JPH08213939A - Echo canceler - Google Patents

Abstract

PURPOSE: To realize high speed convergence and stable operation by introducing a pseudo roll-off filter to the device so as to reduce an impulse width of an adaptive filter of a pseudo echo generator. CONSTITUTION: A single pseudo roll-off filter 23 is inserted between a pseudo echo generator 10 and a subtractor 14. The impulse width of an adaptive filter in the pseudo echo generator 10 is made narrow by inserting the pseudo roll-off filter 23 whose characteristic is close to a frequency roll-off characteristic of a transmission band of an echo pulse, that is, the echo canceler with high quality realizing excellent convergence characteristic and stability is realized, in which the number of taps requiring adaptive control is reduced. A low pass filter whose pass band is flat and having a typical broad band frequency roll-off characteristic for an echo pulse almost depending on, e.g. a PCM coder/decoder or a high pass filter whose pass band is flat and having a typical low frequency roll-off characteristic for an echo pulse is adopted for the pseudo roll-off filter 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an echo canceller device capable of canceling a line echo of a long distance telephone line or the like and an echo contained in an audio signal of a TV conference system.

[0002]

2. Description of the Related Art Here, a conventional technique will be described with a focus on an echo canceller device for a telephone line. FIG.
FIG. 3 shows a basic configuration diagram of a conventional echo canceller device 1 having a 64 kbit / s PCM interface. The input output signal of the echo canceller device 1 is 6
Interfaced with 4 kbit / s (8 kHz sampling, 8-bit non-linear quantization) PCM,
It is assumed that the PCM input signal is converted into a linear digital signal for processing, and the linear digital signal is converted into an 8-bit non-linear quantization PCM signal and output as a PCM output signal. Here, an ordinary 8-bit PCM code quantized by the non-linear quantizer is called a PCM signal, and a linear quantized digital signal is called a linear signal.

In the PCM telephone line, a two-wire to four-wire converter 5 called a hybrid circuit is usually used as shown in FIG. 1 in terms of line structure, but the PCM signal of the far-end speaker is P.
After being converted into an analog signal by the CM decoder 4, a part of the analog signal leaks into the other party (far end) side line as an echo through the two-wire to four-wire converter 5 and is transmitted to the far-end talker.
At this time, if the line delay time is large like a satellite line, the echo significantly deteriorates the call quality, making it difficult to talk. In order to solve this, a method of introducing an echo canceller device 1 as shown in FIG. 1 to cancel an echo is used.

The operation of the apparatus shown in FIG. 1 will be briefly described. The PCM signal of the far-end speaker is input as a PCM reception input signal (Sin) via the reception input terminal 2 of the echo canceller device 1, and is output as it is from the reception output terminal 3 as a PCM reception output signal (Rout). After being converted into a linear digital signal by the linear code converter 12, it is input to the pseudo echo generator 10. After that, the signal is transmitted from the terminal 6 to the near-end speaker through the PCM decoder 4 and the 2-wire to 4-wire converter 5. In addition, a part of the output signal of the PCM decoder 4 is transferred from the 2-wire 4-wire converter 5 to the PCM.
It is mixed as a PCM transmission input signal (Sin) into the transmission input terminal 9 through the encoder 14, and is circulated in the far end direction as a line echo and transmitted to the far end speaker.

Here, the echo path 8 is a transmission system including the 2-wire 4-wire converter 5 when the near end side is seen from both terminals 3 and 9.
Call. In order to cancel the echo generated in the echo path 8, the PCM transmission input signal (Sin) input to the transmission input terminal 9 is converted into the linear transmission input signal by the linear code converter 11 and then obtained from the pseudo echo generator 10. The pseudo echo thus obtained is subtracted via the subtractor 14 to obtain an output as a residual echo at the terminal 21 to reduce the echo. Further, after being PCM converted by the non-linear code converter 16, it is output from the transmission output terminal 17 as a PCM transmission output signal (Sout). A detailed explanation is omitted here, but if the level of the residual echo cannot be suppressed sufficiently,
As shown in FIG. 1, the residual echo may be input to the non-linear code converter 16 after being subjected to non-linear processing by a center clipper or the like via the non-linear processor 15. The residual echo is input to the pseudo echo generator 10 and used to update the filter coefficient of the adaptive filter prepared for the estimation of the impulse response of the echo path 8 in a timely manner.

