CA1219649A - Echo cancelling system comprising cascade-connected local echo cancellers operable independently of one another - Google Patents
Echo cancelling system comprising cascade-connected local echo cancellers operable independently of one anotherInfo
- Publication number
- CA1219649A CA1219649A CA000457200A CA457200A CA1219649A CA 1219649 A CA1219649 A CA 1219649A CA 000457200 A CA000457200 A CA 000457200A CA 457200 A CA457200 A CA 457200A CA 1219649 A CA1219649 A CA 1219649A
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- Prior art keywords
- echo
- signal
- local
- cancelled
- delayed
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-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
- H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
- H04B3/237—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers using two adaptive filters, e.g. for near end and for end echo cancelling
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
- H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
- H04B3/238—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers using initial training sequence
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
- Filters That Use Time-Delay Elements (AREA)
Abstract
Abstract of the Disclosure:
In an echo cancelling system responsive to a send-in signal including an echo signal and to a receive-in signal, a plurality of local echo cancellers are connected in cascade through an additional delay circuit between two adjacent ones of the local echo cancellers to locally cancel parts of the echo signal. The receive-in signal is successively delayed by a delay circuit to be delivered to the local echo cancellers as delayed signals which have different delays relative to the receive-in signal and which correspond to the parts of the echo signal.
Each local echo canceller produces a partially echo-cancelled signal by the use of a transversal filter and an adder. Each partially echo-cancelled signal is sent to the transversal filter of each local echo canceller and to the additional delay circuit. The local echo cancellers are put into operation independently of one another.
In an echo cancelling system responsive to a send-in signal including an echo signal and to a receive-in signal, a plurality of local echo cancellers are connected in cascade through an additional delay circuit between two adjacent ones of the local echo cancellers to locally cancel parts of the echo signal. The receive-in signal is successively delayed by a delay circuit to be delivered to the local echo cancellers as delayed signals which have different delays relative to the receive-in signal and which correspond to the parts of the echo signal.
Each local echo canceller produces a partially echo-cancelled signal by the use of a transversal filter and an adder. Each partially echo-cancelled signal is sent to the transversal filter of each local echo canceller and to the additional delay circuit. The local echo cancellers are put into operation independently of one another.
Description
I
Backgrolmd ox the Invention is :inventioll relates to an echo canceling system for use in a long-distance telephone network all, in particular, in a long-distance conference communication system including a satellite.
Recently, a long-distance conference communication system has been developed which is suitable err a conference concurrently held at distant locations, namely, auditor or conference rooms geographically spaced from one another. Such a system is very effective for saving money, time and labor of participants attending the conference.
In the conference communication system, both a microphone and a loudspeaker should be installed in each auditorium so as to carry out bidirectional communication of an audio or voice signal. Accordingly, unfavorable acoustic coupling inevitably takes place between the loudspeaker and the microphone.
I 9~919 Furthermore, when each auditorium is spaced far from the others and is, in particular, communicable through a satellite with one another, a speaker's voice sent from one microphone in a near-end auditorium is returned back to the near-end auditorium in the form of an echo through acoustic coupling between the loud-speaker and the microphone placed at a far-end auditorium.
Various attempts have been made to cancel such an echo by the use of an echo canceling technique which has been used in a usual telephone communication network.
It should be noted here that a reverberation time of each auditorium is not less than 200 milliseconds and, therefore, the echo resulting from the acoustic coupling between the loudspeaker and the microphone lasts a very long time as compared with an echo sounding in the telephone communication network. In addition, it is general that a higher quality of voice is required in the long-distance conference system than in the tote-phone communication network.
Consideration has been given to an echo canceler comprising a transversal filter in order to cancel the echo in the long-distance conference system. Such an echo canceler can cancel the echo by calculating a simulated echo signal to be subtracted from an actual echo signal at every sampling period and by adaptively controlling tap gains of the transversal filter. However, the transversal filter should have a great number of taps so as to cancel such an echo which lasts a very I
long time. For example, let the sampling period or a sampling frequency be equal to 62.5 microseconds or 16 kHz. Let the echo be canceled over 200 milliseconds.
In this event, the number of the taps becomes equal to 3,200. Accordingly, calculation and control must be carried out in connection with the taps of 3,200 at every sampling period of 62.5 microseconds. The echo canceler becomes very intricate in structure because a great amount of hardware is necessary so as to precut-gaily carry out the above-mentioned calculation and control.
In United States Patent No. 4,377,793 assigned to Communications Satellite Corporation, O. A. Home discloses a digital adaptive transversal filter comprising two or more separate filter units connected in cascade.
With this structure, a large number of tap gains can be controlled by the use of an increased number of Coffey-clients. At any rate, the transversal filter produces an error signal representative of a component finally left uncanceled.
It should be noted in this connection that the error signal is produced at every sampling period and should be fed back to the respective filter units so as to modify each coefficient of the respective filter units at every sampling period. This means that a restrict lion is imposed on the number of the filter units connected in cascade. Therefore, the transversal filter can not cancel a long-term echo which lasts a long time. Moreover, it is difficult for the time being to purchase a large scale into-gyration circuit convenient for the echo canceler.
In Canadian Patent No. 1,178,385, issued on November 20, l98~ in the names of Takahashi Airsick et at, assignors to the present assignee, an echo canceling system is disclosed which comprises a combination of a voice switch and an echo canceler prepared for a high and a low frequency component, respectively.
With this structure, the echo canceler may have a small number of taps and may carry out operation at a low speed of the order of the long-time acoustic coupling. However, a talk or speech is likely interrupted at the beginning and the end thereof on account of the use of the voice switch. In addition, signal separation and signal combination should be carried out for the low and the high frequency components by the use of complicated digital lit-lens.
Summary of the Invention:
_ I-t is therefore an object of this invention to provide an echo canceling system which is convenient for a long-distance conference system.
It is another object of this invention to provide an echo canceling system of the type described, which is capable of canceling an echo even when it lasts a long time.
It is a further object of this invention to provide an echo canceling system of -the type described, wherein an intricate circuit is unnecessary.
It is another object of this invention to provide an echo canceling system of the type described, wherein any interruption of a talk does not occur at the beginning and the end.
