Patent Publication Number: US-8995314-B2

Title: Selectively adaptable far-end echo cancellation in a packet voice system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 12/839,469, filed on Jul. 20, 2010, which is a continuation of U.S. patent application Ser. No. 12/202,634 (now U.S. Pat. No. 7,760,673), filed on Sep. 2, 2008, which is hereby incorporated by reference in its entirety and which is a continuation of U.S. patent application Ser. No. 10/327,747 (now U.S. Pat. No. 7,420,937), filed on Dec. 23, 2002, which is related to U.S. patent application Ser. No. 10/327,781 (now U.S. Pat. No. 7,333,447), entitled “PACKET VOICE SYSTEM WITH FAR-END ECHO,” and U.S. patent application Ser. No. 10/327,773 (now U.S. Pat. No. 7,333,446), entitled “SYSTEM AND METHOD OF OPERATING A PACKET VOICE FAR-END ECHO CANCELLATION SYSTEM,” both filed on Dec. 23, 2002 and both of which are expressly incorporated herein by reference as though set forth in full. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to packet voice communication systems, and more particularly, to far-end echo cancellation in a packet voice system. 
     BACKGROUND 
     Telephony devices, such as telephones, analog fax machines, and data modems, have traditionally utilized circuit-switched networks to communicate. With the current state of technology, it is desirable for telephony devices to communicate over the Internet, or other packet-based networks. Heretofore, an integrated system for interfacing various telephony devices over packet-based networks has been difficult due to the different modulation schemes of the telephony devices. Accordingly, it would be advantageous to have an efficient and robust integrated system for the exchange of voice, fax data and modem data between telephony devices and packet-based networks. 
     An echo canceller is a device that removes the echo present in a communication signal, typically by employing a linear transversal filter. Due to non-linearities in hybrid and digital/analog loops and estimation uncertainties, linear cancellers cannot entirely remove the echo present. A non-linear device, commonly referred to as a non-linear processor (NLP), can be used to remove the remaining echo. This device may be a variable loss inserted into the system, a device that removes the entire signal and injects noise with the correct level, and possibly the correct spectrum, or a combination thereof. 
     Existing echo cancellers in packet voice communication devices endeavor to suppress echo in the ingress signal, that is, the signal that the device sends out over the network. This is typically an echo of the egress signal (the signal that the device receives from the network) that occurs at the device. However, many packet voice transceivers do not have echo cancellers. When a first packet voice transceiver is communicating with a second packet voice transceiver over a network and the second device does not employ echo cancellation on its ingress signal, the first device may receive an egress signal transmitted by the second device that contains echo. Thus it would be advantageous to be able to efficiently suppress echo that is present in such an egress signal. However, cancellation of echo present in the egress signal is problematic because the echo path includes a round-trip journey over the communication network, as well as all of the processing performed on the signal by the packet voice transceiver at the other end of the network. 
     Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is directed to a packet voice transceiver adapted to reside at a first end of a communication network and to send an ingress communication signal comprising voice packets to, and receive an egress communication signal comprising voice packets from, a second packet voice transceiver residing at a second end of the communication network. The packet voice transceiver includes a comfort noise generator and a far-end echo canceller. The comfort noise generator generates comfort noise at times indicated by when the egress communication signal does not contain active voice packets. The far-end echo canceller reduces echo that is present in the egress communication signal. The far-end echo canceller refrains from canceling echo in the egress communication signal at times when the comfort noise generator is generating comfort noise. 
     Another aspect of the present invention is directed to a packet voice transceiver adapted to reside at a first end of a communication network and to send an ingress communication signal comprising voice packets to, and receive an egress communication signal comprising voice packets from, a second packet voice transceiver residing at a second end of the communication network. The packet voice transceiver includes a voice activity detector and a far-end echo canceller. The voice activity detector determines whether the ingress communication signal contains an active voice signal. The far-end echo canceller reduces echo that is present in the egress communication signal. The far-end echo canceller refrains from canceling echo in the egress communication signal at times when the voice activity detector determines that the ingress communication signal does not contain an active voice signal. 
     Another embodiment of the present invention is directed to a method of operating a packet voice transceiver adapted to reside at a first end of a communication network and to send an ingress packet voice signal to, and receive an egress packet voice signal from, a second packet voice transceiver residing at a second end of the communication network. Pursuant to the method, an egress packet voice signal is received. The egress packet voice signal is decoded to produce an egress audio signal. The egress audio signal is monitored to determine if it contains echo that originated at the second end. If the egress audio signal contains echo that originated at the second end, the echo is reduced by subtracting an estimate of the echo from the egress audio signal. If the egress audio signal does not contain echo that originated at the second end, echo is not reduced in the egress audio signal. 
     Another embodiment of the present invention is directed to a packet voice transceiver adapted to reside at a first end of a communication network and to send an ingress communication signal comprising voice packets to, and receive an egress communication signal comprising voice packets from, a second packet voice transceiver residing at a second end of the communication network. The packet voice transceiver includes a lost data element recovery engine and a far-end echo canceller. The lost data element recovery engine estimates a parameter of an unreceived data element. The far-end echo canceller reduces echo that is present in the egress communication signal. The far-end echo canceller refrains from canceling echo in the egress communication signal at times when the lost data element recovery engine is estimating a parameter of an unreceived data element. 
