JPH0374538B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improvement in an echo canceller system capable of effectively eliminating reverberation signals caused by 2-wire to 4-wire conversion of telephone lines.

[Technical background of the invention and its problems]

It is well known that there are two types of telephone lines, 2-line lines and 4-line lines, and that both lines coexist.
The connection between the two is usually made using a hybrid circuit. Ideally, this hybrid circuit can achieve impedance matching with a line, but in reality, impedance mismatching often occurs due to line connection conditions or other reasons. For this reason, signal looping, ie, sidetone, occurs in the hybrid circuit, causing communication disturbances and howling.

Therefore, we estimated the sidetone characteristics, or echo path characteristics, of the above hybrid circuit in the form of an impulse response.
Various echo canceller systems have been researched to eliminate sidetone by generating a pseudo-echo signal according to this impulse response and subtracting it from the echo signal (echo signal), thereby preventing speech interference and howling. There is.

FIG. 1 is a block diagram of an example of a conventional echo canceller system, and numeral 1 in the figure indicates a hybrid circuit. Transmission signal X(+) is sent to A/D converter 2
The signal is sequentially converted into digital data for each sampling period, and is input to an echo canceller 3 consisting of a pseudo echo signal generation circuit and an echo path estimation circuit. The pseudo echo signal generating circuit of the echo canceller 3 is constituted by a variable transversal filter, the tap coefficient of which is set to match the impulse response of the echo path from point a to point b. In this way, the echo canceller 3 outputs a pseudo echo signal. On the other hand, the received signal is digitally converted by the A/D converter 5 and input to the subtracter 4.
After the echo signal component is removed from the received signal in the subtracter 4, a residual signal is outputted via the D/A converter 6 and the low frequency converter 7.

The tap coefficient of the echo canceller 3 is determined by initial training. During training, move switch 9 to the training signal generator 8 side,
The characteristics of the echo canceller 3 are set using the training signal generated thereby. In order to accurately set the characteristics, training is performed when there is no receiving signal. After the training is completed, the characteristics of the echo canceller 3 are fixed, the switch 9 is turned to the transmitting signal side, and a call is started.

It is also possible to consider a configuration in which the echo canceller system is of an adaptive type and the tap coefficient is corrected as needed. Although the adaptive method has the advantage of being able to follow changes in the echo path characteristics, it has the disadvantage that the received signal component prevents the correction operation and tends to cause deterioration of the erasure performance. Therefore, the former training type echo canceller system will be considered here as the premise of the invention. Although the training echo canceller system cannot follow changes in echo path characteristics, there are relatively few cases in which the echo path characteristics change to the extent that this becomes a problem.

The above is an overview of the conventional echo canceller system, but the following points remain as problems.
The level of the reception output R(f) from the hybrid circuit 1 is affected by the amount of line loss from the other party's telephone to the hybrid circuit, and it must be considered that it fluctuates by about 20 dB. If the dynamic range of the A/D converter 5 is set to match the maximum level of R(f), the dynamic range of the A/D converter is only a fraction when R(f) is at the minimum level. This means that only a portion of the system will be used.
This increases the relative quantization noise and increases the SN
There was a problem that the ratio deteriorated.

This is simply a conventional
The problem cannot be solved simply by inserting AGC. what if
Assuming that AGC is inserted before A/D converter 5,
Since the gain of AGC changes every time, the echo path from point a to point b in Figure 1 (including AGC)
The impulse response of will also constantly fluctuate. However, since the training echo canceller fixes the tap coefficient after the initial training, it cannot follow such fluctuations in the impulse response, and its cancellation performance deteriorates significantly.

[Purpose of the invention]

The present invention was made in view of the above-mentioned drawbacks of the conventional training-type echo canceller system, and its purpose is to provide a variable gain amplifier and its gain control circuit that are compatible with the system configuration of the training-type echo canceller. and effectively utilize the dynamic range of the A/D converter to reduce quantization noise generated in the A/D converter,
The object of the present invention is to provide an echo canceller system with good performance.

[Summary of the invention]

The outline of the invention will be explained according to FIG. 1 is a hybrid circuit, 2 is an A/D converter, 3 is an echo canceller consisting of a pseudo echo signal generation circuit and an echo path estimation circuit, 4 is a subtracter, 5 is an A/D converter, 6
7 is a D/A converter, 7 is a low-frequency wave generator, 8 is a training signal generator, and 9 is a switch, which are the same as those in the conventional training type echo canceller system. The present invention adds a variable gain amplifier 10 and its gain control circuit 11 between the hybrid circuit 1 and the A/D converter 5 to compensate for line loss in these conventional training type echo canceller systems. The gain of the variable gain amplifier is set prior to estimating the echo path, and the gain is fixed thereafter.

〔Effect of the invention〕

The problem when using conventional AGC in a training type echo canceller system is that conventional AGC
Since the gain is corrected even after the training operation, the echo path characteristics vary slightly depending on the gain at each point in time, and the echo cancellation performance fluctuates accordingly. In contrast, in the present invention, the gain of the variable gain amplifier 10 is fixed at the time of line connection and once set, so the above-mentioned fluctuations in the echo path characteristics do not occur, and the best echo cancellation performance is achieved. The result is that it is maintained. On the other hand, the main cause of level fluctuations is variation in line loss, and large level fluctuations occur at the moment the line is switched.
Once the line is connected, there are almost no level fluctuations. Therefore, even if the gain of the variable gain amplifier 10 is controlled only when a line is connected as in the present invention, most of the level fluctuations can be absorbed.

