JPH0113774B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in an echo suppression device for compensating for echo phenomena occurring in long-distance communication lines. In particular, the present invention relates to a device using a learning identification method suitable for digital communication signals (PCM signals), which can maintain a large echo return loss improvement (ERLE) when a near-end call is present.

[Conventional technology]

FIG. 1 shows a block diagram of a device using a conventional learning identification method. In FIG. 1, reference numeral 1 denotes an echo suppression device, which has four terminals: a transmission input Sin, a transmission output Sout, a reception input Rin, and a reception output Rout. Reference numeral 2 denotes a transmission line, for example, a domestic transmission line, to the far end of which a subscriber terminal 4 is connected via a two-wire/four-wire converter 3. Terminals Sout and Rin are connected to extremely long distance communication methods such as satellite communications or submarine cables. Here, the above four terminals
The signal passing through Sin, Sout, Rin and Rout is
This is a digital communication signal, for example, a PCM signal with a sampling period of 8 bits and a sampling period of 125 μs.

The signal at the terminal Rin is taken into the delay circuit 10.
This delay circuit 10 is a known delay circuit with a tap.
It is constituted by a register that sequentially stores signal bits at the terminal Rin over a period of time that can cover the round trip delay time of the transmission line 2, and sequentially shifts the signal bits. The readout output of this delay circuit 10 is input to a convolution calculation circuit 11. This circuit 11 is a delay circuit 1
The load value hn^(k), which will be described in detail later, is added to the output x(k) of 0.
, and the results are added over a predetermined number of samplings N.

The output of the convolution arithmetic circuit 11 is input to a subtraction circuit 12, where the difference from the transmission input signal (Sin) is calculated, and the resultant signal is sent out as a transmission output signal (Sout). The output y^(k) of the convolution calculation circuit 11 is
If the signal passing through the terminal Rout is reflected by the two-wire/four-wire converter 3 and is equal to the echo y(k) that arrives at the terminal Sin, then the echo y(k) at the terminal Sout
is canceled out.

In order to control the load value, this subtraction circuit 12
The output of the delay circuit 10 and the output of the delay circuit 10 are taken into the adaptive control circuit 13, which generates a load value correction output Δhn^(k). This load value correction output Δhn^(k) is given to the load value correction circuit 15 via the switching circuit 14. Here, this load value correction output is added to the immediately preceding load value to obtain a new load value hn^(k), and this is provided to the convolution calculation circuit 11.

That is, when the adaptive control circuit 13 successively generates positive or negative load value correction outputs Δhn^(k) so that the echo component included in the signal appearing at the terminal Sout becomes smaller, the operation of this system gradually converges. As a result, the echo at the terminal Sout is maintained at its lowest level.

In this state, when the subscriber terminal 4 sends a call and it arrives at the terminal Sin, it passes through the subtraction circuit 12 and appears at the terminal Sout. This terminal at this time
The Sout signal is not an echo, so it should not be controlled to be small. The near-end call detection circuit 16 detects the presence of a near-end call by detecting the signal levels of the terminals Sin and Sout, and when there is a detection output, opens the opening/closing circuit 14 and corrects the load value of the adaptive control circuit 13. The output is prevented from being given to the load value correction circuit 15. As a result, the load value modification circuit 15 is prohibited from modifying its load value only while near-end communication exists, and continues to send out the immediately previous load value.

FIG. 2 shows changes in the echo return loss improvement amount (ERLE) of such an echo suppression device. Second
When this device is reset at time 0 in the figure, load value correction outputs are sent out one after another as described above.
ERLE gradually increases and reaches the saturation point _E1 . time
When a near-end call is started at _T1 , a new load value correction output is generated until it is detected by the near-end call detection circuit 16 at time _T2 , so that ERLE gradually deteriorates. The switching circuit 14 opens at time T ₂ and since there is no modification of the load value from this time T ₂ onwards, ERLE remains constant at the value E ₂ . When the near-end call disappears at time _T3 , the load value is corrected again and ERLE improves toward the value _E1 .

When a near-end call occurs in this way, there is a phenomenon in which ERLE deteriorates until it is detected. This is because the near-end call detection circuit 16 determines that there is a near-end call based on the signal levels of the terminals Rout and Sin, for example, when the level difference therebetween exceeds a threshold value. In other words, the method of detecting the presence or absence of a near-end call based on this level is an excellent method with accurate operation, but in real call audio, a level that suddenly exceeds the threshold is reached from the start of the call. Since there are only a few, detection will inevitably be delayed, and ERLE will deteriorate accordingly.