     It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the invention are shown and described only by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a functional block diagram representing a communication system in which the present invention may operate. 
         FIG. 1A  is a functional block diagram representing a communication system in which the present invention may operate. 
         FIG. 2  is a functional block diagram illustrating the services invoked by a packet voice transceiver system according to an illustrative embodiment of the present invention. 
         FIG. 3  is a functional block diagram representing a communication system in which the present invention may operate. 
         FIG. 4  is a functional block diagram representing a communication system in which the present invention may operate. 
         FIG. 5  is a functional block diagram representing the functionality of a far-end echo canceller according to an illustrative embodiment of the present invention. 
         FIG. 6  is a functional block diagram illustrating the services invoked by a packet voice transceiver system according to an illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In an illustrative embodiment of the present invention, a signal processing system is employed to interface voice telephony devices with packet-based networks. Voice telephony devices include, by way of example, analog and digital phones, ethernet phones, Internet Protocol phones, interactive voice response systems, private branch exchanges (PBXs) and any other conventional voice telephony devices known in the art. The described preferred embodiment of the signal processing system can be implemented with a variety of technologies including, by way of example, embedded communications software that enables transmission of voice data over packet-based networks. The embedded communications software is preferably run on programmable digital signal processors (DSPs) and is used in gateways, remote access servers, PBXs, and other packet-based network appliances. 
       FIG. 1  is a functional block diagram representing a communication system that enables the transmission of voice data over a packet-based system such as Voice over IP (VoIP, H.323), Voice over Frame Relay (VoFR, FRF-11), Voice Telephony over ATM (VTOA), or any other proprietary network, according to an illustrative embodiment of the present invention. In one embodiment of the present invention, voice data can also be carried over traditional media such as time division multiplex (TDM) networks and voice storage and playback systems. Packet-based network  10  provides a communication medium between telephony devices. Network gateways  12   a  and  12   b  support the exchange of voice between packet-based network  10  and telephony devices  13   a  and  13   b . Network gateways  12   a  and  12   b  include a signal processing system which provides an interface between the packet-based network  10  and telephony devices  13   a  and  13   b . Network gateway  12   c  supports the exchange of voice between packet-based network  10  and a traditional circuit-switched network  19 , which transmits voice data between packet-based network  10  and telephony device  13   c . In the described exemplary embodiment, each network gateway  12   a ,  12   b ,  12   c  supports a telephony device  13   a ,  13   b ,  13   c.    
     Each network gateway  12   a ,  12   b ,  12   c  could support a variety of different telephony arrangements. By way of example, each network gateway might support any number of telephony devices, circuit-switched networks and/or packet-based networks including, among others, analog telephones, ethernet phones, fax machines, data modems, PSTN lines (Public Switching Telephone Network), ISDN lines (Integrated Services Digital Network), T1 systems, PBXs, key systems, or any other conventional telephony device and/or circuit-switched/packet-based network. In the described exemplary embodiment, two of the network gateways  12   a ,  12   b  provide a direct interface between their respective telephony devices and the packet-based network  10 . The other network gateway  12   c  is connected to its respective telephony device through a circuit-switched network such as a PSTN  19 . The network gateways  12   a ,  12   b ,  12   c  permit voice, fax and modem data to be carried over packet-based networks such as PCs (personal computers) running through a USB (Universal Serial Bus) or an asynchronous serial interface, Local Area Networks (LAN) such as Ethernet, Wide Area Networks (WAN) such as Internet Protocol (IP), Frame Relay (FR), Asynchronous Transfer Mode (ATM), Public Digital Cellular Network such as TDMA (Time Division Multiple Access IS-13x), CDMA (Code Division Multiple Access IS-9x) or GSM (Global System for Mobile communications) for terrestrial wireless applications, or any other packet-based system. 
     Another exemplary topology is shown in  FIG. 1A . The topology of  FIG. 1A  is similar to that of  FIG. 1  but includes a second packet-based network  16  that is connected to packet-based network  10  and to telephony device  13   b  via network gateway  12   b . The signal processing system of network gateway  12   b  provides an interface between packet-based network  10  and packet-based network  16  in addition to an interface between packet-based networks  10 ,  16  and telephony device  13   b . Network gateway  12   d  includes a signal processing system which provides an interface between packet-based network  16  and telephony device  13   d.    
       FIG. 2  is a block diagram illustrating the services invoked by a packet voice transceiver system  50  according to an illustrative embodiment of the present invention. In an illustrative embodiment of the present invention, the packet voice transceiver system  50  resides in a network gateway such as network gateways  12   a ,  12   b ,  12   c ,  12   d  of  FIGS. 1 and 1A . In an exemplary embodiment, packet voice transceiver system  50  provides two-way communication with a telephone or a circuit-switched network, such as a PSTN line (e.g. DS0). The transceiver  50  receives digital voice samples  60 , such as a 64 kb/s pulse code modulated (PCM) signal, from a telephone or circuit-switched network. 
