the present invention provides a device , method and system for implementing an echo control on hand - free phones . the primary technological advantage of the present invention is that it deploys an echo canceller in the hand - free phones , determines a call status of a current network through the echo canceller , and cancels the echo in a sending channel through software . in the present invention , a speech signal on a receiving channel is referred to as a remote speech signal r in and a speech signal on a sending channel is a local speech signal s in . the remote speech signal is played by a loudspeaker and fed into a microphone to produce an echo , which is then overlapped with the local signal to generate a local speech signal with the echo s in . in the present invention , a call status of a current network falls into two modes , a remote mode and a local mode . in the remote mode , a remote user of a local hand - free phone is speaking ; whereas , in the local mode , a user of the local hand - free phone is speaking . the following presents a detailed description of the present invention in conjunction with attached drawings . the device described in this method is an echo eliminator , which is used to determine the call status of the current network , and cancel an unnecessary echo in the local speech signal s in through software . as shown in fig2 , the echo eliminator includes a speech signal detector and a half duplex controller . the speech signal detector receives , samples the aforementioned local and remote speech signals , and determines whether the call status of the current network is the remote mode or local mode based on a relationship between estimated short - time energy values of the local speech signal and the remote speech signal obtained through sampling , and transfers the resulting call status to the half duplex controller . the speech signal detector is composed of two modules , a sample module and a call status judgment module . the sample module receives the local and remote speech signals , and samples them in a specified period of time . if the estimated short - time energy value of a current sample point of the local speech signal obtained by the sample module is greater than a defined multiple of a maximum of the estimated short - time energy value of the remote speech signal in the specified period of time , the call status judgment module will consider the call status to be local mode ; otherwise , it considers the call status to be remote mode . the half duplex controller implements a corresponding half duplex echo control measure in the aforementioned local speech signal with the echo s in , according to the call status of the current network from the speech signal detector . if the call status is the local mode , the half duplex controller will output speech sample points on an “ as it is ” basis without restraining the echo in the local speech signal s in . on the other hand , if the call status is the remote mode , it restrains , or mutes , the echo in the local speech signal s in . the half duplex controller includes three modules , a half duplex control module , a speech output module and a mute processing module . the half duplex control module receives the call status of the current network from the speech signal detector , and starts up the speech output module when the local user is speaking ( namely , local mode ) or the mute processing module when the remote user is talking ( namely , remote mode ). the speech output module outputs the speech signals traveling through the aforementioned sending channel on an “ as it is ” basis . the mute processing module mutes and outputs the speech signals traveling through the previously mentioned sending channel . fig3 illustrates the procedure of implementing an echo control on digital hand - free phones as follows : step 3 - 1 : the speech signal detector samples the local and remote speech signals and determines the call status of the current network . the speech signal detector first samples the inputted remote and local speech signals , and determines the call status of the current network depending on the estimated short - time energy values of the speech samples on the sending and receiving channels . if the estimated short - time energy value of the current sample point on the sending channel is greater than a fixed multiple of the maximum of the estimated short - time energy values of the speech sample point on the receiving channel in the specified period of time , the speech signal detector will consider the call status to be local mode , that is , the user of the local hand - free phone is speaking . otherwise , it considers the call status to be remote mode , that is , the remote user of the local hand - free phone is speaking ; here requiring an echo cancel because the echo occurs on the sending channel . in formula 1 , s ( n ) is a sample value of the local speech sample point , ŝ ( n )=( 1 − α )* ŝ ( n − 1 )+ α * s ( n ) is an estimated short - time energy value of the local speech signal sample point , r ( n ) is a sample value of the remote speech sample point , and { circumflex over ( r )}( n )=( 1 − α )*{ circumflex over ( r )}( n − 1 )+ α * r ( n ) is an estimated short - time energy value of the remote speech signal sample ; whereas , α = 1 / 32 is an estimated short - time energy coefficient used to calculate an estimated short - time energy value of 4 ms delay , n is a predefined constant determined by an echo delay δ ( ms ), namely , n = 8 * δ , also referring to the sample number of echo delay . for example , if the echo delay is 30 ms , then n = 30 * 8 = 240 . if formula 1 is satisfied , the speech signal detector will consider the call status of the current network to be local mode . otherwise , it considers the call status to be remote mode . then it will transfer the resulting call status to the half