Regarding this conventional device, see Takahashi, Sakamoto, and Watanabe, ``General Purpose Echo Canceller'', Electronics and Communication Technology Committee Study Group [Communication Methods Study Group], sponsored by the Institute of Electronics and Communication Engineers, December 22, 1981, document number CS-
81-139 has a detailed description.

[Problem that the invention seeks to solve]

In addition, in order to solve the problems caused by the poor detection sensitivity of near-end call detection using the level difference detection described above and the delay in detection, two or more pseudo echo paths that are adaptively controlled are provided. Detects disturbances in the characteristics of near-end calls,
A technique has been proposed to prevent deterioration of ERLE when a sensitive near-end call occurs in a configuration that prohibits adaptive control (Japanese Patent Laid-Open Publication No. 28915/1983).

However, since this method attempts to detect the occurrence of a near-end call by detecting disturbances in a pseudo echo path provided with a variable filter, there is a problem in that the detection is slow even though the detection sensitivity is improved.

The present invention aims to improve this problem and to provide an echo suppression device that quickly detects when a near-end call occurs and reduces the deterioration of echo return loss improvement (ERLE).

[Means for solving problems]

The present invention is configured to, based on the load value correction output obtained at the output of the adaptive control circuit, assume that a near-end call has started when this change is rapid, and prohibit the correction of the load value. It is characterized by

In particular, the present invention is characterized by comprising means for inhibiting the load value correction operation of the load value correction circuit when the function of the absolute value of the load value correction output exceeds a predetermined threshold value.

[Effect]

In the present invention, the degree of change in the load value correction output of the adaptive control circuit is detected, and when the change exceeds a certain value, it is assumed that a near-end call has occurred, and the load value control of the adaptive control is stopped. are doing. The present invention
Since the instantaneous change in the control value is detected, the occurrence of a near-end call can be detected earlier than detecting a change in the convolution integral, and it is possible to effectively prevent deterioration of the echo return loss improvement amount.

〔Example〕

FIG. 3 is a block diagram of an apparatus according to an embodiment of the present invention. Comparing this configuration with the conventional device shown in FIG. The output of 17 is passed through the OR circuit 18 to the near-end call detection circuit 16.
It is characterized in that it is applied to the control input of the switching circuit 14 along with the output of the circuit. The configuration of other parts is similar to the conventional device shown in FIG.

In this example, the control circuit 17 cumulatively adds the absolute value of the load value correction output for a predetermined number of samples (for example, 8), and detects whether or not this cumulatively added value exceeds a preset threshold. When this value is exceeded, a signal is sent to the output, and in this case, the switching circuit 14 is opened and the load value correction circuit 15 is prohibited from making a new correction of the load value.

That is, when the echo suppression device enters a stable operating state, the load value is hardly modified, and the load value equal to the immediately previous load value is sent out from the load value correction circuit 15. When a near-end call occurs in such a state, the adaptive control circuit 13 operates to temporarily cancel the call and generates a load value correction output with a large absolute value. The control circuit 17 monitors this, quickly detects that this is due to the occurrence of a near-end call, and prohibits the load value correction operation.

Returning to FIG. 2 to explain this, when a near-end call occurs at time _T1 , this is detected at time _T2 ', and from this point on, modification of the load value is prohibited. As a result, the echo return loss improvement amount (ERLE) is maintained at the value E ₂ ' (Figure 2 - dashed line).
The disappearance of the near-end call is detected at time T ₃ ′,
From this point onwards, recovery begins. Therefore, the deterioration of ERLE is smaller than in the case of the conventional device, resulting in good characteristics as a whole.

The operation of the control circuit 17 is to calculate _N-1 〓 ⁿ⁼⁰ |Δhn^(k)| (where N is a predetermined number of samples, for example 8), and detect whether or not this exceeds a predetermined threshold. However, it may be calculated from the square value as _N-1 〓 ⁿ⁼⁰ (Δhn^(k)) ² , or the number of samples N may be made smaller or larger. It is easiest to extract absolute values from hardware. In addition, it is possible to detect sudden changes in the load value correction output using various calculation forms from functions based on the absolute value, such as detecting time changes (time differentiation) in the absolute value of the load value correction output. Can be done.

FIG. 4 is a diagram showing an example of the configuration of the control circuit 17. This input contains the absolute value of the load value correction output |Δh ₀ ^(k)|, |Δh ₁ ^(k)|, |Δh ₂ ^(k)
N pieces are given in chronological order as |, ......|Δh _N-1 ^(k)|, . This is cumulatively added every N in the cumulative adder 21, and the comparator 2
2 is given to one input a. A certain threshold value is given to this other input b. Further, this comparator 22 is supplied with a set signal c for controlling its comparison operation. The output d of this comparator 22 is sent to an output terminal via a hangover time setting circuit 23.

