JP2766887B2 - Echo canceller - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly applied to a conference loudspeaker call, and is a reverberation canceling device for canceling a reverberation causing a howling and a hearing impairment. The present invention relates to a band-division type echo canceller that divides an echo for each band.

"Conventional technology" With the spread of audio conference equipment, simultaneous callability has been secured,
It is desired to provide a loudspeaker device with less reverberation.

FIG. 5 shows an example of a conventional band division type echo canceller. From the receiving input terminal 1 receiving the receiving signal x (t), the receiving signal x (t) is converted into N signals x _i (t) (i
= 1,..., N) and a frequency band synthesizing circuit 3 for synthesizing these divided signals x _i (t) again into the reception signal x (t) to the speaker 4. And the output signal z (t) from the microphone 5
Is divided into N signals z _i (t) in the same manner as the frequency band division circuit 2, and a subtraction circuit for subtracting the estimated echo signal y _i (t) from each of the signals z _i (t). q _i , through a frequency band synthesizing circuit 7 for synthesizing the outputs of the subtraction circuits q _i , and a speech system including a transmission system reaching a transmission output terminal 8, a reverberation path 11 from the speaker 4 to the microphone 5 is transmitted. The echo signal y (t) of the received signal x (t) wrapping around and the target signal v (t), which is the near-end transmitted voice, are transmitted through the microphone 5.
And a transmission signal z (t) is formed. On the other hand, the band-divided reception signal x _i (t) is an FIR filter (FIR is fi
nite impulse response, abbreviated as a finite length impulse response, this filter does not include a feedback circuit and therefore has no poles.
_{Via i} , an estimated echo signal y ′ _i (t) for each band is generated.
Estimated echo signal y 'from band-divided transmission signal z (t)
_i band-by-band signal to remove the echo signal by subtracting _(t) by subtraction circuit q _i e _i (t) is obtained, it was synthesized by band synthesis circuit 7 signal e (t) of the FIR filter If the coefficient is equal to the impulse response coefficient for each band of the echo path 11 from the speaker 4 to the microphone 5, it is equal to the near-end transmitted voice (target signal) v (t), and the purpose of canceling the echo signal is achieved. Although there is no need for band division in principle (N = 1), this method is used to realize real-time processing of digital signal processing.

"Problems to be Solved by the Invention" The number of taps (the number of coefficients) of the FIR filter is represented by the product of the sampling frequency f of the signal input thereto and the time length T of the impulse response. However, when the reverberation path 11 is a sound field such as a voice conference apparatus, the impulse response is generally long, for example, 0.5 seconds, and f is 8 kHz.
However, there was a drawback that the number of taps was enormous at 4,000. If the number of taps is large, it becomes difficult to economically configure the hardware, and there is a problem that the adaptive performance for following the fluctuation of the impulse response also deteriorates.

An object of the present invention is to reduce the total number of taps by introducing a part of an IIR type filter into a digital filter constituting a pseudo echo path, thereby realizing economical hardware and improved adaptive performance. It is to provide a device.

[Means for Solving the Problems] The present invention relates to a band split type echo canceller, in which some of the multiple echo reverberations for each band are included.
Instead of the FIR filter, an IIR filter having a feedback loop (IIR is an abbreviation of infinite impulse response, which has a pole) is used. With this configuration, the total number of taps is reduced, and hardware economics and adaptive performance are improved. The difference from the prior art is that an IIR filter is used for a part of a set of pseudo echo paths that are band-divided. Focusing on the physical characteristics of the sound field, the number of poles and zeros of the transfer function of the sound field from a loudspeaker to a microphone in a certain band is divided into bands, and the number of poles and zeros in the band constitutes a quasi-echo path with an FIR filter for that band. If the number of taps is less than the FIR filter, I
By configuring a pseudo echo path with an IR filter, the number of taps can be reduced. Therefore, the number of taps of the filter summed over the entire band can be reduced as compared with the related art. Conversely, the number of poles and zeros of the transfer function of the sound field from the speaker to the microphone in a certain band is smaller than the number of taps of the FIR filter when a pseudo echo path is configured by the FIR filter for that band. As described above, by selecting a band and dividing the band, it is possible to reduce the total number of filter taps over the entire band.

