JPS6252491B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an echo cancellation device for a data modem.

Typically, this type of device uses a transversal filter to
This device generates a pseudo-echo signal from a transmitted signal of 2400 sec) or a transmitted signal with no baud interval, and subtracts the pseudo-echo signal from an incoming received signal that includes an echo, thereby obtaining a received signal that does not include an echo. Then, the tap coefficients of the transversal filter are adaptively modified by the so-called MSE (mean square error) method, which minimizes the average power of the error signal, which will be described later.

Conventional echo canceling devices use a baud-interval transmitted signal as the input signal of the transversal filter, and subtract a pseudo echo signal from an incoming received signal containing an echo as an error signal, and then generate a baud-interval received signal as an error signal. There is one that uses a signal obtained by subtracting . However, in this conventional example, since the pseudo echo signal is a signal with a baud interval, there is a drawback that the echo cannot be sufficiently canceled from the incoming received signal. On the other hand, in other arrival examples, a non-baud-interval transmission signal is used as the input signal of the transversal filter, and a signal obtained by subtracting the pseudo echo signal from the arrival reception signal is used as the error signal. However, in this conventional example, the error signal has a large average power due to the transmission signal component of the opposing modem, so the tap coefficient of the transversal filter becomes unstable and the adaptive ability is lost in the case of double talk. Ta.

SUMMARY OF THE INVENTION An object of the present invention is to provide an echo canceling device for a data modem that can sufficiently cancel echoes from incoming received signals and can also be adapted to double talk.

According to the present invention, there is provided a modem transmission circuit that receives a transmission signal with a baud interval and issues a transmission signal with a non-baud interval to an opposing modem; a first transversal filter for generating a simulated echo signal; a first subtractor for subtracting the simulated echo signal from an incoming received signal; and a modem receiver for generating a baud-spaced received signal from the output signal of the first subtractor. a determination circuit; a second transversal filter that receives the baud-interval received signal and generates an estimated transmission signal that is not the baud-interval of the opposing modem and is controlled by a second control signal; and the first subtraction. a second subtracter that generates an error signal obtained by subtracting an estimated transmission signal that is not at the baud interval of the opposing modem from the output signal of the delay circuit; a first tap coefficient correction circuit for issuing the first control signal for modifying the tap coefficient of the first transversal filter; and a second tap coefficient modification circuit for modifying the tap coefficient of the second transversal filter. There is obtained an echo canceling device for a data modem characterized in that it has a second tap coefficient correction circuit for generating a control signal.

A detailed explanation will be given below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the former conventional echo canceling device described above. 1 is a transmission signal (baud interval) input terminal to the modem, 2 is a modem transmission circuit, 3 is a modem transmission signal output terminal, 4 is a reception signal input terminal, 5 is a modem reception judgment circuit, 6 is a judgment result (baud interval) output terminal,
7 is the first subtractor, 8 is the second subtractor, 9 is the first
10 is a tap coefficient correction circuit, and 10 is a first transversal filter. The modem transmitting circuit 2 receives a baud-interval transmission signal A _b (hereinafter, the subscript b indicates a baud-interval signal), and a non-baud-interval transmission signal A _o (hereinafter, the subscript n indicates a Nyquist interval (for example, , 1/9600sec) or an analog signal). The first transversal filter 10 inputs A _b and outputs a pseudo echo signal E _b (the leading sign 〓 indicates an estimated value). The transmission signal B _o from the opposing modem that is not at the baud interval is subject to distortion etc. due to the transmission medium B _o '
, and the arriving received signal on which the echo E _o is superimposed reaches the received signal input terminal 4. The first subtractor 7 subtracts the pseudo echo signal E _b from the incoming received signal B _o '+E _o . The modem reception determination circuit 5 outputs a baud-interval received signal B _b (estimated signal of the baud-interval transmitted signal B _b of the opposing modem) from the output signal of the first subtracter 7 . second subtractor 8
Now, an error signal obtained by subtracting B _b from the output signal of the first subtractor 7 is output. The first tap coefficient correction circuit 9 outputs a first control signal for correcting the tap coefficient of the first transversal filter 10 using the MSE method that minimizes the average power of the error signal. In this way, since the first transversal filter 10 outputs the pseudo echo signal E _b at the baud interval, the echo cannot be canceled sufficiently.

