JPS61135239A - Echo canceller - Google Patents

Abstract

PURPOSE:To secure the stable working of an echo canceller at all times despite a long-time input of a narrow-band signal, by detecting the narrow-band properties of a received input signal and inhibiting the correction of coefficient of an adaptive filter. CONSTITUTION:An adaptive filter 5 stores the estimated impulse response of an echo route 9 as the coefficient of the filter 5 and filters the input signal received through a terminal 1 to produce an artificial echo signal and to supply this echo signal to a subtractor 6. The subtractor 6 subtracts the echo signal from the signal supplied through an input terminal 3 at the transmission side through a route 9 with cancellation of the echo component to deliver the subtracted signal through an output terminal 4 at the transmission side. A narrow-band signal detector 7 measures the narrow-band properties of the input signal at the reception side and inhibits the correction of the coefficient of the filter 5 when a narrow-band signal is decided. Thus an echo canceller has the stable working with no accumulation of arithmetic errors of the tap coefficient of the filter 5 despite the long-time reception of the narrow-band signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an echo canceller used to eliminate echo signals generated in long-distance telephone lines and the like to improve call quality.

Conventional technology In recent years, echo cancellers have been developed to improve call quality by eliminating echo signals generated in long-distance telephone lines or teleconferencing systems that connect remote points.Proc, ICASS
P 83. PP, 466-469, Apri1198
An echo canceller as described in 3) has been developed. An example of the conventional echo canceller described above will be described below with reference to the drawings.

FIG. 6 is a block diagram of a conventional echo canceller. 6th
In the figure, 23 is a receiving side input signal terminal, 24 is a receiving side output signal terminal, 26 is a transmitting side input signal terminal, 26 is a transmitting side output signal terminal, and 27 is a terminal based on the receiving side input signal and the transmitting side output signal. An adaptive filter 28 includes a variable coefficient transversal filter for creating an estimated value (pseudo echo signal) of an echo signal contained in the input signal on the transmitting side; 29 is an echo canceller, and 30 is an echo path that generates an echo signal from the receiving side input signal to the transmitting side input signal.

The operation of the echo canceller configured as described above will be described below.

In the following explanation, the signal inside the echo canceler will be treated as a time-sampled discrete time signal, but the analog-to-digital converter and digital-to-analog converter necessary for this purpose are well known, and will not be explained below. I will omit it here.

The echo canceller 29 uses the adaptive filter 2a to calculate the impulse response (
corresponding to the filter coefficients of the adaptive filter)h, (t=o,
1. ...-, N-1, N is the number of impulse response samples), and the estimated impulse response is (
△ is a symbol representing an estimated value) and the receiving side input signal as shown in equation (1) (filtering the receiving side input signal with the filter coefficient), the transmitting side input signal Zl is obtained. A pseudo echo signal 11 to be included is created.

Next, the subtracter 28 is used to subtract the pseudo echo signal from the transmitter input signal as shown in equation (2) to create a transmitter output signal from which the echo signal has been eliminated.

ei=”j −yj ””°゛゛゛′°゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛゛) This is done by modifying the filter coefficients at each sampling period according to the algorithm of the learning identification method shown in the equation.

・・・・・・・・・・・・・・・(3) However, α is 0
<α is a constant equal to 2.

As described above, by using an echo canceller in a long-distance telephone port number or a communication conference system, it becomes possible to conduct a natural telephone conversation in which echo signals are eliminated.

Problems to be Solved by the Invention However, in the echo canceller configured as described above, if the receiving side input signal is a narrowband signal such as a modem signal, the operation becomes unstable if the echo path estimation continues for a long time. This has the problem of causing a significant deterioration in the amount of echo cancellation.

That is, the echo canceller 29 uses the adaptive filter 27 to estimate the impulse response of the echo path 30. However, if the input signal on the receiving side to be estimated is a narrowband signal, the frequency band of the input signal on the receiving side Since the echo path cannot be estimated outside, the impulse response cannot be estimated correctly. In fact, the literature Niyamamoto,
Coming to Mt.

Tazai, Chihara, Sakurai, ``Echo canceller algorithm and its characteristics using a fast Kalman filter''.

'rl Ziren (iir academic journal (") + ""
, ' 6 '-By A 4°PP, 318-325.
According to 1978, when the receiving side input signal is an audio signal,
It has been reported that echo path estimation is mainly performed during unvoiced speech periods with little correlation and wide band.

In addition, a calculation error occurs when estimating the echo path due to the finite register length when implementing the echo canceller in hardware, the introduction of noise into the echo path, and the like. Therefore, if the input signal on the receiving side is a narrowband signal, if the echo path estimation is continued for a long time, the calculation errors will gradually accumulate, making the operation unstable and causing a significant deterioration in the amount of echo cancellation. There was a point.

In view of the above problems, the present invention provides an echo canceller that operates stably not only for voice signals but also for narrowband signals such as modem signals.

