JPH02278927A - Simplified lattice filter type echo canceller - Google Patents

Abstract

PURPOSE:To obtain an acoustic echo canceller with less quantity of calculation by providing an adaptive one-channel lattice filter processing a white level to a received input signal. CONSTITUTION:A received input signal Rin(i) is subject to white level processing with an adaptive one-channel lattice filter 30 and a transmission input signal Sin(i) is subject to the same pre-processing for that of the reception signal at a one-channel lattice filter 40 by using a coefficient obtained from the film 30. The transmission input signal subject to pre-processing and the reception input signal subject to white level processing are inputted to an adaptive 2channel lattice filter 10, in which the signal to be sent and the received signal are separated. The coefficient of the filter 10 is used to use a 1st output (signal to be sent) of the filter 10 as a 1st input and zero is given to the 2nd input, the result is inputted to a 2-channel inverse lattice filter 20 and a 1-channel inverse lattice filter 50 eliminates the distortion of the pre-processing of the 1st output of the filter 20 to form the transmission output of the signal to be sent only. Thus, the quantity of calculation is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is an acoustic echo generator that eliminates echoes caused by acoustic coupling in audio transmission devices using speakers and microphones, such as remote conferences and hands-free telephones. It concerns cancellers.

(Prior Art) Generally, it is difficult to prevent acoustic coupling in a two-way audio transmission device using a speaker and a microphone. If this acoustic coupling has a large transmission delay, it will sound like an echo, and if the coupling is even stronger, it will cause howling. To prevent these phenomena, we use a voice switch that inserts loss in the direction where the voice level is low in two-way communication, and an adaptive FIR type filter that estimates the impulse response of the echo path to synthesize pseudo echoes and remove them from the transmitted signal. An acoustic echo canceller is used.

(Problems to be Solved by the Invention) A voice switch cannot, in principle, perform simultaneous two-way communication, leading to interruptions in topics and endings of conversation, resulting in deterioration in call quality.

An acoustic echo canceller was devised as a way to overcome these drawbacks. However, the acoustic coupling of interest has a very long impulse response.

Therefore, in the FIR type echo canceller, the tap length becomes significantly long, and the amount of calculation required for processing is enormous, leading to an increase in hardware.

SUMMARY OF THE INVENTION An object of the present invention is to provide an acoustic echo canceller with a small amount of calculation in order to solve these problems.

(Means for Solving the Problems) The simplified lattice filter type echo canceller of the present invention includes an adaptive one-channel lattice filter that whitens a received input signal, and a transmitting input signal using a coefficient determined by the adaptive one-channel lattice filter. a 1-channel latex filter for filtering a signal; an adaptive 2-channel latex filter having the output of the adaptive 1-channel latex filter as a first input and the output of the 1-channel latex filter as a second input; a two-channel inverse latex filter that takes the first output of the channel latent filter as its first input and its second input as zero, and filters with the coefficients obtained by the adaptive two-channel latent filter; and a first of the two-channel inverse latent filter. and a 1-channel inverse latex filter whose output is filtered by the coefficients determined by the adaptive 1-channel latex filter and whose transmission output is the output.

(Function) An echo canceller using a two-channel adaptive lattice filter is described in "Implementation of system identification method and signal estimation using multi-channel lattice filter" by Zhao et al., IEICE Technical Report IT
87-66. First, the algorithm of the M-order lattice filter is shown below.

(1) Algorithm of M-order filter (a) Calculation of prediction error Zero-order forward prediction error vector eo(i) and zero-order backward prediction error vector r, (i) at time i are calculated using transmission input signal %(i
), then the forward prediction error vector e, (i) of each stage is defined as
and the backward prediction error vector r, (i) are calculated as follows.

(j=0,1.....,M-1) (
1) K, (i-1), K, , (i-1) are the forward prediction coefficient matrix and the backward prediction coefficient matrix.

(b) Calculation of prediction coefficient matrix Correlation matrix RkJ (iXk = 1.2.3) is 1 = 0 (
7) Set the initial value to a zero matrix and calculate as follows.

r3. (i) = R3, (i-1)D + e, (i
)e, (i)' From this correlation matrix, the forward prediction coefficient matrix and the backward prediction coefficient matrix are θ=0.1. ..., M-1) (
3).

C) Calculation of composite signal The forward composite vector d7) and backward composite vector (i) are defined as follows.

菟(i) =To(i) Then, the forward composite vector d of each stage, (i) and the backward composite hector Kaishi(i) are expressed as (i) = Toshi(i)+, (i−υ stop i −υpossible”；a＞
= possible (i-1)-, (i-1)4 (i) (i=o,
1. ..., M-1) (4) Calculate. Transmission output signal S. ut(i) is 5ouL(i)
='To(i) becomes the first term.

