JPH0321122A - Echo canceller circuit for full duplex modem - Google Patents

Abstract

PURPOSE:To simplify the structure of an echo canceller circuit and to effectively cancel near end and far end echoes without being influenced by a time-varying element by individually forming the pseudo signals of the near end and far end echoes through different filters. CONSTITUTION:Application filters are divided into near end and far end echo canceller adaptive filters 21, 22. A bulk delay 20 is connected to the input stage of the filter 22 to hold a time difference required for the return of a far end echo F from a transmission point of time. Thereby, a sending signal S for forming a far end pseudo echo component is effectively inputted to the filter 22. Thereby, the number of taps of the filter 22 can be reduced. The output of the filter 22 is inputted to a phase follow-up circuit 23 to allow the input signal to follow up the frequency of an error signal E. Thereby a pseudo signal, i.e., a far end echo canceling signal SE, formed from the circuit 23 is allowed to further approximate to a practical far end echo. Consequently, a time-varying element such as a frequency offset and a phase jitter to be applied to the far end echo on a transmission line can be effectively canceled.

Description

[Detailed Description of the Invention] [Sanraku] Ni-no-Usatsu Bunmachi] The present invention relates to an F+Yancer circuit for a full-duplex communication motem, and in particular, a device that improves the performance of the echo-Yancer circuit that generates pseudo echoes. Regarding.

[Prior Art] In a two-wire full-duplex modem in which the modems communicate data with each other, there is a near-end echo that occurs when the transmitted signal is reflected by the hybrid in the own modem, and a similar echo that occurs in the hybrid in the other party's station. There are far FF:16 echoes that are generated and circulate, and these interfere with the received signal from the other party's modem and impede normal communication.

In order to eliminate these echoes, conventionally,
- There was an echo canceller circuit disclosed in the 2/12127 publication. The principle will be briefly explained using FIG. 2.

An information signal is input from the terminal side to the own station G, and the code converter 1 converts this into a multi-level transmission signal S.

This transmission signal S is input to the echo canceller 14, while passing through the roll filter 2, modulator 3, D/A converter 4, transmission filter 5, and sent out to the transmission path via the hybrids 1 and 6.

However, the near echo "11 echo N" generated by hybrid 6, and the far end echo F generated by the hybrid in the other station (■) and returned to the receiving side, and the received signal R from the other station I. superimposed on These superimposed signals pass through a reception filter 7, an A/D converter 8, a demodulator 9, and a roll-off filter 10, and reach an adder 19. Here, the pseudo echo S generated by the echo canceller 14 is subtracted. That is, near O:i+: echo N and far end echo F are removed to obtain received signal R. The sum P of the residual echo remaining without being removed and the received signal R passes through a determination circuit 11 and a code converter 12 to obtain a received information signal and is sent to the terminal side.

On the other hand, the difference between the determination result and the sum P of the residual echo and the received signal is obtained by an adder l3, and this is supplied as an error signal E to the coefficient control section of the adaptive filter 16 of the echo canceller 14 and the gain control circuit l7. . Adaptive filter 16
The coefficient control of and the gain control of the gain control circuit 17 are performed so that the error signal E is minimized. echo canceller 14
A fixed filter 15 was provided in the filter 15, and its output and the output of the adaptive filter 16 were guided to an adder 18, and the addition result was further manually input to a gain control circuit 17 for gain control. are doing.

Note that the fixed filter 15 is provided in order to reduce the dynamic amplitude of the echo impulse response of the adaptive filter 16.

[Problems to be Solved by the Invention] However, the conventional echo canceller circuit that uses a common adaptive filter to generate a total pseudo echo of near-end echo and far-end echo has the following drawbacks. Ta.

(1) The filter number increases.

An adaptive filter is constructed by connecting multiple delay elements in series to obtain a weighted sum of their tap outputs, but the time difference between the near-end echo and the far-end echo is long. (flat delay section shown in FIG. 4), the longer the length, the more delay elements are required, and as a result, the number of filter taps increases. As a result, the cost increases by 1.0, which requires a large-scale adaptive filter.

(2) If there is a time-varying element, it will not be canceled sufficiently.