It is another object of this invention to provide an echo canceling system of the type described, wherein signal separation and signal combination are unnecessary.
An echo canceling system according to this invention is responsive to a send-in and a receive-in signal so as to produce a send-out signal, the send-in signal including an echo signal which results from the receive-in signal through an echo path. The echo cancel-lying system comprises first delay means for successively delaying the receive-in signal to produce a plurality of delayed signals which have different delays relative to the receive-in signal and a plurality of Vocal echo cancelers which are successively arranged from a leading one for receiving the send-in signal to a trailing one for producing the send-out signal and which are for locally canceling different parts of the echo signal with reference to the delayed signals to produce partially echo-cancelled signals, respectively. A trailing one of said partially echo-cancelled signals is the send-out signal. The system further comprises second delay means between each pair of a preceding and a succeeding one ; of the local echo cancelers for delaying the partially echo-cancelled signal of the preceding local echo canceler to produce a delayed echo-cancelled signal and to supply I
the delayed echo-cancelled signal to the succeeding local echo canceler.
Brief Description of the Drawing:
Fig. 1 is a block diagram of a conventional echo canceling system;
Fig. 2 is a time chart for use in describing operation of the echo canceling system illustrated in Fig. l;
Fig. 3 is a block diagram of an echo canceling system according to a first embodiment of this invention;
Fig. 4, drawn below Fig. 2, is a time chart for use in describing operation of the echo canceling system illustrated in Fig. 3;
Fig. 5 is a time chart for use in describing an impulse response of an acoustic coupling circuit illustrated in Fig. 3;
Fig. 6 is a block diagram of an echo canceler unit applicable to the echo canceling system illustrated in Fig. 3;
Fig. 7 is a block diagram of an echo canceling system according to a second embodiment of this invention and Fig. 8 is a block diagram of an echo canceling system according to a third embodiment of this invention.
Description of the Preferred Embodiments:
Referring to Fig. 1, description will be made as regards a conventional echo canceling system for a better understanding of this invention. The illustrated lZ~6~
echo canceling system is substantially equivalent to that illustrated in the above-referenced United States Patent No. 4,377,793.
Receive-out and send-in terminals 11 and 12 S are for producing a receive-out signal x and for receiving a send-in signal y, respectively. Let the receive-out and the send-in terminals 11 and 12 be acoustically coupled to each other through an acoustic coupling circuit 15 which may comprise a microphone and a loudspeaker installed in an auditorium. Acoustic coupling between the receive-out and the send-in terminals 11 and 12 gives rise to an echo signal. The echo signal is included in the send-in signal y and supplied -to the send-in terminal 12. It is surmised that the echo signal lasts a duration of 200 milliseconds or more on account of a reverberation time of the auditorium.
A receive-in terminal 16 is for receiving a receive-in signal from a remote party through a receiving path (not shown). The receive-in signal is sent -to the receive-out terminal 11 as the receive-out signal x and is therefore designated by the same reference symbol as the receive-out signal x. A send-out terminal 17 is for delivering a send-out signal to the remote party through a sending path (not shown). The send-out US signal is derived primarily from the send-in signal y and comprises an error signal e which remains uncanceled after echo cancellation carried out by -the echo canceling system. The send-out signal is specified by the error I
signal e in this figure because description is mainly directed to the error signal e.
The illustrated echo canceling system comprises first, second, and third local echo cancelers 21, 22, and 23 which are connected to one another in cascade between the send-in and the send-out terminals 12 and 17. Each of the first through the third local echo cancelers 21 to 23 is substantially equal in structure to one another and includes a transversal filter. As well known in the art, such a transversal filter has a plurality of delay us is for delaying an input signal, a plurality of taps derived from the delay units, tap gain control circuits connected to the respective taps, and an adder connected to the respective tap gain control circuits. Each tap gain control circuit controls a tap gain in accordance with a coefficient modified at every sampling period. Anyway, the transversal filter is operable to equalize the input signal in accordance with a transmission path through which the input signal is transmitted As to the first local echo canceler 21, a first one 26 of the transversal filters is supplied with the receive-in signal x as the input signal so as to deliver a first simulated echo signal Ye to a first adder 27.
The first adder 27 carries out subtraction between the first simulated echo signal Ye and the send-in signal y to cancel the echo signal included in the send-in signal y and to send the second local echo canceler I
22 a first difference signal Do representative of a difference between the echo signal and the first simulated echo signal Ye-A second one 28 of the transversal filters is included in the second local echo canceler 22 and supplied with the receive-in sisal x through the delay units of the first transversal filter 26. Like the first transversal filter 26, the second transversal filter 28 delivers a second simulated echo signal Ye to a second adder 29. The second adder 29 carries out subtraction between the first difference signal and the second Sims-fated echo signal Ye to supply the third local echo canceler 23 with a second difference signal Do represent-alive of a result of subtraction.
A third one 33 of the transversal filters is included in the third local echo canceler 23 and supplied with the recelve-in signal x delayed by the first and the second transversal filters 21 and 22. The third transversal filter 33 produces a third simulated echo signal ye in the manner similar to that illustrated in conjunction with the first and the second transversal filters 21 and 22. A third adder 34 calculates a differ-once between the third simulated echo signal ye and the second difference signal to produce the error signal e representative of the difference.
It should be recollected that the coefficients assigned to the tap gain control circuits of the first through the third transversal filters 26, 28, and 33 I
are modified into modified coefficients at every sampling period. The modified coefficients serve to determine the tap gains in the following sampling period. Such modification should be carried out with reference to the error signal e, as known in the art. In other words, the respective coefficients must be modified into the modified coefficients after production of the error signal e within each sampling period.
Turning to Fig. 2, the first through the third local echo cancelers 21 to 23 are successively operated in a first interval To of time which is followed by second and third intervals To and To of time. Each interval is equal to the sampling period. Each operation of the first through the third local echo cancelers Al to 23 should be completed within each interval of time to modify the coefficients by the use of the error signal e. Otherwise, operation can not be carried out in the following interval.