In addition to this calculation, stability determination is actually required. As described above, when an echo canceller is simply constructed using a lattice filter, the amount of calculation increases as in the case of the conventional adaptive IIR type filter.

If the received input signal is white noise, the second term of the forward prediction error vector is white noise, so the correlation is O except for the same signal, so the second column of R1; (t) is zero, and R3,
In (i), the off-diagonal elements are zero. Therefore, the adaptive operation is calculated as follows.

(2) For white input (a) Calculation of prediction error Zero-order forward prediction error e at time i. (i) and the zero-order backward prediction error vector ro(i) are defined as the transmission input signal S, n(i) and the received amount, then the forward prediction error of each stage e, (i) and the backward prediction eyebrow (i) = η1-1)-η1-1) Town (i) θ=
o, i, ..., M-1) (1)"
Here, (i), Ke, and (i) are a forward prediction coefficient vector and a backward prediction coefficient vector.

(b) Calculation of prediction coefficient vector Correlation matrix R2j(i) and vector Rt;
is calculated as follows, assuming that the initial value of i=o is a zero matrix and a zero vector.

From the above, the forward prediction coefficient vector and backward prediction coefficient vector are %(i) = R,, (i)su1) Uj(i) = element lπ of curse (i) element of i)) θ=
Q, l, ..., M-1) (3)
' is calculated.

C) Calculation of composite signals Forward composite vector dN(i) and backward composite vector q. (
i) is defined as follows.

It is expressed as dN(i)=eN(i). Then, the forward composite signal d of each stage, (i) and the reverse "shoulder (i) = purchase - 1) -": to j (i - 1) (i) (j
=o, 1. ..., M-1). Transmission output signal S. ut(i) is So,,(
g ”do(t).

(4)' In this calculation method, stability can be determined as long as K (i) KT (i) < 1. In this way, when the received signal is white noise, the amount of calculation can be significantly reduced.

However, the audio signal is not a white signal. Therefore, the received input signal is input to a one-channel adaptive lattice filter, and the whitening of the signal and the coefficients of the filter at that time are determined.

After preprocessing the transmission input signal using these coefficients, the transmission input signal and reception input signal are input to a two-channel adaptive lattice filter. At this time, since the received signal is whitened, it is possible to use the above-mentioned estimation algorithm [in the case of white input]. Furthermore, the output of the two-channel inverse latex filter remains distorted due to preprocessing.

This constructs an inverse filter using the coefficients of the preprocessing one-channel latex filter, and removes distortion from the output of the two-channel inverse latex filter.

(Example) Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. The received input signal R,(i) is whitened by a first channel latex filter 30. Transmission input signal S, (iX At this stage, the signal to be transmitted and the echo signal are superimposed)
is subjected to the same preprocessing as the received signal by the one-channel latex filter 40 using the coefficients determined by the adaptive one-channel latex filter 30. The signal to be transmitted and the received signal are separated by an adaptive two-channel latex filter 10, which takes the preprocessed transmission input signal as a first input and the whitened reception reception input signal as a second input.

Using the coefficients of this adaptive 2-channel latex filter 10, the first output of the adaptive 2-channel latex filter 10 (
The signal to be transmitted) is used as the first input, and the second input is used as the inverse lattice filter 50 for the signal to be transmitted by the 2-channel inverse lattice filter 20. The distortion caused by the pre-processing of the first output of 20 is removed to create a transmission output of only the signal to be transmitted.

[Effect 1 of the Invention According to the present invention, it is possible to realize an acoustic echo canceller with a small amount of calculation.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, 1... adaptive 2-channel latex filter, 20... 2-channel reverse latex filter, 30...
・Applicable to 1 channel latex filter, 40...1 channel latex filter, 50...1 channel reverse latex filter.

Claims (1)

[Claims] an adaptive 1-channel lattice filter that whitens the received input signal; a 1-channel lattice filter that filters the transmitted input signal with the coefficients determined by the adaptive 1-channel lattice filter; an adaptive 2-channel lattice filter with one input and the output of the 1-channel lattice filter as the second input; and an adaptive 2-channel lattice filter with the first output of the adaptive 2-channel lattice filter as the first input and the second input as zero. a two-channel inverse Latisse filter that filters with coefficients determined by the adaptive 1-channel Latisse filter, and a transmission output obtained by filtering the first output of the two-channel inverse Latisse filter with the coefficients determined by the adaptive one-channel Latisse filter. An echo canceller having an inverse channel lattice filter.