The far-end echoes traveling along the transmission path may be affected by time-varying elements such as frequency offset and phase jitter on the transmission path, but these time-varying elements are not taken into account, so it does not include time-varying elements. Daen D: tA echo cannot be canceled sufficiently.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and simplify the structure by separately generating a pseudo signal of a near-end echo and a pseudo signal of a far-end echo using different filters. An echo for full-duplex modems that can effectively cancel near-end echoes and far-end echoes without being affected by time-varying elements even if the far-end echo contains time-varying elements. The purpose of the present invention is to provide a canceller circuit.

4 [Means for Solving the Problems] The echo canceller circuit for a full-duplex modem of the present invention produces a received signal in which a near-end echo generated at the own station that has issued the transmission signal and a far-end echo generated at the opposite station are superimposed. The configuration is such that the echo component superimposed on the received signal is removed by subtracting the pseudo echo generated by the adaptive filter from the received signal.

In such a configuration, the adaptive filter described in section 2 is divided into a near-end adaptive filter that creates a near-end echo pseudo signal and a far-end adaptive filter that creates a far-end echo pseudo signal. The input stage of the filter is provided with bulk delay delay means for delaying and inputting the response time transmission signal of the far end echo.

In addition, a synchronization circuit for synchronizing the pseudo signal of the far-end echo with the far-end echo is provided at the output stage of the far-end adaptive filter, and the output of this synchronization circuit and the output of the near-end adaptive filter mentioned above are connected. It is plotted to be subtracted from the received signal.

[Operation] When the transmission signal is input to the delay means for bulk delay, the transmission signal is delayed by the distance ttjf of the far D:I:echo response and is input to the far end adaptive filter at the subsequent stage.

Then, the far-end echo pseudo signal output from the far-end adaptive filter is created immediately from the timing at which the far-end echo responds. For this reason, although the delay time of 1.1 tar for creating the far-end echo pseudo signal remains the same, the delay time until the far-end echo responds can be replaced by the 1,000-stage delay for Hulk Tilley. Become.

Therefore, the response between the near-end echo and the far-end echo I1:! 、．
[Regardless of the time difference between HIs, it is possible to remove the delay number corresponding to the delay time of the delay stage for the Halkti Ray from the delay number of the far-end adaptive filter.

Furthermore, when a far-end echo pseudo signal is output from the far-end application filter, this output is input to the synchronization circuit.

Then, the pseudo signal of the far end echo is the same as the far D#5 echo.
Therefore, even if a time-varying element such as one on the transmission line (: , L'. The pseudo signals of the end echo and the far 'JRa echo match in phase.

Therefore, when the near-end echo pseudo signal and the far-end echo pseudo signal are subtracted from the received signal, the far-end echo component is also sufficiently canceled from the received signal.

[Example] Hereinafter, an example of the present invention will be described using FIGS. 1 and 3 to 5.

FIG. 1 shows an example of an echo canceller circuit for a full-duplex motem according to the present invention. The difference between this embodiment and the conventional example shown in FIG. The explanation will be omitted.

A multi-level transmission signal S converted by the code converter 1 is guided to an echo canceller 14 to be branched, and an adaptive filter 2l for a near-end echo canceller is provided which inputs one of the branched signals, thereby canceling the near-end echo. A signal (pseudo signal of near-end echo) SN is generated.

In addition, the other branch signal is delayed through the Hulk relay 2o, and this delayed signal is input to the remote i, ji,
An adaptive filter 22 for echo canceller is provided. The output of this far-end echo canceller adaptive filter 22 is input to a phase tracking circuit 23 as a synchronization circuit, thereby generating a far-end echo cancellation signal (pseudo signal of far-end echo) Sr.

Then, the near-end echo cancellation signal SN and the far-end echo 1"
The T' cancellation signals 81 to 81 are added to the adder 24 to generate a signal S, which is output from the effect canceller 14.

Here, the control input sections of the two adaptive filters 21 and 22 and the phase tracking circuit 23 in the echo canceller 14 include:
The error signal E obtained by the adder 13 is supplied.