Means for Solving the Problems In order to solve the above-mentioned problems, the echo canceller of the present invention includes an adaptive filter that creates a pseudo echo signal based on a receiving side input signal and a transmitting side output signal, and a transmitting side input signal. a subtractor that creates the transmitter output signal by subtracting the pseudo echo signal from a signal; and a subtractor that measures the narrowband nature of the receiver input signal and prohibits modification of the coefficients of the adaptive filter based on the narrowband nature. It consists of a narrowband signal detector.

The echo canceller of the present invention has the above-described configuration, so that when the input signal on the receiving side is a narrowband signal, the narrowband signal detector detects the narrowband nature of the signal and prohibits modification of the coefficients of the adaptive filter. As a result, the estimation of the echo path is stopped, so that the calculation error described above does not accumulate and the operation is stable. Therefore, the echo canceller of the present invention can always operate stably even when not only audio signals but also narrowband signals such as modem signals are input for a long time.

Embodiment Hereinafter, an echo canceler according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a configuration diagram of an echo canceller in an embodiment of the present invention. In FIG. 1, 1 is a receiving side input signal terminal, 2 is a receiving side output signal terminal, 3 is a transmitting side input signal terminal, 4 is a transmitting side output signal terminal, and 6 is based on a receiving side input signal and a transmitting side output signal. 6 is an adaptive filter consisting of a variable transversal filter for creating a pseudo-echo signal by subtracting the pseudo-echo signal from the transmitting-side input signal; 7 is a narrowband signal detector that measures the narrowband nature of the input signal on the receiving side and prohibits modification of the coefficients of the adaptive filter based on the narrowband nature; 8 is an echo canceller; 9 is the receiving side; An echo path that generates an echo signal from an input signal to the transmitter input signal.

The operation of the echo canceller configured as above will be described below.

First, a pseudo echo signal is created by filtering the receiving side input signal using the adaptive filter 6 which stores the estimated impulse response of the echo path 9 as its filter coefficient.

Next, a subtracter 6 is used to subtract the pseudo echo signal from the transmitter input signal to create a transmitter output signal.

The narrowband signal detector 7 measures the narrowband nature of the input signal on the receiving side, and when it is determined to be a narrowband signal based on the narrowband nature, a signal ( Hereinafter referred to as a filter coefficient modification prohibition signal) is output.

The filter coefficients of the adaptive filter 6 are estimated when the narrowband signal detector 7 does not output the filter coefficient modification prohibition signal and when the input signal on the transmitting side is generated only by the echo signal of the input signal on the receiving side. For example, this is done by successively modifying the filter coefficients according to the algorithm of the learning identification method described above.

The narrowband signal detector 7, which is a feature of the present invention, will be explained in more detail below.

FIG. 2 shows an internal configuration diagram of the first embodiment of the narrowband signal detector 7. As shown in FIG. In FIG. 2, 10 is an input signal terminal on the receiving side, 111 to 11n are band pass filters for dividing the input signal on the receiving side into band signals of a plurality of (n) frequency bands, and 121 to 12n are input signal terminals for the band signals. A short-time energy calculator 13 for calculating the short-time energy measures the narrowband nature of the n band signals based on the short-time energy, and if it is determined to be a narrowband signal, an adaptive filter 5 14 is a filter coefficient modification inhibition signal output terminal.

The operation of the narrowband signal detector configured as above will be explained below.

When a receiving side input signal is input from the receiving side input signal terminal 10, n band signals are obtained by n band pass filters 111 to 11n having frequency characteristics as shown in FIG. Next, the short-time energy calculator 121 to 1
2n is used to calculate the short-time energy of each band signal and output it to the narrowband determiner 13.

When the input signal on the receiving side is a narrowband signal, a short-time energy distribution diagram as shown in FIG. 4, for example, is obtained.

The narrowband determination unit 13 calculates the total short-time energy of the band signal to obtain the total short-time energy, and also calculates the ratio of the short-time energy of the band signal to the total energy, and determines whether the value of the ratio is a predetermined value. If the threshold is exceeded, it is determined that the signal is a narrowband signal with energy concentrated in a specific frequency band, and a filter coefficient modification prohibition signal is output as the filter coefficient modification prohibition signal 14.

As described above, according to this embodiment, the narrowband characteristic of the receiving side input signal is measured from the short-time energy distribution on the frequency axis of the receiving side input signal, and the coefficients of the adaptive filter are corrected based on the narrowband characteristic. To provide an echo canceller that operates stably without accumulating calculation errors during echo path estimation even if a narrowband signal is input as a receiving side input signal for a long time by providing a narrowband signal detector that prohibits echo path estimation. be able to.