duplex controller . step 3 - 2 : according to the call status of the current network , the half duplex controller gives a corresponding echo control treatment to the speech signals on the sending channel . upon receiving the call status from the speech signal detector , the half duplex controller will take corresponding actions . if the call status is in the local mode , it stops the echo and outputs the received local speech signal as the output value s out on an “ as it is ” basis . this is equal to turning on the local microphone . if the call status is in the remote mode , the half duplex controller cancels the echo in the received local speech signal . it controls the inputted local speech signal s in , and replaces the value s in with 0 as the output value s out , that is , it mutes the local speech signal s in . this is equal to the “ software mute ” of the local microphone , or turning off the microphone . in this way , the remote user cannot hear the echo fed back from the local microphone . after the remote user stops speaking and the local users begins to speak , the local microphone is turned on and the speech signals are sampled and sent to a peer for the purpose of restraining echo feedback and eliminating echo on the hand - free phones . the aforementioned devices and methods apply to the digital hand - free phones . fig4 shows an embodiment of the digital hand free phone system described in the present invention , including a speech compression / decompression module , an echo eliminator , a codec chip , a loudspeaker , and a microphone , wherein the echo eliminator is deployed between the speech codec and codec chip . the principle of the digital hand free phone system is described as follows . on the sending channel , when the local user is speaking , the codec chip samples , quantifies and encodes the local and remote speech signals collected by the microphone , generates digital code streams and transfers them to the echo eliminator . the echo eliminator samples the received digital code streams and considers the call status of the current network to be local mode . therefore , it outputs the digital code streams without any change to the speech compression / decompression module . next , the compression / decompression module compresses them in speech frames and transmits the frames to the remote user through a pstn network . on the sending channel , when the remote user is speaking , the codec chip samples , quantifies and encodes the local and remote speech signals collected by the microphone , generates digital code streams and transfers them to the echo eliminator . the echo eliminator samples the received digital code streams and considers the call status of the current network to be remote mode . therefore , it mutes the digital code streams . on the receiving channel , the speech compression / decompression module decompresses the speech frames from a peer , generates digital code streams and transmits them to the codec chip . then , the codec chip transforms the digital code streams into analog electric signals and plays the analog electric signals through the loudspeaker . an embodiment of the method described in the present invention is also available . the software procedure in this embodiment includes : 1 . resets a half duplex control register and makes a call . 2 . based on a frame length specified by an employed speech compression algorithm , the speech signal detector puts the remote speech signal collected by the microphone into a large buffer with a length of n plus the frame length . the following takes g . 723 . 1 and a frame length of 30 ms as an example to describe detailed steps . before processing the speech signal samples for the first time , the speech signal detector needs to wait until the remote speech signal samples are fully filled into the buffer . the samples in the buffer from left to right are x 0 . . . x n + 239 , and the half duplex control register is located in the header of the buffer and its content is x 0 . . . x 239 . 3 . the speech signal detector reads the sample value ( s in ( i )) of the local speech signal from the input signal of the microphone , and then calculates the estimated short - time energy value p sin ( i )=( 1 − α )* p sin ( i − 1 )+ α * sin ( i − 1 ) of s in ( i ) , and the maximum value max ( abs ( p rin )), p rin ( i )=( 1 − α )* p rin ( i − 1 )+ α * rin ( i − 1 ) of the estimated short - time energy value p rin of x 1 . . . x 240 . 4 . sets s out = s in ( i ). 5 . if p sin & gt ; ½ * max ( abs ( p rin )), the call status of the current network is considered to be local mode and step 7 is executed . otherwise , the call status is considered to remote mode and step 6 is executed . 6 . sets s out = s out * 0 , that is , the muted s out serves as the output value of the echo eliminator . 7 . outputs s out as the output value of the echo eliminator without any change . 8 . updates the content of the half duplex control register by adding one to a base pointer of the half duplex control register ( namely , moving it to right by 1 bit ). at this point , the content of the register is x 2 . . . x 240 . 9 . repeats step 3 through step 8 till a frame of data on the left side of the buffer is processed . here the half duplex control register will be moved to the 240 to 479 bits of the buffer , and it needs to be moved to left by 240 bits to the header of the buffer to reset the base pointer . 10 . reads a frame of new samples and fills it in the 240 to 479 bits of the buffer . now a frame of samples is processed completely , and speech signal samples can be processed continuously in a next 30 ms period by repeating this process . it is of course to be understood that the embodiments described herein is merely illustrative of the principles of the invention and that a wide variety of modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims .