Abstract:
This invention provides a technique to reduce echoes not adequately attenuated by echo cancellers during initial periods of voice communication. Signals transmitted across communication networks are often reflected back to the caller resulting in echo. Echo cancellers are employed in the communication system to cancel this effect in order to maintain a high quality transmission. However, echo cancellers require time to detect, adapt, and effectively remove echo, often resulting in echo during the initial moments of the call and thereby degrading the quality of service. By installing an attenuation device into the communication system, all signals that pass through it are reduced to a preset value for a set period of time. This reduces any echo below a detectable threshold. After the period of time expires, the attenuation device allows the signal to pass unaffected, by which time the echo cancellers have adaptively adjusted to any echoes in the system.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to echo reduction. 
   2. Description of Related Art 
   Echoing in signal transmission is a well known phenomena. Devices such as echo cancellers adaptively adjust to transmission medium conditions such as transmission delay, etc., to remove echoes. With increasing network varieties, the transmission medium characteristics require additional improvements to obtain high quality signal transmissions. 
   SUMMARY OF THE INVENTION 
   An echo reduction device reduces echoes starting at the beginning of a communication. The echo reduction is applied until other devices such as echo cancellers can effectively cancel echo signals by adapting to the transmission environment. 
   For example, in a telephone system, when a call is initiated, a finite amount of time occurs between the beginning of the call and when echo cancellers are able to cancel out echo signals. During this time, echo signals may be heard and degrade signal transmission quality. This invention reduces the echo signal by reducing all signals of the call until the echo cancellers are able to cancel the echo signals. 
   The echo may be reduced for a predetermined amount of time after commencement of the communication. For example, a timer may be provided that is initialized to a predetermined value at the commencement of the communication. All the signals of the communication may be attenuated to reduce amplitudes of the echo signals until the timer expires. Other techniques may also be used to control the attenuation of the communication signals. For example, the echo cancellers may output echo canceller signals to an attenuator device to indicate a degree of echo cancellation that has been achieved. Such signals may be based on an error signal commonly used in adaptive weight update process of echo cancellers, for example. 
   In addition, a degree of attenuation may be adjusted during the time that attenuation is applied. For example, in voice communications, a sensitivity of a person to echoes may increase with time from the beginning of a call. Thus, attenuation of all signals may correspondingly increase with time until echo cancellers can cancel echo signals. Also, specific performances of echo cancellers may be known and a profile of percentage of echo cancellation over time from the commencement of the communication may be known. Thus, the degree of attenuation may be varied based on the percentage of echo cancellation that is expected to be achieved by the echo cancellers. For example, no attenuation is needed between the commencement of the communication and a shortest time that an echo signal can be detected. Then, attenuation may increase to a maximum attenuation and then the attenuation may be gradually decreased until the attenuation value reaches zero (i.e., when echo cancellers fully cancel echo signals). In this way, a high quality of signal transmission may be maintained without excessively attenuating the communication signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described, with reference to the following figures, wherein like numerals represent like elements, and wherein: 
       FIG. 1  shows an exemplary communication system; 
       FIG. 2  shows  FIG. 1  in a reduced form; 
       FIG. 3  shows exemplary snap shots of communication and echo signals for the system of  FIG. 2 ; 
       FIG. 4  shows an exemplary communication system that includes echo cancellers; 
       FIG. 5  shows exemplary snap shots of communication and echo signals for the system of  FIG. 4 ; 
       FIG. 6  shows an exemplary amplitude/time diagram of the communication signal and the echo signal with echo cancellation and short signal delay times; 
       FIG. 7  shows an exemplary amplitude/time diagram of the communication signal and the echo signal with echo cancellation and long signal delay times; 
       FIG. 8  shows an exemplary communication system that includes echo cancellers and echo reduction devices; 
       FIG. 9  shows an exemplary amplitude/time diagram for the system of  FIG. 8 ; 
       FIG. 10  shows an exemplary block diagram of an attenuation device; 
       FIG. 11  shows an exemplary diagram where a server/router serves as the echo reduction device; 
       FIG. 12  shows an exemplary diagram of the server/router performing functions of an attenuation device; and 
       FIG. 13  is an exemplary flowchart of a server/router echo reduction functional process. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   When a signal is transmitted over a transmission line, the phenomena “called echoing” occurs due to reflection of the signal caused by impedance discontinuities, hybrid circuits, etc. To remove such echoes, telecommunications networks include devices such as echo cancellers that are installed to cancel the echoing effect so that a high quality signal transmission may be obtained. 