The adaptive filters 21 and 22 may be acyclic digital filters or the like from the viewpoint of convergence and stability. Specifically, as shown in FIG.
An 81 Lanceversal filter configured to arbitrarily amplify and obtain the sum of these weights using an adder Σ is suitable. Note that other filters are not excluded, and it is also possible to use, for example, a recursive tintal filter.

As shown in FIG. 4, the Hulk relay 2o provided at the input stage of the adaptive filter 22 for far smoke echo canceller is
It is responsible for the time difference between the beginning of the transmitted signal that precedes the response of the near-end echo N and the response of the far-end echo F. The reason why the Hulk delay 20 can be used in this way is that there is a flat delay between the impulse responses of the near-end echo N and the far-end echo F. Specifically, the bulk delay 20 can be configured with a multi-stage amplifier, which is the simplest delay means, with only terminals or input/output terminals and no taps in the middle. I do not care.

Also, the length of the tiling is determined to be the optimum value during Motem's training period.

Further, the phase tracking circuit 23 as a synchronization circuit provided at the output stage of the far-end echo canceller adaptive filter 22 is
The method is APC (Automatic
Various methods are possible, such as phase control), crystal filter method (linking method), and burst injection lock method, but here we will exemplify the P L l method that is most commonly used in circuits using ICs, as shown in Figure 5. There is. That is, the output of the adaptive filter for the far-end echo canceller is converted into an analog signal by the modulator MD and guided to the PLL, and the phase difference between this frequency and the frequency of the error signal E is converted into a voltage. This converted voltage is fed back to follow the frequency of the error signal E, and is converted into a digital signal by the demodulator DE to obtain the far-end echo cancellation signal SF. Note that the PLL is generally a primary or secondary loop circuit.

Next, the operation of the above-described configuration will be explained.

The transmission signal S is a multilevel signal, and this signal S is guided to the echo canceller l4 and input to the near-end echo canceller adaptive filter 21, from which the near-end echo cancellation signal S. The first one was created.

Further, the transmission signal S is delayed through a bulk delay 20 and is also input to an adaptive filter 22 for a far-end echo canceller. Adaptive filter 22 for far-end echo canceller
The output is input to the phase tracking circuit 23, where a compensation amount for the frequency offset 1 received on the transmission path is calculated, and a far-end echo cancellation signal S, which includes this compensation amount, is created. The created near-end echo cancellation signal SN and far-end echo cancellation signal SF are added to an adder 24 to generate a total echo cancellation signal S.

On the other hand, the transmission {g signal S passes through a roll-off filter 2, a modulator 3, a D/A converter 4, a transmission filter 5, and is sent out to a transmission path via a hybrid 6. Then, as described above, the superimposed signal of the near-end echo N1, far-end echo F, and reception signal R passes through the reception filter 7, A/D converter 8, demodulator 9, and roll-off filter 10, and then enters the adder 19. reach

This adder 19 generates the ! generated by the echo canceller 14! - The total echo cancellation signal (pseudo echo) S is subtracted from the superimposed signal to obtain a sum signal P of the received signal R and the residual echo. The addition signal PII passes through a determination circuit 11 and a code converter 12 to obtain a received information signal, which is sent to the terminal side.

Further, the addition signal P is guided to the adder 13 together with the determination result of the determination circuit 11, and the difference between them is determined. This difference is used as an error signal E in the near-end echo canceller adaptive filter 21 and the far-end echo canceller adaptive filter 22.
, are input to the phase tracking circuit 23, and are adaptively controlled so that the error signal E is minimized. For example, steepest descent width can be used for this adaptive control.

As described above, according to this embodiment, the applied filter is separated into the near-end echo canceller applied filter 2l and the far-end echo canceller applied filter 22, and Since the bulk delay 20 is inserted to take care of the time difference from the time of transmission until the far-end echo F wraps around and returns, the far-end pseudo echo is sent to the far-end echo canceller adaptive filter 22. The transmission signal S that creates the component is effectively input. Therefore, it is no longer necessary to provide a large number of unit delay elements for the above-mentioned time difference in the number of taps of the adaptive filter 22 for far-end echo canceller, and the adaptive filter 22 for far-end echo canceller
The number of tassops can be significantly reduced. the result,
The number of taps of the adaptive filter 22 for the far-end echo canceller can be made the same as that of the adaptive filter 2l for the near-end echo canceller, and the total number of taps of both filters 21 and 22 is also smaller than that of the conventional common adaptive filter. be able to.