In the pseudo echo generator 10, the echo path 8
In general, the transmission delay time and the transmission frequency characteristic are directly approximated by the FIR type adaptive digital filter, so that the filter coefficient giving the impulse response of the transmission system is adaptively controlled. The filter coefficient is updated using the residual echo and the linearly received input signal under a coefficient adaptive control algorithm called a learning identification method, for example, so that the level of the residual echo obtained from the terminal 21 is minimized. Using the filter coefficient obtained as a result, the filter output for the linear reception input signal is output to the terminal 19 as a pseudo echo.

In addition to the adaptive filter, the pseudo echo estimator 10 includes a filter coefficient register called an H register, an X register for storing a linear reception input signal, a double talk detector, a power calculator and a filter coefficient updater. However, since those functions are not directly related to the present invention, description thereof will be omitted.

In FIG. 1, in the case of a telephone line of PCM multi-link connection, a plurality of PCM code decoders are inserted between the PCM decoder 4 and the 2-wire 4-wire converter 5. On the other hand, a plurality of PCM code decoders are similarly inserted between the 2-line to 4-line converter 5 and the PCM encoder 7.

In general, the transmission frequency characteristic of the echo path 8 has a band pass characteristic of the voice transmission band, and the roll-off characteristic of low and high frequency cutoff is mainly dominated by the frequency roll-off characteristic of the PCM code decoder. Further, the frequency characteristics of the 2-wire to 4-wire converter are dominant in the frequency characteristics in the middle range, and there are cases where the frequency characteristics are flat or slightly wavy. Reception output terminals 3 and 2
When an analog multiplex line is connected between the line / wire converter 5 and between the line / wire converter 5 and the transmission input terminal 9 or a PC
When the M multi-links are connected, the transmission frequency characteristic of the echo path 8 has a steep roll-off characteristic.

The impulse response of the adaptive filter of the pseudo echo generator 10 in the conventional PCM line echo canceller device 1, that is, the filter coefficient sequence is generally as shown in FIG. The portion where the amplitude (coefficient) is very small before the impulse response portion corresponds to the delay time of the echo path and is constant unless the delay of the echo path 8 changes. The impulse response portion has a waveform corresponding to the transmission frequency characteristic of the echo path 8, and the duration of the impulse response portion is called the impulse width. The transmission frequency characteristic of the echo path 8 generally has a bandpass characteristic, and when the frequency roll-off characteristic of low-frequency and high-frequency cutoff is steep, the impulse width becomes very wide, and the frequency roll-off characteristic is This means that many adaptive filter coefficients are needed to approximate. Further, the number of times the adaptive coefficient control is updated also increases, and it takes time for the adaptive filter to accurately approximate the impulse response of the echo path 8, that is, to reach a convergent state.

In particular, when an analog multiplex line or a PCM multiple link line is included, the impulse width becomes very wide due to the abrupt roll-off characteristic of the filter bank.
Further, in such a state, it is difficult to generate an accurate voice pseudo echo because a long convergence time is required for a voice signal whose spectrum is constantly changing, and the echo canceling ability is not enhanced. Therefore, there is a drawback that the residual echo becomes large especially when the operation of the echo canceller device 1 is started or when the double talk is ended.

Further, if the number of filter taps of the adaptive filter increases, the hardware becomes extremely complicated.

[0013]

As described above with respect to the operation of the echo canceller device 1, the impulse width in the adaptive filter depends on the frequency roll-off characteristic of the echo path 8. It becomes long and it takes a convergence time in adaptive coefficient control. Also, when the distribution range of the echo path delay is wide, an adaptive filter having a number of filter taps corresponding to the time width consisting of the maximum delay and the impulse width is required, which requires a significant convergence time and complicated hardware. Is.

An object of the present invention is to provide an echo canceller device having a fast convergence and a stable operation by reducing the impulse width of the adaptive filter of the pseudo echo generator 10.

[0015]

An echo canceller device 2 according to the present invention for solving the above-mentioned problems in the conventional system has a pseudo roll in front of a pseudo echo generator 13 having an adaptive filter for impulse estimation of an echo path. An off-filter 23 is introduced to provide a high-quality echo canceller device having excellent convergence characteristics and stability, which is characterized by reducing a required impulse width in an adaptive filter, that is, reducing the number of taps requiring adaptive control. To do.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration and operation of the echo canceller device according to the present invention will be described in detail below with reference to three embodiments. In addition, in each embodiment, when the constituent element has the same function as that in the conventional apparatus 1, the same reference numeral is attached.