   Echo cancellers are adaptive devices and may set weights based on a current transmission condition so that echoes may be detected and removed. Echo cancellers take time to adaptively set its weights. Such time is dependent upon signal propagation time for a particular transmission medium. Usually, the time required to achieve effective echo cancellation is short and any echoes that are not canceled during initial moments of a signal transmission do not significantly affect the quality of the signal transmission. 
   For example, if the communication system is a telephone system, the echo cancellers begin adapting to the connection circuit immediately after the connection between the calling and the called parties is made. The time required for the echo cancellers to adequately cancel the echoes is less than a detectable threshold so that the calling and called parties do not notice the echoes that escape cancellation. Thus the quality of the communication between the calling and called parties is not affected. However, if the time required for the echo cancellers to adequately cancel the echoes is increased (i.e., as dictated by the conditions of the transmission medium) to exceed the detectable thresholds, the quality of service degrades. This invention provides a technique to reduce the echoes that are not canceled during an initial period of a communication before the echo cancellers are able to cancel the echoes. 
     FIG. 1  one shows an exemplary communication system  100  that includes a network  102  and terminals  104 – 106 . While the network  102  is shown as a single entity, in actuality, such a network  102  may include many networks interconnected to each other. When the terminal  104  desires to communicate with the terminal  106 , the communication may be routed through the network  102  via paths that may traverse many different types of networks. For example, one of the networks may be a data network using the Internet Protocol (IP). When such a network is used to transmit information, the delay time may be significantly increased so that an amount of time required for echo cancellers to adaptively adjust the weights to adequately cancel out echo signals may be relatively long (e.g., in the order of 0.5 second). When this occurs, the effect of echoes may be detected by callers in a telephone network system, for example. 
     FIGS. 2–5  provide pictorial examples of the echo effect.  FIG. 2  condenses the network into a single line so that the terminals  104  and  106  are connected by the line  102  (network).  FIG. 3  shows snap shots  210 – 216  of a signal  200  transmitted by the terminal  104  to the terminal  106  and an echo signal  202  that is reflected back from a portion of the network  102  near the terminal  106 , for example. 
   At snap shot  210 , the signal  200  (e.g., a voice signal such as “hello”) is transmitted from the terminal  104  to the terminal  106 . At snap shot  212 , the signal  200  reaches the end of the network  102  at the terminal  106 . However, because of the reflection effect, the signal the  200  is reflected back from the end of the network  102  at the terminal  106  as an echo signal  202  shown in snap shot  214  traveling from the terminal  106  to the terminal  104 . At snap shot  216 , the echo signal  202  reaches the terminal  104 . 
   As may be noted in  FIG. 3 , the echo signal  202  has a similar shape as the original signal  200  but at a much reduced amplitude. Because there is a time separation between the snap shot  210  and the snap shot  216 , the terminal  104  receives a faint replica (echo signal  202 ) of the original signal  200  at a later time. 
     FIG. 4  shows the network  102  with echo cancellers  300  and  302  added. In such a case, the echo cancellers  300  and  302  adapt to the conditions of the network  102  so that echo signals may be canceled. 
   As shown in  FIG. 5 , snap shots  310 – 316  show the transmission of the signal  200  and the echo  202  through the echo cancellers  300  and  302 . At snap shot  310 , the signal  200  is transmitted from the terminal  104  to the terminal  106 . At snap shot  314 , the echo  202  is reflected from the end of the network  102  at the terminal  106  to the terminal  104 . However, at the snap shot  316 , the echo canceller device  302  removes the echo  202  so that the echo signal  202  is not transmitted to the network  102  and thus the terminal  104  does not receive the echo signal. 
     FIG. 6  shows an amplitude/time diagram of the transmitted signal  200  and the echo signal  202  at the end of the network  102  near the terminal  104 . The line  328  represents an amplitude of the transmitted signal  200  and the line  324  represents an amplitude of the echo signal  202 . Throughout the communication, the transmitted signal  200  is transmitted at an amplitude level  330 . After a time interval T 0 , as indicated by the dashed line  320 , the echo signal  202  arrives at the transmitting terminal having an amplitude level  332 . After a time period T 3 , as indicated by the dashed line  342 , the echo canceller  300  begins to adapt to the transmission conditions until time period T 1 , as indicated by the dashed line  322 , when the amplitude of the echo signal  202  is reduced to below a steady state portion of a threshold  326 . 
   The threshold  326  may be a threshold below which a human ear cannot detect the echo signal  202 , for example. As shown, the threshold  326  may be represented by a line that changes value with time because the detector may be non-linear over time such as a human ear. For example, a person&#39;s ability to detect an echo signal improves with time from the commencement of a communication. At the beginning of a call, for example, a person expects some sounds (clicks, etc.) that are connection-related. In addition, it takes time for a person to adapt to the voice channel. However, after an initial time period (T 0 ), a person becomes more attentive and, perhaps, the ear (and the neural processes) becomes more adapted to the connection and is able to better detect signals on the line. This phenomena may be represented by a decrease of the threshold  326  until the steady state portion is reached where signals below the threshold  326  are below a noise level and are essentially undetectable by the human ear. 