Furthermore, the output of the far-end echo canceller adaptive filter 22 is input to the phase tracking circuit 23 provided at the subsequent stage, so that the filter output follows the frequency of the error signal E, which is affected by time-varying elements. , a pseudo signal generated from the phase tracking circuit 23, that is, a far-end echo cancellation signal SP
can approximate the actual far-end echo even more. Therefore, time-varying elements such as frequency offset and phase jitter added to the far-end echo on the transmission path can be effectively canceled.

In this way-52-. ．． 4! 、． 'Z. j-. ,i
,1i・Example(,,{phrase!,H: VI Easy Hanorectilei adapted 7・I 几く・ as a substitute for some of the valleys, and also included time-varying elements - I-2U-, J, Ru As long as the disturbance is not reduced, it is possible to obtain a double motem at a low cost.

In the embodiment described in 43 and L, the manual signal to the phase tracking circuit is changed to erroneous If the credit card contains A], for example, the receiver (11 signal R
Either can be used, or an added signal I) with a residual echo added to the received signal can be used.

Effects of the Invention 1 According to the present invention, the following effects can be achieved. (1) An adaptive filter is used to create a pseudo signal of a near-end echo. ;+) Pseudo 13 - Applicability ratio for making a bow, filter and divided into ゛, ding, more,
The trick: The echo component superimposed on the received signal can be removed from the received f8 signal.

(2) By providing a delay means for the Hulk Tilley with a simple structure and no intermediate delay, the delay time until the far-end echo responds is taken over by the delay means. Todai has a complicated structure;! 1: River j7 1
．． a: Without reducing the number of taps the filter has, the body structure can be made more frugal.

(3) By providing a synchronization circuit that synchronizes the pseudo signal of the far-end echo with the far-end echo, even if the far-end echo has a time-varying element, it will not be affected by the time-varying element. , near-end echo and far-end echo can be effectively canceled.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an echo canceller circuit for a full-duplex modem according to the present invention, and FIG. 2 is a block diagram showing an example of an echo canceller circuit for a full-duplex modem according to the present invention.
Figures 7 and 3 show the adaptive filter used in this example.
Figure 1 is an explanatory diagram showing an echo impulse response, and Figure 5 is a block diagram showing an example of a phase tracking circuit used in this embodiment. 14 is an echo canceller, 19 is a received signal by subtracting l5 pseudo echo by 1 < force r + calculator, 20 is a Hulk delay which is a delay means for Hulk delay, and 21 is a near 6W
a adaptive filter for the echo canceller, 22 an adaptive filter for the far end echo canceller, 23 a phase tracking circuit as a synchronization circuit, S the transmission signal, G the own station, N the near end j':
・1, I is the other station, rr is the far end echo, R is the reception {alarm signal, SN is the near end echo 1'l which is the light signal of the near end echo
The li'4 L signal, S, is a far end echo imprint signal of the far b:6j echo pseudo signal. 16 Proceeding City Authorization (Volunteer) October 30, 1989

Claims (1)

[Claims] By subtracting the pseudo echo generated by the adaptive filter from the received signal, which is a superimposition of the near-end echo generated at the own station that issued the transmission signal and the far-end echo generated at the other station, the received signal is In an echo canceller circuit for a full-duplex modem that removes superimposed echo components, the above adaptive filter is used as a near-end adaptive filter that creates a pseudo signal of the near-end echo, and a far-end adaptive filter that creates a pseudo signal of the far-end echo. The input stage of the far-end adaptive filter is provided with bulk delay delay means that delays the response time transmission signal of the far-end echo, and the output stage of the far-end adaptive filter is A synchronization circuit for synchronizing the pseudo signal of the end echo with the far end echo is provided, and the output of the synchronization circuit and the output of the near end adaptive filter are subtracted from the received signal.
Echo canceller circuit for heavy modem.