(Embodiment 1) FIG. 2 shows a configuration example as a first embodiment of the present invention. This is a system in which a single pseudo roll-off filter 23 is inserted between the pseudo echo generator 10 and the subtractor 14. The operation will be described in detail below.

As the pseudo roll-off filter 23, for example, a low-pass filter having a flat wide pass band having a typical wide frequency roll-off characteristic of the echo path 8 which is almost determined by a PCM code decoder, or a typical echo path 8 is used. A high-pass filter having a flat low pass band having a low frequency roll-off characteristic or a typical band pass filter having a low frequency band and a wide band frequency roll-off characteristic may be used. Recently, the PCM of the line is rapidly progressing, the number of PCM links is 2-3 links or less, and the difference in frequency roll-off characteristics of the echo path 8 is small depending on the line, and the pseudo roll that approximates these characteristics is small. Setting the off-filter 23 is relatively easy. By inserting such a pseudo roll-off filter 23, the frequency roll-off characteristic to be approximated by the pseudo echo generator 10 becomes slow, and the impulse width is narrowed as compared with the case where it is not inserted as shown in FIG. 3B. And the convergence speed of the adaptive filter is improved.

(Embodiment 2) FIG. 4 shows a configuration example as a second embodiment of the present invention. A single pseudo roll-off filter 23 is inserted between the pseudo echo generator 13 and the subtractor 14, and the pseudo roll-off filter 23 output and the linear code converter 11 output accumulated in the pseudo echo generator 13 Is a system in which a pseudo echo generator 13 having a fixed delay circuit set according to the estimated echo path delay is inserted. The operation will be described in detail below.

The echo path delay estimator 25 correlates, for example, the correlation between the linear transmission input signal at the start of operation of the echo canceller and the pseudo roll-off filtered linear reception input signal stored in the pseudo echo generator 13. The delay amount of the echo path 8 is estimated by. The estimated echo path delay amount is input to the pseudo echo generator 13, and a fixed delay circuit (which may be set to zero the filter coefficient of the corresponding adaptive filter) is set in the preceding stage part of the adaptive filter. Further, since the output of the pseudo roll-off filter 23 is input to the pseudo echo generator 13 as in the first embodiment, the pseudo echo generator 13 has substantially slow frequency roll-off characteristics as the echo path 8 to be approximated. Become. Therefore, the impulse width to be approximated becomes narrower, and the filter tap coefficient corresponding to the estimated echo path delay does not need to be adaptively controlled, so the number of taps that need to be adaptively controlled can be greatly reduced compared to the conventional method. High-speed convergence can be realized. Also, since the number of coefficients that are erroneously corrected at the start of double talk is small, a very stable operation is possible.

(Embodiment 3) FIG. 5 shows a configuration example as a third embodiment of the present invention. An optimum pseudo roll-off filter for inserting a plurality of, for example, two pseudo roll-off filters 27, 28 between the pseudo echo generator 13 and the subtractor 14 and selecting one of the outputs of the pseudo roll-off filters 27, 28. This is a system in which the selector 29 is added to the configuration of the second embodiment. The operation will be described in detail below.

As the pseudo roll-off filter, for example, a filter 27 having a very steep frequency roll-off characteristic assuming an analog multiplex line and a PCM of 2-3 links are used.
A pseudo roll-off filter having a plurality of different frequency roll-off characteristics, such as a filter 28 assuming the frequency roll-off characteristic of the line, is prepared. For example, a pseudo roll-off filter having the largest correlation with the roll-off filter output is selected, and the filter output is input to the pseudo echo generator 13. Then, the echo path delay estimator 25 estimates the delay amount of the echo path 8 between the linear transmission input signal and the pseudo roll-off filtered linear reception input signal stored in the pseudo echo generator 13, for example, by correlation. To do. The estimated echo path delay amount is input to the pseudo echo generator 13, and a fixed delay circuit (which may be set to zero the filter coefficient of the corresponding adaptive filter) is set in the preceding stage part of the adaptive filter. Since the optimum pseudo roll-off filter is selected, the pseudo echo generator 13 has the slowest frequency roll-off characteristic as the echo path 8 to be approximated, and the impulse width to be approximated is minimum. Also,
Since a fixed delay circuit with a tap coefficient of zero corresponding to the estimated echo path delay is inserted, the number of taps that need to be adaptively controlled can be greatly reduced compared to the conventional method, and high-speed convergence can be realized.