"Embodiment" FIG. 1 shows an embodiment of the present invention, and the same signals are assigned to the portions corresponding to those in FIG. In this example, for the M bands on the low frequency side in the frequency bands divided into N, a pseudo echo path (IIR pseudo echo path 12i (t)
= 1,..., M), and for the remaining NM bands on the high frequency side, a pseudo echo path (FIR type pseudo echo path 10j (j = M + 1,. N)).

To operate this, N signals x _i (t) (i = 1,...,...) For each band are received from the receiving input terminal 1 receiving the receiving signal X (t).
N), the frequency band division circuit 2 again divides the reception signal x
Speaker 4 through a frequency band synthesizing circuit for synthesizing (t)
, And a transmission system from the microphone 5 via the frequency band dividing circuit 6 and the frequency band synthesizing circuit 7 to the transmission output end 8 in the echo path 11 from the speaker 4 to the microphone 5. Received signal x wrapping around
The echo signal y (t) of (t) and the target signal v (t) which is a near-end transmission signal are synthesized by the microphone 5 and the transmission signal z
(T) is formed. On the other hand, x _i (t) (i = 1,..., M) of the band-divided reception signal and the frequency band
The transmission signal z _i (t) (i = 1,...,
M) is supplied to the IIR type pseudo echo path 12i to generate an estimated echo signal y ′ _i (t) (i = 1,..., M) for each band.
Further, x _j (t) (j = M) among the band-divided reception signals.
+1... N) are supplied to the FIR pseudo echo path 10j to generate band-wise estimated echo signals y'j (t) (j = M + 1,..., N). In each of the bands from i = 1 to N,
Band split transmission signal z _i (t) from the band-by-band estimation signal y 'i _(t) a subtraction circuit q band-by-band signal to remove the echo signal by subtracting at _i e _i (t) is obtained, These are combined by the band combining circuit 7, and the combined output signal e (t)
Is a target signal which is a near-end transmitted voice if each pseudo echo path (12 _{1 to} 12 _M , 10 _{M + 1} to 10 _N ) is equal to an echo path for each band of the sound field from the speaker 4 to the microphone 5. Equal to v (t), the goal of canceling the echo signal is achieved. The filter characteristics of the FIR pseudo echo path 10 _j are It is represented by Here, z is a delay element, bn is the number of delay taps, b _k is the tap coefficients.

Specific examples of the IIR type pseudo echo path 12 _i in Figure 2. It becomes more and film 201 series filter 202 of the feedback loop is the output of the band dividing circuit 6 to the filter 201 in the feedback loop z _i
(T) is input, and its filter characteristic is It is represented by Here, z is a delay element, an is the number of delay taps, and a _k is a tap coefficient. The characteristics of the series filter 202 And the output x _i (t) of the band division circuit 2 is input. Filter 2 of feedback loop from output of series filter 202
The output of 01 is subtracted by the subtraction circuit 203, and the output is supplied to the subtraction circuit 9 _i as an estimated echo signal y _i (t). This JJR type pseudo echo path is a conventional IIR without band division.
This is similar to the shape echo path.

Further, the configuration of the present invention may be the configuration of FIG. 3 instead of FIG. The difference between the first figure and FIG. 3 is an input signal of the IIR type pseudo echo path 12 _i, which the only signal x _i (t) is supplied, detailed configuration of the IIR type estimated echo path 12i is first 2
FIG. 4 is used instead of the figure. The difference between FIG. 2 and FIG. 4 is the input of the feedback loop filter 201. In the configuration of FIG. 4, the output y ′ _i (t) of the pseudo echo path itself is filtered by the filter 201.
It is used as an input for

As a method of selecting bands 1 to M using the IIR type pseudo-echo path from the N bands divided into bands, paying attention to the physical characteristics of the sound field, the band divided sound from a speaker to a microphone in a certain band is focused. If the number of poles and zeros of the field transfer function is smaller than the number of taps of the FIR filter in that band, an IIR filter is used instead of the FIR filter. Alternatively, the number of poles and zeros of the transfer function of the sound field from the loudspeaker to the microphone in a certain band should be smaller than the number of taps of the FIR filter in that band, and the number of band divisions in the entire band should not increase. A band is selected and the band is divided, and an IIR filter is used in that band instead of the FIR filter.