     The incoming PCM signal  60  is initially processed by a near-end echo canceller  70  to remove far-end echoes that might otherwise be transmitted back to the far-end user. As the name implies, echoes in telephone systems are the return of the talker&#39;s voice resulting from the operation of the hybrid with its two-four wire conversion. If there is low end-to-end delay, echo from the far end is equivalent to side-tone (echo from the near-end), and therefore, not a problem. Side-tone gives users feedback as to how loud they are talking, and indeed, without side-tone, users tend to talk too loud. However, far end echo delays of more than about 10 to 30 msec significantly degrade the voice quality and are a major annoyance to the user. 
     For the purposes of this patent application, the user from which the ingress PCM signal  60  is received will be referred to as the near-end user. Thus the outgoing (egress) PCM signal  62  is provided to the near-end user. The user that receives the ingress packet voice signal  132 , and that transmits the egress packet voice signal  133 , will be referred to as the far-end user. However, it is to be understood that the “near-end” user, that sends and receives PCM signals  60  and  62 , respectively, may reside either at a local device (such as a telephone) or at a device located across a circuit switched network. 
     Near-end echo canceller  70  is used to remove echoes of far-end speech present on the incoming PCM signal  60  before routing the incoming PCM signal  60  back to the far-end user. The near-end echo canceller  70  samples an outgoing PCM signal  62  from the far-end user, filters it, and combines it with the incoming PCM signal  60 . In an exemplary embodiment, the near-end echo canceller  70  is followed by a non-linear processor (NLP)  72  which may mute the digital voice samples when far-end speech is detected in the absence of near-end speech. The NLP  72  may also inject comfort noise, which, in the absence of near end speech, may be roughly at the same level as the true background noise or at a fixed level. 
     After echo cancellation, the power level of the digital voice samples is normalized by automatic gain control (AGC)  74  to ensure that the conversation is of an acceptable loudness. Alternatively, the AGC can be performed before the near-end echo cancellation  70 . However, this approach would entail a more complex design because the gain would also have to be applied to the sampled outgoing PCM signal  62 . In the described exemplary embodiment, the AGC  74  is designed to adapt slowly in normal operation, but to adapt more quickly if overflow or clipping is detected. In one embodiment, the AGC adaptation is held fixed if the NLP  72  is activated. 
     In the voice mode, the transceiver  50  invokes three services, namely call discrimination  120 , packet voice exchange  124 , and packet tone exchange  122 . The call discriminator analyzes the digital voice samples to determine whether a 2100 Hz tone (as in the case when the telephony device is a fax or a modem), a 1100 Hz tone or V.21 modulated high-level data link control (HDLC) flags (as in the case when the telephony device is a fax) are present. If a 1100 Hz tone or V.21 modulated HDLC flags are detected, a calling fax machine is recognized. The voice mode services are then terminated and the packet fax exchange is invoked to process the call. If a 2100 Hz tone is detected, the voice mode services are terminated and the packet data exchange is invoked. In the absence of a 2100 Hz tone, a 1100 Hz tone, or HDLC flags, the digital voice samples are coupled to the encoder system  124  and tone detection  122 . The encoder system illustratively includes a voice encoder, a voice activity detector (VAD) and a comfort noise estimator. Tone detection  122  illustratively comprises a dual tone multi-frequency (DTMF) detector and a call progress tone detector. The outputs of the call discriminator  120 , tone detection  122  and voice encoder  124  are provided to a packetization engine  130  which packetizes the data and transmits the packets  132  over the packet voice network. 
     Typical telephone conversations have as much as sixty percent silence or inactive content. Therefore, high bandwidth gains can be realized if digital voice samples are suppressed during these periods. In an illustrative embodiment of the present invention, a voice activity detector (VAD), operating under the packet voice exchange  124 , is used to accomplish this function. The VAD attempts to detect digital voice samples that do not contain active speech. During periods of inactive speech, a comfort noise estimator, also operating under the packet voice exchange  124 , provides silence identifier (SID) packets to the packetization engine  130 . The SID packets contain voice parameters that allow the reconstruction of the background noise at the far end. 
     From a system point of view, the VAD may be sensitive to the change in the NLP  72 . For example, when the NLP  72  is activated, the VAD may immediately declare that voice is inactive. In that instance, the VAD may have problems tracking the true background noise level. If the NLP  72  generates comfort noise during periods of inactive speech, it may have a different spectral characteristic from the true background noise. The VAD may detect a change in noise character when the NLP  72  is activated (or deactivated) and declare the comfort noise as active speech. For these reasons, in an illustrative embodiment of the present invention, the VAD is disabled when the NLP  72  is activated, as indicated by a “NLP on” message  72   a  passed from the NLP  72  to the voice encoding system  124 . 
     The voice encoder, operating under the packet voice exchange  124 , can be a straight 16-bit PCM encoder or any voice encoder which supports one or more of the standards promulgated by ITU (International Telecommunication Union). The encoded digital voice samples are formatted into a voice packet (or packets) by the packetization engine  130 . These voice packets are formatted according to an application protocol and outputted to the host (not shown). The voice encoder is invoked only when digital voice samples with speech, are detected by the VAD. 