From this fact, it is readily understood that the number of the local echo cancelers is restricted by the number of the coefficients which can be modified during each sampling period, as mentioned in the preamble of the instant specification.
referring to Fig. 3, an echo canceling system according to a first embodiment of this invention comprises similar parts designated by like reference numerals and symbols. In Fig. 3, first, second, and third local echo cancelers depicted at 36, 37, and 38 and may be ~Z~6~9 similar in structure to those illustrated in Fig. l.
Accordingly, the first through the third local echo cancelers 36 to 38 comprise the first through the third transversal filters 26, 28, and 33 coupled to the first through the third adders 27, 29, and 34, respectively.
It should be noted here that the error signal e is fed back to the third local echo canceler 38 alone and that the first and the second difference signals Do and Do are given to the first and the second transversal filters 26 and 28, respectively.
Let each of the first through the third transversal filters 26, 28, and 33 have first through n-th taps, (n + lath through 2n-th taps, and (on -I lath through 3n-th taps, respectively, where n represents an integer.
Thus, the taps of the first through the third transversal filters 26, 28, and 33 are equal in number to one another.
A combination of the first through the third transversal filters 26, 28, and 33 may be called a transversal filter having taps, on in number. The number n may be a maximum number, within which each transversal filter can carry out operation in each sampling period. The number n may be, for example, 400.
It is seen that the tap gain circuits and the delay units of each transversal filter are independently operated from one another.
In Fig. 3, the receive-in signal x is supplied to a delay circuit 40 for successively delaying the receive-in signal x by a predetermined duration at maximum.
~2~9~9 In the example being illustrated, the predetermined duration is equal to (on + 2) times the sampling period, as will later become clear. Thus, the delay circuit 40 has first through (on + Thea taps. First and second delayed signals Al and x2 are derived from the (n + lath and the (on + Thea taps, respectively, and delayed relative to the receive-in signal x by (n + 1) and (on + 2) times the sampling period, respectively. The delay circuit 40 may be referred to as a first delay circuit.
In the example being illustrated, the receive-in signal x is sent to the first transversal filter 26 without being delayed but may be considered one of the delayed signals for convenience of description.
The receive-in signal x, the first delayed signal Al, and the second delayed signal x2 are sent to the delay units of the first through the third transversal filters 26, 28, and 33 as input signals, respectively.
Each input signal is successively delayed Boone+ 1) times the sampling period.
Referring to Fig. 4 afresh and Fig. 3 again, the first transversal filter 36 carries out operation in response to the receive-in signal x to calculate the first simulated echo signal Ye during the first interval To of time and to deliver the first simulated echo signal Ye to the first adder 27. The first difference signal Do is produced from the first adder 27 in the manner similar to that described with reference to Fig.
1.
The first difference signal Do is fed back to -the first transversal filter 26 during the first interval To to modify the coefficients of the tap gain control circuits included in the first transversal filter 26.
Thus, each coefficient of the first transversal filter 26 is modified at every sampling period. Simultaneously, the first difference signal Do is sent through a first additional delay circuit 41 to the second local echo circuit 37. The first additional delay circuit 41 provides a unit delay equal to the sampling period. Therefore, the first difference signal Do is delayed by the sampling period relative to the receive-in signal x and given as a first delayed difference signal to the second adder 29.
On the other hand, the second transversal filter 28 is supplied with the first delayed signal Al derived from the (n + lath -tap and is therefore operated during the second interval To of time in the manner similar to the first transversal filter 26 to produce the second simulated echo signal Ye. Responsive to the second simulated echo signal Ye and the first delayed difference signal, the second adder 29 calculates the second differ-once signal Do to deliver the same to the second transversal filter 28 during the second interval To.
Thus, the second transversal filter 28 modifies the respective coefficients at every sampling period.
The second difference signal Do is also given to a second additional delay circuit 42 having the unit ox delay equal to the sampling period, like the first additional delay circuit 41, and is delivered as a second delayed difference signal to the third local echo callce.ller 38 durillg the -third interval 13 owe time. I've third local echo callceller 38 produces the sencl-out signal, nalllely, the error signal e in the following interval of time in response to the second delayed difference signal and the second delayed signal x2 derived from the (on + Thea tap owe the delay circuit 40 in the maimer similar to each of the first and the second local echo cancelers 36 and 37. A combination of the first and the second additional delay circuits 41 and 42 may collectively be termed a second delay circuit.
As mentioned above, the first through the -third local echo cancelers 36 to 38 are put into operation during the first through the third intervals To to 13, respectively, in response to the send-in signal y received at the first interval To of time.
Referring to Figure 5, it will be assumed that the acoustic coupling circuit 15 (Figure 3) exhibits an impulse response over first through third durations To to To. The exemplified impulse response is divisible into first, second, and third partial impulse responses 46, 47, and 48 in the first through the third durations To to To respectively. The first through the third transversal filters 26, 28 and 33 figure 3) identify or simulate the first through the third partial impulse responses 46 to 48 by production of the first through the third simulated echo signals Ye to ye to make the first through the third subtracters 27, 29, and I cancel them, respectively.
In order to clarify the above-mentioned operation, let the impulse response of the acoustic coupling circuit 15 and the receive-out signal be represented by k and I, respectively. The echo signal included in the send-in signal y is generally given by integration of convolution between k and I. That is:
o hex (1) to m where to represents a sampling instant.
The formula (1) is expanded into:
0 0 -t hex = hex + hex + --to to to tm_l + hex. (2) to m From Equation (2), it is readily understood that the echo signal shown by the formula (1) can be canceled by canceling each partial echo signal repro-sensed by the respective terms on the right hand side of Equation (2). Cancellation of each partial echo signal is possible by producing simulated echo signals corresponding to the respective partial echo signals, by the use of transversal filters.
As to the echo canceling system illustrated in Fig. 3, the echo signal is divided into first, second, and third partial echo signals which appear during the . .
firs~--hrough the Idea dourness I to I respectively.
The first through the third partial echo signals are canceled by the first through the third local echo cancelers 36 to 38, respectively. In other words, the first through the n-th taps of the first local echo canceler 36 are controlled to cancel the first partial echo signal while the (n + lath through the 2n-th taps and the (on + lath through the 3n-th taps of the second and the third local echo cancelers 37 and 38 are con-trolled to cancel the second and the third partial echo signals, respectively. From this fact, it is understood that each of the first and the second difference signals Do and Do may be called a partially echo-cancelled signal.
Referring to Fig. 6, an echo canceler unit 50 is applicable to the echo canceling system illustrated in Fig. 3 and comprises a combination of a local echo canceling portion 51, a first delay unit 52, and a -second delay unit 53. The local echo canceling portion So has a transversal filter 56 of n taps and an adder 57, as is the case with each of the first through the third local cancelers 36 to 38. The transversal filter 56 is supplied with a first input signal IN, such as the receive-in signal x while the adder 57 is supplied with a second input signal IN, such as the send-in signal y or the difference signal Do or Do.
The first delay unit 52 gives the first input signal In a delay equal to (n + l) times the sampling period to supply the following echo canceler unit with LO I
a first delayed signal Dull. The second delay unit 53 delays an output signal of the adder 57 by a single sampling period to produce a second delayed signal DL2.
The echo canceling system illustrated in Fig.
3 can be constituted by connecting first through third stages of the echo canceler units 50 in cascade without the first and the second delay units 52 and 53 in the third stage.
Similarly, the number of the echo canceler units 50 may optionally be changed because it is not restricted by the number of coefficients modified in each sampling period in each transversal filter. Theoretic gaily, an infinite number of the echo canceler units 50 may be connected in cascade. Practically, the number of the echo canceler units 50 should be determined in consideration of a propagation delay time of a trays-mission path. Let the echo canceling system be applied to the long-distance conference communication system including a satellite. In this event, the number of the echo canceler units 50 may be equal to twenty when each echo canceler has 400 taps and the sampling frequency is equal to 16 kHz. More than twenty of the echo canceler units 50 may be connected in cascade because the propaga-lion delay time in the long-distance conference communication systems very long in comparison with the sampling period. Anyway, echo cancellation is possible even when the echo signal lasts a duration as long as 500 milliseconds.
~L2~96,49 Referring to Fig. 7, an echo canceling system according -to a second embodiment of this invention is similar to that illustrated in Fig. 3 except that the illustrated delay circuit, indicated at 40', has first S through 2n-th taps and that the first, the second, and the third transversal filters 36, 37, and 38 are connected to the 2n-th tap, the (n + lath tap, and the second tap, respectively.
The delay circuit 40' successively delays the receive-in signal x at every sampling period to deliver delayed signals to the taps. As a result, the second tap is supplied with a first one of the delayed signals delayed by twice the sampling period relative to the receive-in signal x. Likewise, the (n + lath and the 2n-th taps are supplied with second and third ones of the delayed signals with delays which are equal to (n + 1) and on times the sampling period. ?
With this structure, the third local echo canceler 38 serves to identify or simulate the first partial impulse response 46 illustrated in Fig. S to cancel the first partial echo signal appearing the first interval To of time. The third adder 34 is given the second delayed difference signal delayed through the first and the second additional delay circuits 41 and 42 by twice the sampling period. The receive-in signal x should therefore be delayed by twice the sampling period so as to cancel the first partial echo signal. Under the circumstances, the first delayed signal is delivered I
from the second tap to the third transversal filter 33.
Louses, the second and the first local echo cancelers 37 and 36 serve to identify the second and the third partial impulse responses 47 and 48, respect lively. To this end, the second and the third delayed signals are delivered from the (n + lath and the 2n-th taps to the second and the first transversal filters 37 and 36, respectively.
Finally referring to Fig. 8, an echo canceler system according to a third embodiment of this invention is similar to that illustrated in Fig. 7 except that the delay circuit, illustrated at 40", has first through (on + lath taps and that the n-th and the (on + lath taps of the delay circuits 40" are connected to the first and the second transversal filters 26 and 27, respectively. ?.
With this structure, the first local echo canceler 36 is operable to identify the second partial impulse response 47 (Fig. 5) while the second local echo canceler 37 is operable to identify the third partial impulse response 48.
As mentioned above, a relationship between each local echo canceler and a partial echo signal canceled thereat may be selected in consideration of each delay of the partial echo signal. This is because the echo signal is regarded as a sum of that integration of convolu-lion between each partial impulse response and the go receive out signal which is calculated in a limited integration path.
While this invention has thus far been described in conjunction with a few embodiments thereof, it will readily be possible for those swilled in the art to put this invention into practice in various other manners.
For example, the delay of each of the first and the second additional delay circuits 41 and 42 is not always be equal to the sampling period. In Figs. 7 and 8, at least one additional local echo canceler may be attached to the third local echo canceler I In this event, each delay circuit 40' or 40" should have taps dependent on the number of additional delay circuits, such as 41 and 42, and the number of the local echo cancelers. It is necessary to select the taps of the delay circuit to be connected to the local echo cancelers.
? .
Backgrolmd ox the Invention is :inventioll relates to an echo canceling system for use in a long-distance telephone network all, in particular, in a long-distance conference communication system including a satellite.
Recently, a long-distance conference communication system has been developed which is suitable err a conference concurrently held at distant locations, namely, auditor or conference rooms geographically spaced from one another. Such a system is very effective for saving money, time and labor of participants attending the conference.
In the conference communication system, both a microphone and a loudspeaker should be installed in each auditorium so as to carry out bidirectional communication of an audio or voice signal. Accordingly, unfavorable acoustic coupling inevitably takes place between the loudspeaker and the microphone.
I 9~919 Furthermore, when each auditorium is spaced far from the others and is, in particular, communicable through a satellite with one another, a speaker's voice sent from one microphone in a near-end auditorium is returned back to the near-end auditorium in the form of an echo through acoustic coupling between the loud-speaker and the microphone placed at a far-end auditorium.
Various attempts have been made to cancel such an echo by the use of an echo canceling technique which has been used in a usual telephone communication network.
It should be noted here that a reverberation time of each auditorium is not less than 200 milliseconds and, therefore, the echo resulting from the acoustic coupling between the loudspeaker and the microphone lasts a very long time as compared with an echo sounding in the telephone communication network. In addition, it is general that a higher quality of voice is required in the long-distance conference system than in the tote-phone communication network.
Consideration has been given to an echo canceler comprising a transversal filter in order to cancel the echo in the long-distance conference system. Such an echo canceler can cancel the echo by calculating a simulated echo signal to be subtracted from an actual echo signal at every sampling period and by adaptively controlling tap gains of the transversal filter. However, the transversal filter should have a great number of taps so as to cancel such an echo which lasts a very I
long time. For example, let the sampling period or a sampling frequency be equal to 62.5 microseconds or 16 kHz. Let the echo be canceled over 200 milliseconds.
In this event, the number of the taps becomes equal to 3,200. Accordingly, calculation and control must be carried out in connection with the taps of 3,200 at every sampling period of 62.5 microseconds. The echo canceler becomes very intricate in structure because a great amount of hardware is necessary so as to precut-gaily carry out the above-mentioned calculation and control.
In United States Patent No. 4,377,793 assigned to Communications Satellite Corporation, O. A. Home discloses a digital adaptive transversal filter comprising two or more separate filter units connected in cascade.
With this structure, a large number of tap gains can be controlled by the use of an increased number of Coffey-clients. At any rate, the transversal filter produces an error signal representative of a component finally left uncanceled.
It should be noted in this connection that the error signal is produced at every sampling period and should be fed back to the respective filter units so as to modify each coefficient of the respective filter units at every sampling period. This means that a restrict lion is imposed on the number of the filter units connected in cascade. Therefore, the transversal filter can not cancel a long-term echo which lasts a long time. Moreover, it is difficult for the time being to purchase a large scale into-gyration circuit convenient for the echo canceler.
In Canadian Patent No. 1,178,385, issued on November 20, l98~ in the names of Takahashi Airsick et at, assignors to the present assignee, an echo canceling system is disclosed which comprises a combination of a voice switch and an echo canceler prepared for a high and a low frequency component, respectively.
With this structure, the echo canceler may have a small number of taps and may carry out operation at a low speed of the order of the long-time acoustic coupling. However, a talk or speech is likely interrupted at the beginning and the end thereof on account of the use of the voice switch. In addition, signal separation and signal combination should be carried out for the low and the high frequency components by the use of complicated digital lit-lens.
Summary of the Invention:
_ I-t is therefore an object of this invention to provide an echo canceling system which is convenient for a long-distance conference system.
It is another object of this invention to provide an echo canceling system of the type described, which is capable of canceling an echo even when it lasts a long time.
It is a further object of this invention to provide an echo canceling system of -the type described, wherein an intricate circuit is unnecessary.
It is another object of this invention to provide an echo canceling system of the type described, wherein any interruption of a talk does not occur at the beginning and the end.
It is another object of this invention to provide an echo canceling system of the type described, wherein signal separation and signal combination are unnecessary.
An echo canceling system according to this invention is responsive to a send-in and a receive-in signal so as to produce a send-out signal, the send-in signal including an echo signal which results from the receive-in signal through an echo path. The echo cancel-lying system comprises first delay means for successively delaying the receive-in signal to produce a plurality of delayed signals which have different delays relative to the receive-in signal and a plurality of Vocal echo cancelers which are successively arranged from a leading one for receiving the send-in signal to a trailing one for producing the send-out signal and which are for locally canceling different parts of the echo signal with reference to the delayed signals to produce partially echo-cancelled signals, respectively. A trailing one of said partially echo-cancelled signals is the send-out signal. The system further comprises second delay means between each pair of a preceding and a succeeding one ; of the local echo cancelers for delaying the partially echo-cancelled signal of the preceding local echo canceler to produce a delayed echo-cancelled signal and to supply I
the delayed echo-cancelled signal to the succeeding local echo canceler.
Brief Description of the Drawing:
Fig. 1 is a block diagram of a conventional echo canceling system;
Fig. 2 is a time chart for use in describing operation of the echo canceling system illustrated in Fig. l;
Fig. 3 is a block diagram of an echo canceling system according to a first embodiment of this invention;
Fig. 4, drawn below Fig. 2, is a time chart for use in describing operation of the echo canceling system illustrated in Fig. 3;
Fig. 5 is a time chart for use in describing an impulse response of an acoustic coupling circuit illustrated in Fig. 3;
Fig. 6 is a block diagram of an echo canceler unit applicable to the echo canceling system illustrated in Fig. 3;
Fig. 7 is a block diagram of an echo canceling system according to a second embodiment of this invention and Fig. 8 is a block diagram of an echo canceling system according to a third embodiment of this invention.
Description of the Preferred Embodiments:
Referring to Fig. 1, description will be made as regards a conventional echo canceling system for a better understanding of this invention. The illustrated lZ~6~
echo canceling system is substantially equivalent to that illustrated in the above-referenced United States Patent No. 4,377,793.
Receive-out and send-in terminals 11 and 12 S are for producing a receive-out signal x and for receiving a send-in signal y, respectively. Let the receive-out and the send-in terminals 11 and 12 be acoustically coupled to each other through an acoustic coupling circuit 15 which may comprise a microphone and a loudspeaker installed in an auditorium. Acoustic coupling between the receive-out and the send-in terminals 11 and 12 gives rise to an echo signal. The echo signal is included in the send-in signal y and supplied -to the send-in terminal 12. It is surmised that the echo signal lasts a duration of 200 milliseconds or more on account of a reverberation time of the auditorium.
A receive-in terminal 16 is for receiving a receive-in signal from a remote party through a receiving path (not shown). The receive-in signal is sent -to the receive-out terminal 11 as the receive-out signal x and is therefore designated by the same reference symbol as the receive-out signal x. A send-out terminal 17 is for delivering a send-out signal to the remote party through a sending path (not shown). The send-out US signal is derived primarily from the send-in signal y and comprises an error signal e which remains uncanceled after echo cancellation carried out by -the echo canceling system. The send-out signal is specified by the error I
signal e in this figure because description is mainly directed to the error signal e.