If the volume of the room where the reverberation path is placed is V (m ³ ) and the sound velocity is c (m / s), the number of sound field modes included from 0 to fHz is (Eg, R. Lyon, “Machinery Noise and
Diagnostics ", Butterworth, 1987).
In order to construct a filter, the number of poles is necessary for this number of modes, and it is necessary to expect the same number of zeros.
The required order (the number of taps) of the filter can be estimated as twice the number of modes. Therefore, for example, consider applying an echo canceller with a band of 400 Hz to a room with a reverberation time of 50 m ³ and a reverberation time of 0.5 seconds. If the sampling frequency is set to 800 Hz (since the sampling frequency must be at least twice the required bandwidth) and if one FIR filter is used without band division, 0.5 (sec) x 400 (Hz) = 400 taps are required. Next, from 0 Hz to 200 Hz, and 200 Hz to 4
If the frequency band is divided into frequencies up to 00 Hz and both FIR filters are used, 0.5 (sec) × 40 (Hz) × 2 = 400, and the total number of taps does not change. Next, if you try to configure both with IIR filters, 42 in the band from 0Hz to 200Hz (the number of modes)
× 2 = 84, 294 (the number of modes) × 2 = 588 in the band from 200 Hz to 400 Hz, and the total number of taps is 672, which is larger than that constituted by the FIR filter. However, if the IIR filter is applied in the band from 0 Hz to 200 Hz and the FIR filter is applied in the band from 200 Hz to 400 Hz, the total number of taps is 84 + 2.
00 = 284, and the total number of taps can be reduced as compared with any of the above-described configurations.

In general, as in this example, an IIR type pseudo echo path is used on the low frequency side and an FIR type pseudo echo path is used on the high frequency side, but the II bandwidth is also reduced by reducing the bandwidth on the high frequency side.
Sometimes an R-shaped pseudo echo path is used.

As is clear from the above, compared to the conventional configuration in which only the FIR filter is used for the pseudo echo path divided into bands,
This makes it possible to provide a device that reduces the total number of taps and realizes economical hardware and improved adaptive performance.

[Effects of the Invention] As described above, according to the present invention, in a band split echo canceller having a plurality of pseudo echo paths in parallel by dividing a signal into bands, some of the pseudo echo paths are fed back. Comprising an IIR pseudo-echo path with a loop and the remaining pseudo-echo path with no feedback loop, reducing the total number of taps, making the hardware economical and adaptable There is an advantage that a device that achieves improved performance can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the apparatus of the present invention.
FIG. 3 is a detailed view of the IIR pseudo echo path 12 _i (i = 1,..., M) in FIG. 1, FIG. 3 is a block diagram showing another embodiment of the apparatus of the present invention, and FIG. The IIR pseudo echo path 12 _i (i =
1, M), and FIG. 5 is a block diagram showing a conventional echo canceller.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification symbol FI H04B 3/23 G10K 11/16 H (56) References JP-A-64-29094 (JP, A) JP-A-1-114150 ( JP, A) JP-A-64-16030 (JP, A) JP-A-62-51836 (JP, A) JP-A-3-106208 (JP, A) JP-A-1-125113 (JP, A)

Claims (1)

(57) [Claims] 1. A receiving system for receiving a receiving signal, and a transmitting system for receiving the output of the receiving system via a reverberation path as an echo signal, receiving a target signal, and outputting a transmitting signal. It is used in a speech system, and generates a pseudo echo path by estimating a transfer characteristic of the echo path from the received signal to the echo path and the echo signal, and uses the received signal as an input of the pseudo echo path. By generating an estimated reverberation signal obtained by subtracting the estimated reverberation signal from the transmission signal, the reverberation signal is eliminated, and a reverberation elimination device configured to extract a target signal added to the transmission system and In a band-division echo canceller that divides a signal into frequency bands and includes a plurality of pseudo echo paths in parallel, some of the pseudo echo paths are constituted by an IIR type pseudo echo path having a feedback loop, and the remaining The above pseudo counter Road is echo canceller characterized in that it is composed of a FIR type estimated echo path which does not have a feedback loop.