   Returning to  FIG. 6 , the dashed line  340  indicates a time threshold T 2  before which the echo signal  202  is assumed to be undetectable at the transmitting terminal (i.e., the echo signal amplitude is always less than the threshold  326 ). For example, if the transmitting terminal is a telephone station and a user of the telephone station is the detector, then any echo signals that occur in a time period less than the time threshold T 2  would not be detectable by the user. T 2  may be about 500 ms, for example. Thus, the conditions shown in  FIG. 6  represent an acceptable performance of the echo cancellers which reduce the echo signals  202  to below a detectable amplitude threshold  326  after a time period T 1  that is less than the time threshold T 2 . 
     FIG. 7  shows conditions of the echo signal  202  when a larger delay is experienced over the transmission medium so that the time period T 0  is extended as shown. Due to the larger delay, the echo signals  202  are also delayed and arrive at the terminal  104  after the longer time period T 0 . The echo cancellation of the echo canceller  300  also appears delayed so that time periods T 1  and T 3  appear much later from the commencement of the communication than before, as shown in  FIG. 6 . Unfortunately, the threshold  326  remains the same relative to the start of the communication, so that after a time period T 4 , the echo signals  202  are detectable until the time period T 1 , when echo canceller  300  is able to reduce the echo signals  202  to below the threshold  326 . This invention reduces the amplitude of echo signal  202  between the time periods T 0  and T 1  so that the quality degradation during this time is also reduced. 
     FIG. 8  shows an attenuation device  400  disposed between the echo cancellers  300  and  302 . The attenuation device  400  attenuates all signals transmitted or received via the network  102  for a period of time that is required for the echo cancellers  300  and  302  to adequately reduce the echo signal  202 . 
     FIG. 9  shows an amplitude/time diagram of a transmitted signal  350  and an echo signal  352  when the attenuation device  400  is used. As shown, both the signal  350  and echo signal  352  are reduced in amplitude during the time period T 5 . For the example shown in  FIG. 9 , the amount of attenuation is set so that the amplitude of the echo signal  352  is reduced to be below the threshold  326  during the period T 5 . After the period T 5 , the attenuation is removed so that signal amplitude of the signal  350  and the echo signal  352  return to their non-reduced level. Since the echo canceller  300  has adequate to time to reduce the amplitude of the echo signal  352  to below the threshold  326 , the amplitude of the echo signal  352  never exceeds the threshold  326 . Thus, the quality of transmission during the period T 1  is not reduced by the echo signal  352 . 
   While  FIG. 9  shows that the attenuation is sufficient to reduce the echo signal  352  to below the threshold  326 , the amount of the attenuation may be set to other values so that any desirable quality of transmission may be achieved. Because the transmitted signal  350  is also attenuated along with the echo signal  352 , it may be desirable to tolerate a small amount of echo signal  352  so that the transmitted signal may not be overly attenuated. Also, while the amount of attenuation is shown to be a fixed value over the time period T 3 , variable attenuation values may be used. 
   For example, the signal does not need to be attenuated at all during the time period between T 0  and T 4  (see  FIG. 7 ) since any echo signals occurring in this period would not affect the quality of transmission. After the period T 4 , the echo signal  202  may be reduced with an increased amount of attenuation to follow the falling threshold  326 . After time period T 3 , the amount of attenuation may be reduced because the echo canceller  300  may cancel a progressively greater amount of the echo signal  352  toward the end of the period T 1 . Thus, depending on the specific circumstances, the amount of attenuation may be varied throughout the period T 5  to achieve a desirable performance. 
     FIG. 10  shows an exemplary block diagram of the attenuation device  400 . The attenuation device  400  may include an attenuator control device  404  and an attenuator  406 . The attenuator control device  404  may be implemented in many different ways. For example, if the attenuation device  400  is adapted for a simple analog application, the attenuator control device  404  may be simply a timer that generates a time delay corresponding to the period T 5  shown in  FIG. 9 , for example. Thus, during the period T 5 , the attenuator control device  404  (timer) outputs a signal that sets an attenuation value of the attenuator  406  to be a preset amount. When a communication begins, a signal is received from the signal line  408  to start the attenuator control device  404  (timer). During the period T 5 , the attenuator control device  404  outputs a signal to the attenuator  406  to attenuate all signals passing through the transmission medium  410  to a preset attenuation value. After the period T 5  expires, the attenuator control device  404  either outputs a different signal to the attenuator  406  or resets the signal that was outputted to the attenuator  406 , thus causing the attenuator  406  to reduce the attenuation value to a smaller value, such as 0 attenuation value, for example. 