The illustrated echo canceling system comprises first, second, and third local echo cancelers 21, 22, and 23 which are connected to one another in cascade between the send-in and the send-out terminals 12 and 17. Each of the first through the third local echo cancelers 21 to 23 is substantially equal in structure to one another and includes a transversal filter. As well known in the art, such a transversal filter has a plurality of delay us is for delaying an input signal, a plurality of taps derived from the delay units, tap gain control circuits connected to the respective taps, and an adder connected to the respective tap gain control circuits. Each tap gain control circuit controls a tap gain in accordance with a coefficient modified at every sampling period. Anyway, the transversal filter is operable to equalize the input signal in accordance with a transmission path through which the input signal is transmitted As to the first local echo canceler 21, a first one 26 of the transversal filters is supplied with the receive-in signal x as the input signal so as to deliver a first simulated echo signal Ye to a first adder 27.
The first adder 27 carries out subtraction between the first simulated echo signal Ye and the send-in signal y to cancel the echo signal included in the send-in signal y and to send the second local echo canceler I
22 a first difference signal Do representative of a difference between the echo signal and the first simulated echo signal Ye-A second one 28 of the transversal filters is included in the second local echo canceler 22 and supplied with the receive-in sisal x through the delay units of the first transversal filter 26. Like the first transversal filter 26, the second transversal filter 28 delivers a second simulated echo signal Ye to a second adder 29. The second adder 29 carries out subtraction between the first difference signal and the second Sims-fated echo signal Ye to supply the third local echo canceler 23 with a second difference signal Do represent-alive of a result of subtraction.
A third one 33 of the transversal filters is included in the third local echo canceler 23 and supplied with the recelve-in signal x delayed by the first and the second transversal filters 21 and 22. The third transversal filter 33 produces a third simulated echo signal ye in the manner similar to that illustrated in conjunction with the first and the second transversal filters 21 and 22. A third adder 34 calculates a differ-once between the third simulated echo signal ye and the second difference signal to produce the error signal e representative of the difference.
It should be recollected that the coefficients assigned to the tap gain control circuits of the first through the third transversal filters 26, 28, and 33 I
are modified into modified coefficients at every sampling period. The modified coefficients serve to determine the tap gains in the following sampling period. Such modification should be carried out with reference to the error signal e, as known in the art. In other words, the respective coefficients must be modified into the modified coefficients after production of the error signal e within each sampling period.
Turning to Fig. 2, the first through the third local echo cancelers 21 to 23 are successively operated in a first interval To of time which is followed by second and third intervals To and To of time. Each interval is equal to the sampling period. Each operation of the first through the third local echo cancelers Al to 23 should be completed within each interval of time to modify the coefficients by the use of the error signal e. Otherwise, operation can not be carried out in the following interval.
From this fact, it is readily understood that the number of the local echo cancelers is restricted by the number of the coefficients which can be modified during each sampling period, as mentioned in the preamble of the instant specification.
referring to Fig. 3, an echo canceling system according to a first embodiment of this invention comprises similar parts designated by like reference numerals and symbols. In Fig. 3, first, second, and third local echo cancelers depicted at 36, 37, and 38 and may be ~Z~6~9 similar in structure to those illustrated in Fig. l.
Accordingly, the first through the third local echo cancelers 36 to 38 comprise the first through the third transversal filters 26, 28, and 33 coupled to the first through the third adders 27, 29, and 34, respectively.
It should be noted here that the error signal e is fed back to the third local echo canceler 38 alone and that the first and the second difference signals Do and Do are given to the first and the second transversal filters 26 and 28, respectively.
Let each of the first through the third transversal filters 26, 28, and 33 have first through n-th taps, (n + lath through 2n-th taps, and (on -I lath through 3n-th taps, respectively, where n represents an integer.
Thus, the taps of the first through the third transversal filters 26, 28, and 33 are equal in number to one another.
A combination of the first through the third transversal filters 26, 28, and 33 may be called a transversal filter having taps, on in number. The number n may be a maximum number, within which each transversal filter can carry out operation in each sampling period. The number n may be, for example, 400.
It is seen that the tap gain circuits and the delay units of each transversal filter are independently operated from one another.
In Fig. 3, the receive-in signal x is supplied to a delay circuit 40 for successively delaying the receive-in signal x by a predetermined duration at maximum.
~2~9~9 In the example being illustrated, the predetermined duration is equal to (on + 2) times the sampling period, as will later become clear. Thus, the delay circuit 40 has first through (on + Thea taps. First and second delayed signals Al and x2 are derived from the (n + lath and the (on + Thea taps, respectively, and delayed relative to the receive-in signal x by (n + 1) and (on + 2) times the sampling period, respectively. The delay circuit 40 may be referred to as a first delay circuit.
In the example being illustrated, the receive-in signal x is sent to the first transversal filter 26 without being delayed but may be considered one of the delayed signals for convenience of description.
The receive-in signal x, the first delayed signal Al, and the second delayed signal x2 are sent to the delay units of the first through the third transversal filters 26, 28, and 33 as input signals, respectively.
Each input signal is successively delayed Boone+ 1) times the sampling period.
Referring to Fig. 4 afresh and Fig. 3 again, the first transversal filter 36 carries out operation in response to the receive-in signal x to calculate the first simulated echo signal Ye during the first interval To of time and to deliver the first simulated echo signal Ye to the first adder 27. The first difference signal Do is produced from the first adder 27 in the manner similar to that described with reference to Fig.
1.
The first difference signal Do is fed back to -the first transversal filter 26 during the first interval To to modify the coefficients of the tap gain control circuits included in the first transversal filter 26.
Thus, each coefficient of the first transversal filter 26 is modified at every sampling period. Simultaneously, the first difference signal Do is sent through a first additional delay circuit 41 to the second local echo circuit 37. The first additional delay circuit 41 provides a unit delay equal to the sampling period. Therefore, the first difference signal Do is delayed by the sampling period relative to the receive-in signal x and given as a first delayed difference signal to the second adder 29.
On the other hand, the second transversal filter 28 is supplied with the first delayed signal Al derived from the (n + lath -tap and is therefore operated during the second interval To of time in the manner similar to the first transversal filter 26 to produce the second simulated echo signal Ye. Responsive to the second simulated echo signal Ye and the first delayed difference signal, the second adder 29 calculates the second differ-once signal Do to deliver the same to the second transversal filter 28 during the second interval To.
