FIG. 2 is formed. By shaping the waveform with FIR and adding the gain and phase together, the transfer function of the entire echo canceller can be better approximated as shown in FIG. Since only a very limited band is targeted, it is possible to match the gain and phase with a small number of FIR taps, and it is also possible to remove high-frequency components that could not be removed by the IIR. In particular, when a low frequency band is targeted, even if there is an error in the phase component, the influence on the signal is very small, so that high accuracy is not required for the phase estimation. In addition, since there are generally 0 points in the DC component in the echo path, matching between the drop at the band edge and the fall of the LPF can be easily achieved as shown in FIG.

FIG. 4 shows a more general configuration example of the present invention. In the present invention, in addition to a general echo canceller 15, a plurality of band-limited subband echo cancellers 20 are provided. By limiting the band in this way, it is possible to intensively compensate only the transfer function in a region where the frequency response changes rapidly, such as a band edge. In FIG. 4, parts having the same functions as those in the configuration example shown in FIG. 1 are denoted by the same reference numerals as those in FIG.

The simplest and most efficient method for estimating the band-limited pseudo echo signal is to limit the band of the transmission signal, shape the waveform with a filter or the like, and adjust the gain and phase. When limiting the band, II
If an R filter is used, efficient band limitation can be performed with a small order.

In the waveform shaping, since the object is a signal whose band is limited, even when a single gain and phase are adjusted, the waveform can be shaped relatively accurately. FI with short tap number as waveform shaping means
If R is used, gain and phase can be adjusted by training using the LMS method or the like, and components remaining outside the limited band can be cut.

When the target of band limitation is a low band, the DC component is always 0 in a general echo path, so that the band limitation means can be a simple LPF. Further, in the low-frequency component, even if there is a phase error, the influence on the signal error is reduced, and the accuracy of the estimated echo signal can be easily kept high.

When performing band limitation for low-frequency components, even if downsampling is performed before band limitation, components do not disappear. On the other hand, by performing downsampling, the amount of calculation can be reduced. Alternatively, it is possible to perform a steeper band limitation while maintaining the calculation amount.

When performing waveform shaping for low frequency components, even if downsampling is performed before waveform shaping, the components do not disappear. On the other hand, by performing downsampling, the amount of calculation can be reduced. Alternatively, it is possible to increase the frequency resolution while maintaining the calculation amount, and it is possible to more accurately approximate the transfer function of the echo path.

Although the above description has been made assuming echo cancellation in a communication line such as a modem, the present invention can be easily applied to a case where a transmission signal goes around a reception path in space such as a speakerphone. is there.

[0037]

According to the first aspect of the present invention, the use of the sub-band echo canceller for estimating the band-limited pseudo echo signal allows the frequency response of the transfer function to rapidly change, such as near the band edge. A good transfer function approximation can be performed, and it is possible to reduce the echo difference signal generated due to the problem of the frequency resolution.

Advantageous Effects of Claim 2 By generating a pseudo echo signal by using the band limitation of the transmission signal and the waveform shaping of the band, it is possible to easily estimate the band-limited pseudo echo signal. .

Operation and Effect to Claim 3: By using IIR as the band limiting means, it is possible to effectively limit the band with a small amount of calculation.

Operation and Effect to Claim 4: By performing gain adjustment and phase adjustment as the waveform shaping means, the waveform can be easily shaped.

Operation and effect to claim 5: By using an FIR filter as the waveform shaping means, the gain, phase and the like can be adjusted for each band, so that the transfer function can be more accurately approximated.

Action and Effect to Claim 6 By adaptively changing the coefficients of the FIR filter using the LMS method or the like, it is possible to estimate the transfer function according to the state of the echo path.

Effect of Claim 7 By limiting the band to low-frequency components, the influence of the phase error of the estimated echo signal is reduced, and it is possible to accurately generate the estimated echo signal whose band is limited.

Operation and effect on claim 8: By downsampling before band limitation, the amount of calculation of the band limitation means can be reduced without affecting the performance. Alternatively, it is possible to perform a steeper band limitation using the same calculation amount.

Operation and effect to claim 9: By down-sampling the band-limited transmission signal, the amount of calculation of the waveform shaping means can be reduced without affecting the performance. Alternatively, waveform shaping with high frequency resolution can be performed using the same amount of calculation, and the approximation accuracy of the transfer function can be further increased.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of the present invention.

FIG. 2 is a diagram showing an example of an LPF using an IIR.

FIG. 3 is a diagram showing an approximation of a transfer function at a band edge.

FIG. 4 is a diagram showing another configuration of the present invention.

FIG. 5 is a diagram illustrating a relationship between a frequency resolution and a transfer function at a band edge.

[Explanation of symbols]

11: transmission unit, 12: D / A circuit, 13: reception unit, 14
... A / D circuit, 15 ... Echo canceller, 16, 17 ...
Hybrid circuit, 18 ... LPF, 19 ... FIRFil
ter, 20 ... Band-limited echo canceller.

Claims (9)

[Claims] An echo canceller for removing an echo generated when a transmission signal wraps around a reception path, and adaptively estimates a pseudo echo signal using the transmission signal and the received echo signal. In addition to filters,
An echo canceller characterized by having a sub-band echo canceller for estimating only a band-limited pseudo echo signal. 2. The echo canceller according to claim 1, wherein the sub-band echo canceller comprises a band limiting means for a transmission signal and a waveform shaping means for the band. 3. The echo canceller according to claim 2, wherein an IIR filter is used as the band limiting means. 4. The echo canceller according to claim 2, wherein the waveform shaping means performs gain adjustment and phase adjustment. 5. The echo canceller according to claim 2, wherein an FIR filter is used as the waveform shaping means. 6. The echo canceller according to claim 5, wherein coefficients of the FIR filter are adaptively changed according to an echo path. 7. The echo canceller according to claim 2, wherein a band is limited to a low frequency component. 8. The echo canceller according to claim 7, wherein downsampling is performed before band limitation. 9. The echo canceller according to claim 7, wherein down-sampling is performed on the band-limited transmission signal before entering the waveform shaping means.