   As discussed above, the attenuator control device  404  may also implement more complex schemes such as varying the attenuation value of the attenuator  406  throughout the period T 5  so that desirable signal quality may be achieved. 
   In addition, the attenuator control device  404  may receive a signal from an associated echo canceller  300 ,  302  so that the echo canceller  300 ,  302  may inform the attenuator control device  404  of a more accurate time when an adequate echo cancellation has been achieved. The echo canceller  300 ,  302  may also output an estimate of percentage of echo cancellation that is achieved at any moment (e.g., error signal) so that the appropriate attenuation value may be set for the attenuator  406 . 
     FIG. 11  shows a configuration where the attenuation device  400  may be a server/router  500  implemented in a data network when the communication is routed through the data network such as voice over IP. In such a case, the function of the attenuation device  400  may be performed by a server or a router of the data network, for example, when the server/router  500  is in the path of the transmission medium that is providing communication between the terminals  104  and  106 . The server/router  500  may attenuate the signals as they are received. 
   For example, if the signal transmitted through the server/router  500  is expressed in binary values, then the attenuation device  400  implemented using the server/router  500  may multiply the binary value by an attenuation value before forwarding the signal to its destination. The attenuation amount is the multiplying factor which may be set to a value of 1 for no attenuation or a fractional value for reducing the amplitude of the signal. 
   The attenuation may also be performed by making a number of right shifts of the binary value to achieve a power of two attenuation. For example, if the attenuation factor is one half, then the binary value may be shifted right by one bit. 
     FIG. 12  shows an exemplary block diagram of the server/router  500  which may include a controller  502 , a memory  504 , and an attenuator  506 , a network interface  508  and a terminal interface  510  (optional). The above components may be coupled together via bus  512 . 
   While  FIG. 12  shows the attenuator  506  as a separate unit, the functions performed by the attenuator  506  may be performed by the controller  502 . The terminal interface  510  is required if the server/router  500  interfaces with a terminal  104 ,  106 . If the server/router  500  does not interface with a terminal  104 ,  106 , then the terminal interface  510  is not needed. 
   The signals transmitted between the terminals  104  and  106  may be received by the server/router via the network interface  508 . The controller  502  may determine if the received signals (now in digital data form) should be processed by the attenuator  506 . If attenuation should be performed, the controller  502  sends the data portion of the received data to the attenuator  506 . 
   The controller  502  may determine whether attenuation should be performed based on transmission conditions. For example, if a timer process is implemented, the controller  502  initializes a timer when a new communication commences. The data of the communication is sent to the attenuator  506  for attenuation until the timer expires. After the timer expires, the controller  502  sends the received data to other units along the transmission path without any attenuation. 
   As discussed above, other techniques may also be applied such as receiving a signal from the echo cancellers  300 ,  302  to determine when to stop attenuating the transmitted data; sending an attenuation parameter to the attenuator  506  based on a time since the commencement of the communication so that a variable attenuation value based on a desired level of transmission quality may be implemented; and/or receiving parameters from the echo canceller  300 ,  302  relating to a percentage of reduction of the echo signal  202  so that an appropriate attenuation value may be sent to the attenuator  506 . Any combination of the above techniques may be selected based on specific circumstances of a particular implementation. 
     FIG. 13  shows an exemplary flowchart of a process of the server/router  500  that implements a timer for timing a period of signal attenuation from commencement of a communication to a time when the echo canceller  300 ,  302  adequately reduces the echo signal  202 . In step  1000 , the controller  502  determines whether a communication has commenced. If commenced, the controller  502  goes to step  1002 ; otherwise, the controller  502  returns to step  1000 . In step  1002 , the controller  502  sets a timer and goes to step  1004 . 
   In step  1004 , the controller  502  attenuates the received signal value by sending the signal value to the attenuator  506  and goes to step  1006 . In step  1006 , the controller  502  decrements the timer and goes to step  1008 . In step  1008 , the controller  502  determines whether the timer has expired. If expired, the controller  502  transmits the remaining communication signals to the destination device without attenuating the signal and goes to step  1010 . If the timer has not expired, the controller  502  goes to step  1004 . 
   In step  1010 , the controller  502  forwards the received signal data to the destination device and goes to step  1012 . In step  1012 , the controller  502  determines whether the communication has ended. If ended, the controller  502  goes to step  1014  and ends the process; otherwise, the controller  502  returns to steps  1010 . 
   While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.