Thus, the second transversal filter 28 modifies the respective coefficients at every sampling period.
The second difference signal Do is also given to a second additional delay circuit 42 having the unit ox delay equal to the sampling period, like the first additional delay circuit 41, and is delivered as a second delayed difference signal to the third local echo callce.ller 38 durillg the -third interval 13 owe time. I've third local echo callceller 38 produces the sencl-out signal, nalllely, the error signal e in the following interval of time in response to the second delayed difference signal and the second delayed signal x2 derived from the (on + Thea tap owe the delay circuit 40 in the maimer similar to each of the first and the second local echo cancelers 36 and 37. A combination of the first and the second additional delay circuits 41 and 42 may collectively be termed a second delay circuit.
As mentioned above, the first through the -third local echo cancelers 36 to 38 are put into operation during the first through the third intervals To to 13, respectively, in response to the send-in signal y received at the first interval To of time.
Referring to Figure 5, it will be assumed that the acoustic coupling circuit 15 (Figure 3) exhibits an impulse response over first through third durations To to To. The exemplified impulse response is divisible into first, second, and third partial impulse responses 46, 47, and 48 in the first through the third durations To to To respectively. The first through the third transversal filters 26, 28 and 33 figure 3) identify or simulate the first through the third partial impulse responses 46 to 48 by production of the first through the third simulated echo signals Ye to ye to make the first through the third subtracters 27, 29, and I cancel them, respectively.
In order to clarify the above-mentioned operation, let the impulse response of the acoustic coupling circuit 15 and the receive-out signal be represented by k and I, respectively. The echo signal included in the send-in signal y is generally given by integration of convolution between k and I. That is:
o hex (1) to m where to represents a sampling instant.
The formula (1) is expanded into:
0 0 -t hex = hex + hex + --to to to tm_l + hex. (2) to m From Equation (2), it is readily understood that the echo signal shown by the formula (1) can be canceled by canceling each partial echo signal repro-sensed by the respective terms on the right hand side of Equation (2). Cancellation of each partial echo signal is possible by producing simulated echo signals corresponding to the respective partial echo signals, by the use of transversal filters.
As to the echo canceling system illustrated in Fig. 3, the echo signal is divided into first, second, and third partial echo signals which appear during the . .
firs~--hrough the Idea dourness I to I respectively.
The first through the third partial echo signals are canceled by the first through the third local echo cancelers 36 to 38, respectively. In other words, the first through the n-th taps of the first local echo canceler 36 are controlled to cancel the first partial echo signal while the (n + lath through the 2n-th taps and the (on + lath through the 3n-th taps of the second and the third local echo cancelers 37 and 38 are con-trolled to cancel the second and the third partial echo signals, respectively. From this fact, it is understood that each of the first and the second difference signals Do and Do may be called a partially echo-cancelled signal.
Referring to Fig. 6, an echo canceler unit 50 is applicable to the echo canceling system illustrated in Fig. 3 and comprises a combination of a local echo canceling portion 51, a first delay unit 52, and a -second delay unit 53. The local echo canceling portion So has a transversal filter 56 of n taps and an adder 57, as is the case with each of the first through the third local cancelers 36 to 38. The transversal filter 56 is supplied with a first input signal IN, such as the receive-in signal x while the adder 57 is supplied with a second input signal IN, such as the send-in signal y or the difference signal Do or Do.
The first delay unit 52 gives the first input signal In a delay equal to (n + l) times the sampling period to supply the following echo canceler unit with LO I
a first delayed signal Dull. The second delay unit 53 delays an output signal of the adder 57 by a single sampling period to produce a second delayed signal DL2.
The echo canceling system illustrated in Fig.
3 can be constituted by connecting first through third stages of the echo canceler units 50 in cascade without the first and the second delay units 52 and 53 in the third stage.
Similarly, the number of the echo canceler units 50 may optionally be changed because it is not restricted by the number of coefficients modified in each sampling period in each transversal filter. Theoretic gaily, an infinite number of the echo canceler units 50 may be connected in cascade. Practically, the number of the echo canceler units 50 should be determined in consideration of a propagation delay time of a trays-mission path. Let the echo canceling system be applied to the long-distance conference communication system including a satellite. In this event, the number of the echo canceler units 50 may be equal to twenty when each echo canceler has 400 taps and the sampling frequency is equal to 16 kHz. More than twenty of the echo canceler units 50 may be connected in cascade because the propaga-lion delay time in the long-distance conference communication systems very long in comparison with the sampling period. Anyway, echo cancellation is possible even when the echo signal lasts a duration as long as 500 milliseconds.
~L2~96,49 Referring to Fig. 7, an echo canceling system according -to a second embodiment of this invention is similar to that illustrated in Fig. 3 except that the illustrated delay circuit, indicated at 40', has first S through 2n-th taps and that the first, the second, and the third transversal filters 36, 37, and 38 are connected to the 2n-th tap, the (n + lath tap, and the second tap, respectively.
The delay circuit 40' successively delays the receive-in signal x at every sampling period to deliver delayed signals to the taps. As a result, the second tap is supplied with a first one of the delayed signals delayed by twice the sampling period relative to the receive-in signal x. Likewise, the (n + lath and the 2n-th taps are supplied with second and third ones of the delayed signals with delays which are equal to (n + 1) and on times the sampling period. ?
With this structure, the third local echo canceler 38 serves to identify or simulate the first partial impulse response 46 illustrated in Fig. S to cancel the first partial echo signal appearing the first interval To of time. The third adder 34 is given the second delayed difference signal delayed through the first and the second additional delay circuits 41 and 42 by twice the sampling period. The receive-in signal x should therefore be delayed by twice the sampling period so as to cancel the first partial echo signal. Under the circumstances, the first delayed signal is delivered I
from the second tap to the third transversal filter 33.
Louses, the second and the first local echo cancelers 37 and 36 serve to identify the second and the third partial impulse responses 47 and 48, respect lively. To this end, the second and the third delayed signals are delivered from the (n + lath and the 2n-th taps to the second and the first transversal filters 37 and 36, respectively.