     In the described exemplary embodiment, voice activity detection is applied after the AGC  74 . This approach provides optimal flexibility because the VAD and the voice encoder are integrated into some speech compression schemes such as those promulgated in ITU Recommendations G.729 with Annex B VAD (March 1996)—Coding of Speech at 8 kbits/s Using Conjugate-Structure Algebraic-Code-Exited Linear Prediction (CS-ACELP), and G.723.1 with Annex A VAD (March 1996)—Dual Rate Coder for Multimedia Communications Transmitting at 5.3 and 6.3 kbit/s, the contents of which is hereby incorporated by reference as through set forth in full herein. 
     Operating under the packet tone exchange  122 , a DTMF detector determines whether or not there is a DTMF signal present at the near end. The DTMF detector also provides a pre-detection flag which indicates whether or not it is likely that the digital voice sample might be a portion of a DTMF signal. If so, the pre-detection flag is relayed to the packetization engine  130  instructing it to begin holding voice packets. If the DTMF detector ultimately detects a DTMF signal, the voice packets are discarded, and the DTMF signal is coupled to the packetization engine  130 . Otherwise the voice packets are ultimately released from the packetization engine  130  to the host (not shown). The benefit of this method is that there is only a temporary impact on voice packet delay when a DTMF signal is pre-detected in error, and not a constant buffering delay. In one embodiment, whether voice packets are held while the pre-detection flag is active is adaptively controlled by the user application layer. 
     A call progress tone detector also operates under the packet tone exchange  122  to determine whether a precise signaling tone is present at the near end. Call progress tones are tones that indicate what is happening to dialed phone calls. Conditions like busy line, ringing called party, bad number, and others each have distinctive tone frequencies and cadences assigned them. The call progress tone detector monitors the call progress state, and forwards a call progress tone signal to the packetization engine  130  to be packetized and transmitted across the packet-based network. The call progress tone detector may also provide information regarding the near end hook status which is relevant to the signal processing tasks. If the hook status is on hook, the VAD should preferably mark all frames as inactive, DTMF detection should be disabled, and SID packets should only be transferred if they are required to keep the connection alive. 
     The decoding system of the packet voice transceiver system  50  essentially performs the inverse operation of the encoding system. The decoding system comprises a depacketizing engine  131 , a call discriminator  121 , tone generation functionality  123  and a voice decoding system  125 . 
     The depacketizing engine  131  identifies the type of packets received from the host (i.e., voice packet, DTMF packet, call progress tone packet, SID packet) and transforms them into frames that are protocol-independent. The depacketizing engine  131  then provides the voice frames (or voice parameters in the case of SID packets) to the voice decoding system  125  and provides the DTMF frames and call progress tones to the tone generation functionality  123 . In this manner, the remaining tasks are, by and large, protocol independent. 
     The voice decoding system  125  illustratively includes a jitter buffer that compensates for network impairments such as delay jitter caused by packets not arriving at the same time or in the same order in which they were transmitted. In addition, the jitter buffer compensates for lost packets that occur on occasion when the network is heavily congested. In one embodiment, the jitter buffer for voice includes a voice synchronizer that operates in conjunction with a voice queue to provide an isochronous stream of voice frames to the voice decoder. 
     In addition to a voice decoder and a jitter buffer, the voice decoding system  125  also illustratively includes a comfort noise generator, a lost frame recovery engine, a VAD and a comfort noise estimator. Sequence numbers embedded into the voice packets at the far end can be used to detect lost packets, packets arriving out of order, and short silence periods. The voice synchronizer analyzes the sequence numbers, enabling the comfort noise generator during short silence periods and performing voice frame repeats via the lost frame recovery engine when voice packets are lost. SID packets can also be used as an indicator of silent periods causing the voice synchronizer to enable the comfort noise generator. Otherwise, during far end active speech, the voice synchronizer couples voice frames from the voice queue in an isochronous stream to the voice decoder. The voice decoder decodes the voice frames into digital voice samples suitable for transmission on a circuit switched network, such as a 64 kb/s PCM signal for a PSTN line. The output of the voice decoder is provided to the far-end echo canceller  110 . 
     The comfort noise generator of the voice decoding system  125  provides background noise to the near end user during silent periods. If the protocol supports SID packets, (and these are supported for VTOA, FRF-11, and VoIP), the comfort noise estimator at the far end encoding system should transmit SID packets. Then, the background noise can be reconstructed by the near end comfort noise generator from the voice parameters in the SID packets buffered in the voice queue. However, for some protocols, namely, FRF-11, the SID packets are optional, and other far end users may not support SID packets at all. In these systems, the voice synchronizer must continue to operate properly. In the absence of SID packets, the voice parameters of the background noise at the far end can be determined by running the VAD at the voice decoder  125  in series with a comfort noise estimator. 
     The tone generation functionality  123  illustratively includes a DTMF queue, a precision tone queue, a DTMF synchronizer, a precision tone synchronizer, a tone generator, and a precision tone generator. When DTMF packets arrive, they are depacketized by the depacketizing engine  131 . DTMF frames at the output of the depacketizing engine  131  are written into the DTMF queue. The DTMF synchronizer couples the DTMF frames from the DTMF queue to the tone generator. Much like the voice synchronizer, the DTMF synchronizer provides an isochronous stream of DTMF frames to the tone generator. The tone generator of the tone generation system  123  converts the DTMF signals into a DTMF tone suitable for a standard digital or analog telephone, and provides the DTMF signal to the far-end echo canceller  110 . 