FIG. 5 is an operational waveform diagram of each part of the control circuit 17. 5A to 5E show the waveforms at the points in FIGS. 4A to 4E. That is, when a line connection response signal is received at time t=0 and the load value correction circuit 15 is cleared, the echo suppression device starts operating from the time _t1 has elapsed. At this time, the output of the cumulative adder 21 is zero. In subsequent times, the load value is modified significantly one after another and gradually becomes stable. After time t ₂ elapses, the amount of correction becomes smaller and stabilized.

After stabilization, a set signal c is sent to the comparator 22.
is given to start the comparison operation. After this stable state continues, the output a of the cumulative adder 21 increases and when it exceeds the threshold value b, the comparator 22
An output d is sent from. This is sent out as output e in circuit 23 with a slight delay of time t ₃ and prohibits modification of the load value.

When the output of the cumulative adder 21 becomes small again and falls below the threshold value b, the output signal e is stopped with a so-called hangover time _t4 .
This hangover time is chosen to correspond to a syllable break in a call, and is provided to avoid unnecessary repeated activations and stops in the middle of a call. This is time t ₅ when modification of the load value is prohibited in the operation shown in FIG.

In the above embodiment, the sampling synchronization T (T in FIG. 5) is 125 μs.

The cumulative adder 21 has a configuration in which a delay circuit whose delay time is equal to the period T is connected to the output of the adder, and the output of this delay circuit is feedback-connected to the input of the adder. This delay circuit is reset by a predetermined cumulative adder.

The embodiment device described in FIG. 3 is equipped with a near-end call detection circuit 16 based on level detection, which is also included in the conventional device. This circuit 16 is effective when the near-end call is a voice that suddenly rises at a high level, and its operation is stable. Therefore, this circuit 16 can be used in combination with the control circuit 17 as in the above embodiment. is desirable.

The above explanation assumes that the call signal is a digital signal, but even if the echo suppression device operates digitally, the call signal can be processed by inserting an AD converter or an AD converter into each transmitting and receiving signal. It can also be applied when the signal is an analog signal.

〔Effect of the invention〕

As explained above, according to the device of the present invention,
A device can be obtained that can reduce deterioration in echo return loss improvement (ERLE) when a near-end call occurs. In particular, the present invention has an excellent effect of quickly and accurately detecting the occurrence of near-end calls whose initial level is low.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional device. Figure 2 is a diagram showing an example of the change in echo return loss improvement amount (ERLE) over time; the solid line is that of the conventional device;
The dashed-dotted line shows the example of the present invention. FIG. 3 is a block diagram of an apparatus according to an embodiment of the present invention. FIG. 4 is a block diagram of the main parts of the control circuit. FIG. 5 is an explanatory diagram of the operation. DESCRIPTION OF SYMBOLS 1...Echo suppression device, 2...Transmission line (for example, domestic transmission line), 3...Two-wire/four-wire converter, 4...Subscriber terminal.

Claims (1)

[Claims] 1. Delay circuit 1 that takes in and delays a received input signal.
0, a convolution calculation circuit 11 that multiplies the output of this delay circuit by a load value and adds the result of the multiplication over a predetermined number of samples, and subtracts the output of this calculation circuit from the transmission input signal to obtain a transmission output signal. an adaptive control circuit 13 that generates a load value correction output as a function of the output of the delay circuit so as to reduce the echo appearing in the output of this arithmetic circuit; A load value correction circuit 15 that adds the value to a new weight value, and a near-end call detection circuit that detects the signal level difference between the received input signal and the transmitted output signal to detect the occurrence of a near-end call. 16, and a switching circuit 1 that prohibits the load value correction operation of the load value correction circuit based on the output of this near-end call detection circuit.
4, an echo suppression device comprising: an accumulative adder 21 that cumulatively adds the load value correction output from the adaptive control circuit 13 for a certain period of time; a comparator 22 that outputs a pulse;
a control circuit means 17 comprising a hangover circuit 23 for delaying the pulse output of the comparator by a hangover time after its disappearance; and a logical OR of the output of the control circuit means 17 and the output of the near-end call detection circuit 16. An OR circuit is added to stop the operation of the load value correction circuit when the cumulative addition value of the load correction value output exceeds a predetermined value or when a signal from the near-end call detection circuit is input. An echo suppression device characterized by: 2. The echo suppression device according to claim 1, wherein the cumulative adder 21 in the control circuit means 17 cumulatively adds the square value of the load value correction output for a certain period of time.