Finally referring to Fig. 8, an echo canceler system according to a third embodiment of this invention is similar to that illustrated in Fig. 7 except that the delay circuit, illustrated at 40", has first through (on + lath taps and that the n-th and the (on + lath taps of the delay circuits 40" are connected to the first and the second transversal filters 26 and 27, respectively. ?.
With this structure, the first local echo canceler 36 is operable to identify the second partial impulse response 47 (Fig. 5) while the second local echo canceler 37 is operable to identify the third partial impulse response 48.
As mentioned above, a relationship between each local echo canceler and a partial echo signal canceled thereat may be selected in consideration of each delay of the partial echo signal. This is because the echo signal is regarded as a sum of that integration of convolu-lion between each partial impulse response and the go receive out signal which is calculated in a limited integration path.
While this invention has thus far been described in conjunction with a few embodiments thereof, it will readily be possible for those swilled in the art to put this invention into practice in various other manners.
For example, the delay of each of the first and the second additional delay circuits 41 and 42 is not always be equal to the sampling period. In Figs. 7 and 8, at least one additional local echo canceler may be attached to the third local echo canceler I In this event, each delay circuit 40' or 40" should have taps dependent on the number of additional delay circuits, such as 41 and 42, and the number of the local echo cancelers. It is necessary to select the taps of the delay circuit to be connected to the local echo cancelers.
? .
Claims (3)
1. An echo cencelling system responsive to a send-in and a receive-in signal for producing a send-out signal, said send-in signal including an echo signal which results from said receive-in signal through an echo path, said echo cancelling system comprising:
first delay means for successively delaying said receive-in signal to produce a plurality of delayed signals which have different delays relative to said receive-in signal;
a plurality of local echo cancellers which are successively arranged from a leading one for receiving said send-in signal to a trailing one for producing said send-out signal and which are for locally cancelling different parts of said echo signal with reference to said delayed signals to produce partially echo-cancelled signals, respectively, a trailing one of said partially echo-cancelled signals being said send-out signal; and second delay means between each pair of a preceding and a succeeding one of said local echo cancellers for delaying the partially echo-cancelled signal of the preceding local echo canceller to produce a delayed echo-cancelled signal and to supply said delayed echo-cancelled signal to the succeeding local echo canceller.
first delay means for successively delaying said receive-in signal to produce a plurality of delayed signals which have different delays relative to said receive-in signal;
a plurality of local echo cancellers which are successively arranged from a leading one for receiving said send-in signal to a trailing one for producing said send-out signal and which are for locally cancelling different parts of said echo signal with reference to said delayed signals to produce partially echo-cancelled signals, respectively, a trailing one of said partially echo-cancelled signals being said send-out signal; and second delay means between each pair of a preceding and a succeeding one of said local echo cancellers for delaying the partially echo-cancelled signal of the preceding local echo canceller to produce a delayed echo-cancelled signal and to supply said delayed echo-cancelled signal to the succeeding local echo canceller.
2. An echo cancelling system as claimed in Claim 1, wherein said delayed signals are delivered from said first delay means to said local echo cancellers in consideration of the delayed echo-cancelled signals and said parts of said echo signal which are to be cancelled by said local echo cancellers, respectively.
3. An echo cancelling system as claimed in Claim 2, wherein each of said local echo cancellers is for producing each of the partially echo-cancelled signals in a predetermined interval of time and comprises:
a transversal filter responsive to said each partially echo-cancelled signal and a selected one of said delayed signals for producing a simulated echo signal which simulates each part of said echo signal corresponding to said selected delay signal; and means for cancelling said each part of said echo signal by said simulated echo signal to produce-each partially echo-cancelled signal in the following interval of time.
a transversal filter responsive to said each partially echo-cancelled signal and a selected one of said delayed signals for producing a simulated echo signal which simulates each part of said echo signal corresponding to said selected delay signal; and means for cancelling said each part of said echo signal by said simulated echo signal to produce-each partially echo-cancelled signal in the following interval of time.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP113811/1983 | 1983-06-24 | ||
| JP11381183A JPS605634A (en) | 1983-06-24 | 1983-06-24 | Echo canceller |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1219649A true CA1219649A (en) | 1987-03-24 |
Family
ID=14621643
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000457200A Expired CA1219649A (en) | 1983-06-24 | 1984-06-22 | Echo cancelling system comprising cascade-connected local echo cancellers operable independently of one another |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS605634A (en) |
| CA (1) | CA1219649A (en) |
| FR (1) | FR2549320B1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5471527A (en) | 1993-12-02 | 1995-11-28 | Dsc Communications Corporation | Voice enhancement system and method |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL8603247A (en) * | 1986-12-22 | 1988-07-18 | At & T & Philips Telecomm | ADAPTIVE TIME-DISCREET FILTER FOR FORMING A COMPENSATION SIGNAL FROM SYNCHRONOUS DATA SYMBOLS. |
| JPH05227110A (en) * | 1992-02-14 | 1993-09-03 | Nippon Television Network Corp | Side tone suppressing device for sng |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1180141A (en) * | 1980-11-15 | 1984-12-27 | Kenzo Takahashi | Echo canceller system |
| JPS5864830A (en) * | 1981-10-15 | 1983-04-18 | Nippon Telegr & Teleph Corp <Ntt> | Echo erasing device |
| JPS5784633A (en) * | 1980-11-15 | 1982-05-27 | Nippon Telegr & Teleph Corp <Ntt> | Echo eraser |
-
1983
- 1983-06-24 JP JP11381183A patent/JPS605634A/en active Granted
-
1984
- 1984-06-22 FR FR8409895A patent/FR2549320B1/en not_active Expired
- 1984-06-22 CA CA000457200A patent/CA1219649A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5471527A (en) | 1993-12-02 | 1995-11-28 | Dsc Communications Corporation | Voice enhancement system and method |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6343015B2 (en) | 1988-08-26 |
| FR2549320A1 (en) | 1985-01-18 |
| JPS605634A (en) | 1985-01-12 |
| FR2549320B1 (en) | 1986-07-11 |
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