     When call progress tone packets, arrive, they are depacketized by the depacketizing engine  131 . Call progress tone frames at the output of the depacketizing engine  131  are written into the call progress tone queue of the tone generation functionality  123 . The call progress tone synchronizer couples the call progress tone frames from the call progress tone queue to a call progress tone generator. Much like the DTMF synchronizer, the call progress tone synchronizer provides an isochronous stream of call progress tone frames to the call progress tone generator. The call progress tone generator converts the call progress tone signals into a call progress tone suitable for a standard digital or analog telephone, and provides the DTMF signal to the far-end echo canceller  110 . 
     Far-end echo canceller  110  is used to remove echoes of near-end speech present on the outgoing PCM signal  62  before providing the outgoing PCM signal  62  to the near-end user or circuit-switched network. The far-end echo canceller  110  samples an ingress PCM signal  80  from the near-end user, filters it, and combines it with the egress PCM signal  85 . In an exemplary embodiment, the far-end echo canceller  110  is followed by a non-linear processor (NLP)  73  which may mute the digital voice samples when near-end speech is detected in the absence of far-end speech. The NLP  73  may also inject comfort noise, which, in the absence of near end speech, may be roughly at the same level as the true background noise or at a fixed level. In an alternative embodiment, the NLP  73  suppresses the samples by a fixed or variable gain. In yet another embodiment, the NLP combines these two schemes. 
     The NLP  73  provides the echo-cancelled PCM signal to automatic gain control (AGC) element  108 . AGC  108  normalizes the power level of the digital voice samples to ensure that the conversation is of an acceptable loudness. Alternatively, the AGC can be performed before the far-end echo cancellation  110 . In the described exemplary embodiment, the AGC  108  is designed to adapt slowly in normal operation, but to adapt more quickly if overflow or clipping is detected. In one embodiment, the AGC adaptation is held fixed if the NLP  73  is activated. The AGC  108  provides the normalized PCM signal to the PCM output line  62 . 
       FIG. 2  shows two echo cancellers: near-end echo canceller  70  and far-end echo canceller  110 . In most typical systems, the transceiver systems on both ends of a communication would have a “near-end” echo canceller, i.e., an echo canceller that cancels echo of the egress far-end signal that is present in the ingress near-end signal before transmitting the ingress near-end to the far end.  FIG. 3  is a functional block diagram representing an illustrative communication. In  FIG. 3 , the voice from talker 1 ( 300 ) is processed by transceiver system 1 ( 310 ), which transmits a packetized signal over packet network  320  to transceiver system 2 ( 330 ), which processes the packet signal and provides an audio signal to talker 2 ( 340 ). Similarly, the voice from talker 2 ( 340 ) is processed by transceiver system 2 ( 330 ), which transmits a packetized signal over packet network  320  to transceiver system 1 ( 310 ), which processes the packet signal and provides an audio signal to talker 1 ( 300 ). The near-end echo canceller in system 1 ( 310 ) operates on behalf of talker 2 ( 340 ). In other words, if the echo canceller in system 1 ( 310 ) is disabled, then talker 2 ( 340 ) will perceive echo (assuming the round trip delay in the packet network  320  is larger than about 10-20 msec or so). The near-end echo canceller in system 2 ( 330 ) operates on behalf of talker 1 ( 300 ). Thus, if the echo canceller in system 2 ( 330 ) is disabled, then talker 1 ( 300 ) will perceive echo. The near-end echo cancellers are referred to as such because they cancel echo generated on the near end. That is, the near-end echo canceller in system 1 removes echo generated between system 1 ( 310 ) and talker 1 ( 300 ), echo that the far-end (talker 2) would perceive. 
     Now, for purposes of illustration, assume that system 2 ( 330 ) doesn&#39;t have an echo canceller. This might be true for a variety of reasons, including for example, cost reasons, because the designer of system 2 ( 330 ) thought the delay would be low and an echo canceller wouldn&#39;t be necessary, or because the echo canceller in system 2 ( 330 ) is ineffective. To cope with this situation, the present invention provides a transceiver system that cancels echo in both directions. The near-end echo canceller, such as echo canceller  70  of  FIG. 2 , cancels “near-end” echo for the benefit of the far-end user. The far-end echo canceller, such as echo canceller  110  of  FIG. 2 , cancels “far-end” echo for the benefit of the near-end user. 
     Another example would be a device which bridged two different networks. i.e., a bridge between ATM and IP networks.  FIG. 4  is a functional block diagram representing another communication system in which the present invention could be employed. In the communication shown in  FIG. 4 , talker 1 ( 400 ) accesses a packet voice network  410  via a device that doesn&#39;t have echo control. Talker 2 ( 440 ) accesses a voice over IP (VoIP) system  430  via a device without echo control. 
     In an illustrative embodiment of the present invention, the transceiver system  420  that transcodes between voice over IP and voice over ATM has two echo cancellers. However, it does not make a lot of sense to call one “near end” and one “far end”. Both are operating over a packet voice network, and the concept of “near” and “far” which is ambiguous. For purposes of explanation in the present application, the two echo cancellers in such a transceiver are sometimes referred to as a near-end echo canceller and a far-end echo canceller. However, it is to be understood that in certain implementations of the present invention, the terms “near-end” and “far-end” hold little, if any literal meaning. 