FIG. 2 is a block diagram showing the configuration of the echo canceling device according to the latter conventional example mentioned above. As shown in the figure, the first transversal filter 10 inputs A _o and outputs a pseudo echo signal E _o . The error signal input to the first tap coefficient correction circuit 9 is the output signal of the first subtracter 7, that is, the signal obtained by subtracting E _o from the incoming received signal B _o '+E _o . Therefore, even if the echo is almost completely canceled (E _o 〓E _o ), the error signal has a large average power due to the transmitted signal component B _o ' of the opposing modem, so the tap coefficient is unstable. I tended to. To prevent this, measures have been taken to set the correction coefficient of the tap coefficient of the first transversal filter 10 to zero or to an extremely small value during double talk.
However, double talk is common in data modems, and conventional devices such as those described above almost always lose the ability to adapt to changes in the echo propagation path, which is a serious problem in practice.

FIG. 3 is a block diagram showing the configuration of one embodiment of the echo canceling device according to the present invention.
1 (or 2), 11 is a delay circuit, 12 is a second tap coefficient correction circuit, and 13 is a second transversal filter. With this configuration, the error signal used by the first tap coefficient correction circuit 9 is the received signal B that has arrived at the first subtracter 7.
This is a signal obtained by subtracting not only the pseudo echo signal E _o of the first transversal filter 10 but also the estimated transmission signal B' _o of the opposing modem output from the second transversal filter 13 from o'+ _{E o} . . In order to obtain this estimated transmission signal B' _o , the output signal B _b of the modem reception determination circuit 5 is input to the second transversal filter 13 . and,
The tap coefficient of the second transversal filter 13 is adaptively determined by the second control signal of the second tap coefficient correction circuit 12, that is, by the MSE method using the output signal of the second subtracter 8 as an error signal. will be corrected. Further, the delay circuit 11 includes a modem reception determination circuit 5 and a second transversal filter 1.
This is used to correct the amount of delay at 3. Therefore, even in the case of double talk, there is no need to reduce the correction coefficient in the first tap coefficient correction circuit 9.

Although the above embodiments are directed to baseband transmission systems, it goes without saying that the present invention can also be applied to passband transmission systems.

As is clear from the above description, according to the present invention, it is possible to realize an echo canceling device for a data modem that can almost completely eliminate echo from an incoming received signal and has a high adaptability even in the case of double talk. The benefits are great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a conventional example of an echo canceling device for a data modem, FIG. 2 is a block diagram showing the configuration of another conventional example of an echo canceling device for a data modem, and FIG. 3 is a block diagram showing the configuration of a conventional example of an echo canceling device for a data modem. 1 is a block diagram showing the configuration of an embodiment of an echo canceling device for a data modem according to the present invention. Explanation of symbols: 1 is the transmission signal input terminal, 2 is the modem transmission circuit, 3 is the transmission signal output terminal, 4 is the reception signal input terminal, 5 is the modem reception judgment circuit, 6 is the judgment result output terminal, 7 is the first 8 is a second subtracter; 9 is a first tap coefficient correction circuit; 10 is a first transversal filter; 11 is a delay circuit; 12 is a second tap coefficient correction circuit; 13 is a second tap coefficient correction circuit; Each represents a transversal filter.

Claims (1)

[Claims] 1 A modem transmission circuit that receives a transmission signal with a baud interval and emits a transmission signal with a non-baud interval to the opposing modem;
a first transversal filter for emitting a pseudo-echo signal controlled by a control signal; and a first transversal filter for subtracting the pseudo-echo signal from an incoming received signal.
a subtracter, and a modem reception determination circuit that generates a baud-interval received signal from the output signal of the first subtracter;
a second transversal filter that receives the baud-spaced received signal and generates an estimated non-baud-spaced transmitted signal of the opposing modem controlled by a second control signal; and an output signal of the first subtractor. a second subtracter that generates an error signal by subtracting an estimated transmission signal that is not at the baud interval of the opposing modem from the output signal of the delay circuit; a first tap coefficient correction circuit for generating the first control signal for correcting the tap coefficient of the universal filter; and a first tap coefficient correction circuit for correcting the tap coefficient of the second transversal filter in response to the error signal. 1. An echo canceling device for a data modem, comprising: a second tap coefficient correction circuit for emitting a second control signal.