Although not described in detail, also for the echo canceller for a conference, it is possible to realize an echo canceller device that converges at high speed by similarly introducing a pseudo roll-off filter that approximates the frequency roll-off characteristic of the echo path. it can.

[0024]

As is apparent from the above, the present invention has the following effects.

(1) By inserting a pseudo roll-off filter that approximates the frequency roll-off characteristic of the transmission band of the echo path 8, the impulse width of the adaptive filter in the pseudo echo generators 10 and 13 is narrowed, and the convergence speed and the echo are reduced. It is possible to realize a high-quality echo canceller device with improved erasing ability and stable operation.

(2) A pseudo roll-off filter is inserted, the echo path delay is estimated using the output of the pseudo roll-off filter and the linear transmission input signal, and the pseudo echo generator 13 is provided with a fixed delay circuit corresponding to the echo path delay. By setting the number, the number of coefficient adaptive control taps required for the adaptive filter can be greatly reduced, the hardware can be simplified, and an echo canceller device having a large high-speed convergence characteristic and performing stable operation can be realized.

(3) The impulse width of the pseudo echo generator 13 can be minimized by preparing a plurality of pseudo roll-off filters and selecting and using a filter having an optimum frequency roll-off characteristic from among them. By setting the fixed delay circuit in the same manner as in 2), the hardware can be simplified and an echo canceller device having a much higher speed convergence and a very high echo canceling ability can be realized.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a conventional echo canceller device 1 applied to a PCM line.

FIG. 2 is a configuration example of an echo canceller device 22 according to a first embodiment of the present invention.

FIG. 3 is an example of an impulse response waveform of an adaptive filter in a pseudo echo generator.

FIG. 4 is a configuration example of an echo canceller device 24 according to a second embodiment of the present invention.

FIG. 5 is a configuration example of an echo canceller device 26 according to a third embodiment of the present invention.

[Explanation of symbols]

1 Conventional echo canceller device for PCM line 2 PCM reception input terminal (Rin) 3 PCM reception output terminal (Rout) 4 PCM decoder 5 2 line 4 line converter 6 2 line input / output terminal 7 PCM encoder 8 echo path 9 PCM Transmission input terminal (Sin) 10, 13 Pseudo echo generator 11, 12 Linear code converter 14 Subtractor 15 Non-linear processor 16 Non-linear code converter 17 PCM transmission output terminal (Sout) 18 Input terminal of the pseudo echo generator 10 Output terminal 20 of pseudo echo generator 10 Input terminal of subtractor 14 Output terminal of subtractor 14 22 Echo canceller device for PCM line 23, 27, 28 Pseudo roll-off filter 24 in Embodiment 1 of the present invention Implementation of the present invention Echo canceller device for PCM line in Example 2 25 Echo path delay estimator 26 Echo Canceller Device for PCM Line in Embodiment 3 of the Present Invention 29 Optimal Pseudo Roll-Off Filter Selector

Claims (4)

[Claims] 1. A subtractor inserted into a transmission circuit for removing an echo component from a transmission signal, and a pseudo echo component is supplied to the subtractor according to a relationship between a reception signal and a residual echo of an output of the subtractor. An echo canceller device having a pseudo echo generating means, comprising a pseudo roll-off filter for inputting the received signal, providing the pseudo echo generating means with an output of the pseudo roll-off filter, and generating a pseudo echo. Echo canceller device. 2. The echo canceller device according to claim 1, further comprising an echo path delay estimating unit which is coupled to the transmitting circuit and the pseudo echo generator to estimate an echo delay time,
An echo canceller device characterized in that a fixed delay circuit corresponding to the output of said means is provided in said pseudo echo generating means. 3. The echo canceller device according to claim 1, further comprising a plurality of said pseudo roll-off filters, selecting an optimum pseudo roll-off filter, and outputting the output thereof to said pseudo-roll-off filter. An echo canceller device provided to an echo generation means. 4. A pseudo roll-off filter having the largest correlation between the transmission signal and the output of each pseudo roll-off filter,
An echo canceller device according to claim 3, wherein an optimum pseudo roll-off filter selector is provided which selects as the optimum pseudo roll-off filter.