     Referring again to  FIG. 2 , there are two echo cancellers shown: one referred to as near-end echo canceller  70  and one referred to as far-end echo canceller  110 . The near-end canceller  70  monitors the samples  62  that are sent towards the phone. These samples go towards the phone and are echoed back. The echo is substantially always present and the non-linearities in that path are minimal. There is no (or very little) time-varying component. The echo (which is almost linear) is almost completely removed by the linear component of the echo canceller  70 . The fact that it is nearly linear and non-time-varying makes removing the echo easier. 
     The far-end echo canceller  110  monitors the samples  80  going out of the AGC  74  towards the packet network. These samples get compressed by the voice coder  124  and sent across the packet network. At the far end they illustratively go through the jitter buffer, voice decoder, get echoed at the end device, AGC, VAD, voice coder, etc. Furthermore, the far-end device might not have a (near-end) echo canceller/NLP, or might have an ineffective echo canceller/NLP. Then, at the near end, the packets (potentially with far-end speech+echo) go through the jitter buffer, packet loss concealment, and voice decoder of voice decoding system  125 . Far-end echo canceller  110  then attempts to remove the far-end echo. There are numerous sources of non-linearities, variable delay (jitter buffers) and variable attenuation (due to AGC at the far end) in the echo path. Once the echo model is estimated by the echo canceller  110 , it may change immediately. Furthermore, the echo model is (usually) linear, and there are numerous non-linear devices within the system. The present invention endeavors to cope with these problems. 
       FIG. 5  is a functional block diagram representing the functionality of far-end echo canceller  110 . R in  and R out  are samples from the output of AGC  74  ( FIG. 2 ). S out  is provided to the AGC  108  ( FIG. 2 ) and S in  is provided from some combination of the voice decoder  125  ( FIG. 2 ) and the tone generator  123  ( FIG. 2 ). 
     The voice encode block  521  and voice decode blocks  501 ,  522  are meant to take into account any non-linearities due to the network format. For example, if ITU-T standard G.711 is used to represent the TDM samples, then the echo canceller takes into account the non-linearity introduced by the encoding and decoding of G.711 on both the ingress  500 ,  501  and egress  521 ,  522  path. The transcoding on the receive path (Rin to Rout) is taken into account by having voice decode operation  501  available prior to the transversal filter  510 ,  511 . This transcoding also may be present on the send path (S in  to S out ) and is modeled in voice encode block  521  and voice decode block  522 . 
     In a far-end echo canceller, the voice encode/decode operation  500 ,  501  could be a low bit rate voice coder (such as ITU-T standard G.729). As such, the encode and decode operation would be a G.729 transcoding (potentially with VAD). The encode operation in blocks  521  and  522  may not be the same encode/decode operation as that in blocks  500  and  501 . Given that the encode operation is performed on the ingress path the echo canceller only needs to decode the encoded bit stream output by voice decoder  124  of  FIG. 2 . This is shown in  FIG. 6 . 
     Because accounting for encoding and decoding operations with decode blocks  501  and  522  and encode block  521  may overly complicate system operation, in an alternative embodiment of the present invention, the far-end echo canceller  110  does not include decode blocks  501  and  522  and encode block  521 . In this alternative embodiment, the reference signal is applied by the output of  74  as shown in  FIG. 2 . 
     Any known (minimum) fixed delay in the system between R out  and S in  is incorporated into a bulk delay  502 . This simply ensures the echo canceller can cancel over the greatest possible delay range. 
     Tone detection  503  detects the presence of continuity test (COT) tones (1780 Hz, 2010 Hz, 2400 Hz, 2600 Hz, 2400+2600 Hz) dial tone, and some modem tones. Presence of these tones may place the echo canceller in a bypass mode  512  or may control the aggressiveness of the NLP  519 . 
     The level estimators  504 ,  505 ,  506  calculate peak power levels, average power levels over 5 msec and 35 msec rectangular windows; and minimum background noise levels (using a non-linear minimum tracking algorithm). Level estimator  504  operates on the ingress signal, R out . Level estimator  505  operates on the egress signal, S in . Level estimator  506  operates on the egress signal after cancellation. The outputs of the level estimators are used for doubletalk detection for adaptation  515 , NLP  514 , ERL and ERLE estimation  513 , and the bypass control  512 . 
     The short-term (ingress signal) spectral estimate  507  is illustratively a spectral estimate over the length of the tail of the echo canceller or 16 msec, whichever is greater. The estimate is used in the tone detectors  503 , the doubletalk detector  514  for NLP  519 , and in bypass control  512 . In an illustrative embodiment, the short-term spectral estimate is a 6th order LPC (linear predictive coding) autocorrelation method. The autocorrelation values are computed based on a rectangular window recursively. The long-term spectral estimate  508  is illustratively a 6th order spectral estimate computed using a normalized LMS (least mean squares) algorithm (with a small step size). The estimate is intended to be the spectral estimate of the background noise. In an illustrative embodiment of the present invention, the long-term spectral estimate  508  is frozen if the egress or ingress level is high. 
     The peak level estimator  509  illustratively computes the peak level over a sliding window of duration 5-30 msec over the tail length of the echo canceller. For example, for a 128 msec echo canceller, the peak level is the peak power using a 5-30 msec window over a sliding window over the full 128 msec. 