As described above, according to the present invention, it is possible to absorb level fluctuations in a received signal without causing any hindrance to the operation of the training type echo canceller system. The benefits are significant, such as improving the utilization rate of the microwave oven and improving the characteristics of the echo canceller system.

[Embodiments of the invention]

FIG. 3 shows an example of a variable gain amplifier and a gain control circuit used in the present invention. Reference numeral 10 denotes a variable gain amplifier, which is composed of a ladder resistor 101, a switch group 102, and an amplifier 103. The other part is a gain control circuit, 11
1 is a peak detection circuit, 112 is an A/D converter, 1
13 and 114 are I/O ports, 115 is a bus, 1
16 is a microprocessor. The configuration of the variable gain amplifier 10 itself is not particularly new;
It has already been commercialized.

Therefore, although a detailed explanation of the variable gain amplifier 10 is omitted, the switch 102 controls the amplifier 10.
The basic principle is to control the current flowing into the amplifier 103 and perform current-to-voltage conversion using the amplifier 103.

Next, the operation of the gain control circuit will be described.

When the line is connected to the hybrid circuit, the peak value of the received signal R(t) including the echo signal is first detected by the peak detection circuit 111, and this peak value is detected by the A/D converter 112, I/O port 113,
The data is read into the microprocessor 116 via the microprocessor 116. The microprocessor 116 compares the detected reception level with a reference level given in the program and calculates the gain to be given to the variable gain amplifier 10. Various algorithms for calculating the gain can be considered, but simply G=V ₀ /Vd However, the gain may be determined based on the detected reception level where Vd is the detected reception level and V ₀ is the reference level. Once the gain is determined in this way, the gain is applied to the variable gain amplifier 10 via the I/O port, and thereafter the gain is fixed. This gain setting is performed before the echo canceller training, as described above.

Note that the algorithm for calculating the gain can be changed as appropriate depending on the application. For example, if the present invention is
Consider application to an echo canceller for a wire duplex modem. It is convenient to measure the received signal level using a control signal exchanged for initial setting of the modem when the line is connected. However, since these control signals are sent and received alternately, it is not possible to measure the received signal level and the echo signal level at the same time. In this case, for example, both the received signal level and the echo signal level may be measured individually, and the gain may be determined based on whichever is greater.

In this way, the gain setting algorithm can be modified and implemented in various ways without departing from the spirit of the present invention.

FIG. 4 shows another embodiment of the variable gain amplifier and gain control circuit. 10 is a variable gain amplifier having the same configuration as in FIG. 3, 111 is a peak detection circuit,
112 is an A/D converter, 117 is a comparator, 118
is an up/down counter.

The operation of the gain control circuit of this embodiment is as follows. First, the output level of the variable gain amplifier 10 is detected by the peak detection circuit 111. this is,
After being converted into a digital value by the A/D converter 112, it is compared with a reference value in the comparator 117. Depending on the result of this comparison, the up/down counter 118 is set to +1 or -1, and a new gain is given to the variable gain amplifier 10. For example, if the detected level is less than the reference value, the up/down counter is set to +1 and the gain is increased. When the above operation is repeated and the output level of the variable gain amplifier 10 reaches the reference level, the enable signal of the up/down counter is set to L, and subsequent gain modification is prohibited. Needless to say, the above gain setting is performed before the echo canceller starts operating.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional example of a training type echo canceller system, FIG. 2 is a configuration diagram of an echo canceller system of the present invention, and FIG. 3 is a configuration example of a variable gain amplifier and its gain control circuit. figure,
FIG. 4 is a diagram showing another example of the configuration of a gain amplifier and its gain control circuit. 1-hybrid circuit, 2-A/D converter, 3
~Echo canceller, 4~Subtractor, 5~A/D converter, 6~D/A converter, 7~Low frequency converter, 8~
Training signal generator, 9~ switch, 10~
Variable gain amplifier, 11~gain control circuit, 101~
Ladder resistor, 102 - switch group, 103 - amplifier, 111 - peak detection circuit, 112 - A/D converter, 113, 114 - I/O port, 115 -
bus, 116 - microprocessor, 117 - comparator, 118 - up/down counter.

Claims (1)

[Scope of Claims] 1. A device for canceling echo signals caused by 2-wire to 4-wire conversion of a telephone line, the echo path estimation circuit estimating the echo path characteristics of at least the 2-wire to 4-wire conversion circuit. a pseudo-echo signal generation circuit that generates a digitized pseudo-echo signal according to the estimated echo path characteristics; an A/D converter that converts the received signal from analog to digital; In the training type echo canceller, which is equipped with a subtraction circuit that subtracts from the output of the converter, estimates the echo path characteristics only in a specially set training period, and then fixes the characteristics thereafter. A variable gain amplifier for line loss compensation and a control circuit for setting the gain are provided between the circuit and the A/D converter, and the gain is set before the echo path characteristics are determined. An echo canceller system that is characterized by keeping the . 2. The echo canceller system according to claim 1, wherein the gain of the variable gain amplifier is determined by measuring the level of the received signal including the echo signal. 3. The echo canceller system according to claim 1, wherein the gain of the variable gain amplifier is determined by individually measuring both the level of the echo signal and the level of the received signal and determining the greater of the two. .