FIG. 5 shows an internal configuration diagram of a second embodiment of the narrowband signal detector. In FIG. 5, 15 is a receiving side input signal terminal, 16 is a zero crossing frequency calculator for calculating the zero crossing frequency of the receiving side input signal, 17 is a memory for storing the zero crossing frequency, and 18 is a memory 17. a maximum value detector 19 for detecting the maximum value of the number of zero crossings stored in the memory; 19 a minimum value detector for detecting the minimum value of the number of zero crossings stored in the memory; 20 the zero crossing Subtractor for determining the difference between the maximum value and the minimum value of the number of times, 21
determines the narrowband nature of the input signal on the receiving side based on the difference between the maximum value and the minimum value of the number of zero crossings, and outputs a filter coefficient modification prohibition signal to the adaptive filter when it is determined to be a narrowband signal. The determiner 22 is a filter coefficient modification inhibition signal output terminal.

The operation of the narrowband signal detector configured as above will be explained below.

When the receiving side input signal is input from the receiving side input signal terminal 15, the zero crossing number calculator 16 calculates the number of zero crossings in a predetermined time (hereinafter referred to as a frame), and calculates the number of zero crossings in a predetermined time (hereinafter referred to as a frame), and is written into the memory 17 for storing the number of zero crossings. Next, the contents stored in the memory 17 are read using the maximum value detector 18 and the minimum value detector 19, and the maximum value and minimum value of the number of zero crossings are respectively detected. Next, the subtracter 2o is used to find the difference between the maximum and minimum values of the number of zero crossings. If the difference between the maximum value and the minimum value of the number of zero crossings is smaller than a predetermined threshold value, that is, if the number of zero crossings continues within a predetermined range for a predetermined period of time, the narrowband determination unit 21 determines whether the receiving side It determines that the input signal is a narrowband signal and outputs a filter coefficient modification inhibition signal to the filter coefficient modification inhibition signal output terminal 22.

As described above, according to this embodiment, the narrowband characteristic of the receiving side input signal is measured from the parameter on the time axis such as the number of zero crossings of the receiving side input signal, and the coefficients of the adaptive filter are determined based on the narrowband characteristic. To provide an echo canceller that operates stably without accumulating calculation errors during echo path estimation even if a narrowband signal is input as a receiving side input signal for a long time by providing a narrowband detector that prohibits modification. I can do it. In addition, in the second embodiment, since the narrowband property is determined using the easily calculated parameter of the number of zero crossings, the amount of hardware and calculations are smaller than in the first embodiment. It has the characteristic of being good.

Effects of the Invention As described above, the echo canceller of the present invention measures the narrowband nature of the input signal on the receiving side on the frequency axis or the time axis, and prohibits modification of the coefficients of the adaptive filter based on the narrowband nature. By providing a band detector, it can operate stably not only for audio signals but also for narrowband signals.
Its scope of application can be expanded.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an echo canceller in a first embodiment of the present invention, FIG. 2 is an internal configuration diagram of a narrowband signal detector in the first embodiment, and FIG. 3 is a frequency diagram of the bandpass filter. 4 is a diagram showing the short-time energy distribution of a band signal when a narrow band signal is used as an input signal. FIG. 6 is an internal configuration diagram of the narrow band signal detector of the second embodiment. FIG. 6 is a block diagram of a conventional echo canceller. 1... Receiving side input signal terminal, 2... Receiving side output signal terminal, 3...... Transmitting side input signal terminal, 4...... Transmitting side output signal Terminal, 6...
Adaptive filter, 6... Subtractor, 7...
Narrowband signal detector, 8...Echo canceller,
9...Echo path, 1o...Receiving side input signal terminal, 111-11n...Band pass filter, 121-12n...Short-time energy calculator , 13... Narrowband determiner, 14...
・Filter coefficient modification inhibition signal output terminal, 15...
- Receiving side input signal terminal, 16... Zero crossing frequency calculator, 17... Memory, 18... Maximum value detector, 19... Minimum value Detector, 20...
. . . Subtractor, 21 . . . Narrowband determination device, 22
...Filter coefficient modification inhibition signal output terminal. Name of agent: Patent attorney Toshio Nakao and one other person wJ
Figure 1 Figure 2

Claims (3)

[Claims] (1) An adaptive filter that creates a pseudo-echo signal based on a receiving-side input signal and a transmitting-side output signal; and a subtracter that creates the transmitting-side output signal by subtracting the pseudo-echo signal from the transmitting-side input signal. and a narrowband signal detector that measures the narrowband nature of the input signal on the receiving side and prohibits modification of the coefficients of the adaptive filter based on the narrowband nature. (2) The narrowband signal detector divides the receiving side input signal into band signals of a plurality of frequency bands, measures the narrowband nature of the receiving side input signal based on the band signal, and measures the narrowband nature of the receiving side input signal. 2. The echo canceller according to claim 1, wherein modification of the coefficients of the adaptive filter is prohibited based on the echo canceller. (3) The narrowband signal detector measures the number of zero crossings of the receiving side input signal, measures the narrowband nature of the receiving side input signal based on the number of zero crossings, and adapts based on the narrowband nature. 2. The echo canceller according to claim 1, wherein modification of filter coefficients is prohibited.