     The tone canceller  510  is a short tail length echo canceller designed to work for periodic or near periodic signals. If the signal at Rin is periodic or nearly periodic, then a short tail length echo canceller will perform suitably well. In an illustrative embodiment of the present invention, if the short tail canceller  510  performs well, the long tail canceller  511  (the main canceller) adaptation process can be inhibited to minimize divergence (and reduce processing requirements). Typical sources of echoes are limited to about 4 to 12 msec of dispersion (and typically less than 8 msec). Due to delays in the echo path, these locations of these echoes may be anywhere within the 128 msec echo tail. 
     The main (foreground) canceller  511  is a sparse canceller. In an illustrative embodiment, the main canceller  511  has a total of about 24 msec (192 taps) of coefficients. The coefficients are specified by a starting location and a duration. This will allow the sparse echo canceller  511  to cancel up to three sources of echo, which is the maximum number of distinct reflectors expected to be encountered. 
     The bypass logic  512  detects when it is better to use the tone canceller  510 , the foreground (main) canceller  511  or to bypass the entire cancellation process. 
     ERL and ERLE estimation  513  computes the echo return loss (ERL) and echo return loss enhancement (ERLE) based on the power level estimators  504 ,  505 ,  506  and peak-level power estimator  509 . The ERL is the level at R out  minus the level at S in  in the absence of speech at S in . The ERL estimator tracks the level difference from R out  to S in  while limiting the change in the estimator  513  when a signal (speech or high level noise) at S in  is detected. In an illustrative embodiment of the present invention, the ERL estimator is only run when it appears the signal at R in  is active (when the level at R in  is appreciably high). 
     The ERLE is the level at S in  minus the level at the input to the NLP  519  again in the absence of speech at S in  with appreciable speech at R in . (In a far end echo canceller, this would be the near end talker active with the far end talked inactive. In a near end echo canceller, this would be the near end talker inactive with the far end talker active). The ERLE is a measure of how well the linear portion (transversal filter  510  or  511 ) of the echo canceller  110  is working. 
     In an illustrative embodiment of the present invention, the far-end echo canceller  110  includes independent doubletalk detection for the NLP  519  and for background canceller adaptation  516 . Keeping these separate simplifies interactions between the NLP  519  and background canceller adaptation  516 , and each can be tuned for the different criteria required. 
     In an illustrative embodiment of the present invention, the doubletalk detector  514  for NLP  519  detects when a signal with a significant level is present at S in  or when NLP  519  is not required, and subsequently disables the NLP  519 . This is essentially done when the level at the output of the digital subtractor  530  is significantly higher than the level at R out  minus the ERL and ERLE estimates  513 . In other words, if the echo level after linear removal of the echo is lower than the estimated talker level at S in  (not including the echo) the NLP  519  should not be activated. 
     Doubletalk detection  515  for background canceller adaptation  516  is relatively conservative. Due to the dual-canceller feature, if the background canceller  511  diverges the update control would limit divergence. In an illustrative embodiment of the present invention, unless there is proof that there is far end present (in a far end echo canceller), adaptation takes place when the level at R out  is significantly high. 
     In an illustrative embodiment of the present invention, background canceller adaptation  516  is based on a two-stage approach. In stage one, a downsampler reduces the rate of the egress and ingress signals. A full tail canceller is then run on the downsampled signal. A peak picking method is then used on the full tail canceller coefficients in order to determine the most likely windows of significant coefficients. Once these windows are determined, a sparse weighted block-oriented LMS algorithm is used. Since the number of coefficients in this canceller is relatively small, and due to the weighting used, fast convergence is attained. 
     The short tail canceller  510  is adapted based on tone adaptation  517 , which, in an illustrative embodiment of the present invention is an 8-tap LMS algorithm. 
     Update control  518  is a key portion of the algorithm. The update control is aggressive (likely to copy the coefficients from the background canceller to the foreground canceller), when performance metrics of the echo canceller (namely, ERL, ERLE, and combinations thereof) are indicative of poor performance. For example, if the echo canceller is completely unconverged, coefficients are copied from the background to foreground canceller whenever the short term ERLE of the background canceller is better than the foreground canceller. Once convergence is attained (higher ERLE), copying coefficients from the background canceller to the foreground canceller is delayed. For example, it may take up to 100 msec for the coefficients to be copied if the performance (as per ERL and ERLE is good). Delay is also added when tones are detected, doubletalk is detected, and so on. One component of the invention is to delay the copying of coefficients by a larger time period when performance metrics indicate that performance is good. It is also possible for the background canceller to diverge (perhaps badly) in doubletalk. Although this will not impact the performance of the foreground canceller (if coefficients are not copied) it may impact future adaptation or tracking. As such, if the foreground canceller is significantly better than the background canceller, a copy from the foreground canceller to the background canceller may be performed. 
     As previously mentioned, the activation of the NLP  519  is controlled by the doubletalk detector  514 . The actual implementation of the NLP  519  can be based on a variety of methods. In one embodiment of the present invention, the NLP  519  includes a spectral comfort noise generator that generates comfort noise when the NLP  519  is activated. In another embodiment, when the NLP  519  is activated, it removes the signal and replaces it with silence. In another embodiment, the NLP  519  includes a dynamic compressor that dynamically compresses the level of signal down to the background noise level. In one embodiment of the present invention, any of the above-described schemes are selectable by configuration registers. In another embodiment, an adaptable switched scheme is employed which uses either the spectral comfort noise generator, the dynamic compress, or a combination of both depending on the estimated noise characteristics. For example, if the spectrum of the noise is relatively stationary, then the spectral comfort noise generator is used. If the noise is very dynamic, the dynamic compressor is used. Otherwise, some mixture of the two is used. 
     Referring again to  FIG. 2 , and as previously mentioned, the comfort noise generator of the voice decoding system  125  provides background noise to the near end user during silent periods. When the comfort noise generator is active there can be no echo in the egress signal  85 . Thus, in an illustrative embodiment of the present invention, the comfort noise generator (CNG) communicates with the far-end echo canceller  110 . When the comfort noise generator is active, it provides a “CNG on” flag to the echo canceller  110 . In one embodiment of the invention, when the echo canceller  110  receives the “CNG on” flag, the echo canceller  110  stops canceling echo in the egress signal  85 . In one embodiment, the “CNG on” flag is provided to the bypass controller  512  ( FIG. 5 ) of the echo canceller  110 . In response thereto, the bypass controller  512  causes the echo cancellation process to be bypassed. In an alternative embodiment, when the comfort noise generator is active, the far-end echo canceller  110  freezes adaptation of the echo path model. 
     As previously mentioned, the voice activity detector (VAD) of the voice encoding system  124  detects whether the digital voice samples in ingress signal  80  contain active speech. When the VAD of encoding system  124  declares that the ingress signal  80  does not contain active voice samples, there can be no echo in the egress signal  85 . Thus, in an illustrative embodiment of the present invention, the VAD of voice encoder  124  communicates with the far-end echo canceller  110 . When the VAD is declaring that the ingress signal  80  is inactive, it provides a “no voice” flag to the echo canceller  110 . In one embodiment of the invention, when the echo canceller  110  receives the “no voice” flag, the echo canceller  110  stops canceling echo in the egress signal  85 . In one embodiment, the “no voice” flag is provided to the bypass controller  512  of the echo canceller  110 . In response thereto, the bypass controller  512  causes the echo cancellation process to be bypassed. In an alternative embodiment, when the VAD is declaring “no voice,” the far-end echo canceller  110  freezes adaptation of the echo path model. In an illustrative embodiment of the invention, there is a delay from the time when the ingress signal  80  switches from active to inactive to the time that the far-end echo canceller  110  is turned off (or adaptation is frozen). This is due to the round trip delay of the echo path. Thus the delay is equal to an estimate of the round trip delay. 
     In an illustrative embodiment of the present invention, the far-end echo canceller  110  detects when the far-end hybrid disappears and acts accordingly. This is to detect far-end suppressers. When the hybrid, and thus the echo, disappears, the echo path is open. In one embodiment of the present invention, convergence is maintained by preserving the set of echo canceller coefficients that represented the echo path prior to the disappearance of the echo. Thus a set of open echo path coefficients are maintained that represent the open echo path. When these open echo path coefficients perform well, i.e., cancel echo well, i.e., result in less residual energy over some time period, the saved coefficients are not adapted. 
     For example, take a far-end echo canceller, such as echo canceller  110  of  FIG. 5 , having a foreground canceller  511 , a background canceller  510  and an open echo path model (selectable by bypass controller  512 ). In an illustrative embodiment of the present invention, the background canceller  510  is adapted and copied to the foreground canceller  511  if (1) the background canceller  510  is performing better than the foreground canceller  511 , and (2) the background canceller  510  is significantly better than the open echo path model. This scheme can be extended to multiple foreground models. 
     Referring again to  FIG. 2 , and as previously mentioned, the lost frame recovery engine of the voice decoding system  125  attempts to reconstruct frames that were transmitted by the far end but never received by the voice packet transceiver  50 . In one embodiment this is accomplished by estimating the characteristics of the lost frame based on received frames that were transmitted in proximity to the lost frame. When the lost frame recovery engine is active, there is no echo in the egress signal  85 . Thus, in an illustrative embodiment of the present invention, the lost frame recovery engine communicates with the far-end echo canceller  110 . When the comfort noise generator is active, it provides a “LFR on” flag to the echo canceller  110 . In one embodiment of the invention, when the echo canceller  110  receives the “LFR on” flag, the echo canceller  110  stops canceling echo in the egress signal  85 . In one embodiment, the “LFR on” flag is provided to the bypass controller  512  ( FIG. 5 ) of the echo canceller  110 . In response thereto, the bypass controller  512  causes the echo cancellation process to be bypassed. In an alternative embodiment, when the lost frame recovery engine is active, the far-end echo canceller  110  freezes adaptation of the echo path model. 
     Although a preferred embodiment of the present invention has been described, it should not be construed to limit the scope of the appended claims. For example, the present invention is applicable to any real-time media, such as audio and video, in addition to the voice media illustratively described herein. Those skilled in the art will understand that various modifications may be made to the described embodiment. Moreover, to those skilled in the various arts, the invention itself herein will suggest solutions to other tasks and adaptations for other applications. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.