Abstract:
A telephone system comprising an analog telephone and a full-duplex speakerphone adapter is disclosed. The analog telephone comprises an ordinary analog telephone subscriber circuit for transmitting and receiving analog signals and a handset for users. The full-duplex speakerphone adapter is coupled between the analog telephone and a central office, uses a subscriber loop interface circuit through the first telephone line to couple to the ordinary analog telephone subscriber circuit of the analog telephone and uses a telephone hybrid interface circuit through the second telephone line to couple to a wall jack to communicate with the central office.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/906,134, filed on Mar. 9, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hand free telephone system, and in particular relates to a telephone system with an ordinary analog telephone and a full-duplex speakerphone adapter. 
     2. Description of the Related Art 
     Hands-free speakerphone usages have become a common and important part of business conferences and daily telecommunications. Demand for good voice quality to facilitate efficient communications have increased, and the full-duplex feature for hands-free speakerphones has become highly desirable for business and home communications. 
     Conventionally, the most common speakerphones offer the ability to converse either in a handset mode or hands-free mode.  FIG. 1A  shows a conventional speakerphone  100  with a handset  72 , a microphone  50  and a loudspeaker  51 . A switch  191  on the speakerphone  100  determines whether the speakerphone  100  operates in the handset mode or hands-free mode. In the hands-free mode, users can be at a distance away from the speakerphone  100 . The loudspeaker  51  conducts a telephone conversation without users holding the handset  72 . Hands-free mode allows multiple participants to join the conversation in an audio-conference meeting, and also allows users to free up their hands for operating computer keyboards or other activities.  FIG. 1B  shows a conventional speakerphone  101  without a handset. In recent years, speakerphones  101  without handsets have become common, operating only in hands-free mode. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     An embodiment of telephone system is provided. The telephone system comprises an analog telephone and a full-duplex speakerphone adapter. The analog telephone comprises an ordinary analog telephone subscriber circuit for transmitting and receiving analog signals and a handset for users. The full-duplex speakerphone adapter is coupled in between the analog telephone and a central office, uses a subscriber loop interface circuit through a first telephone line to couple to the ordinary analog telephone subscriber circuit of the analog telephone and uses a telephone hybrid interface circuit through a second telephone line to couple to a wall jack to communicate with the central office. 
     Another embodiment of a full-duplex speakerphone adapter is provided. The full-duplex speakerphone adapter comprises a subscriber loop interface circuit, a telephone hybrid interface circuit, and a telephone signal processing unit. The subscriber loop interface circuit through a first telephone line is coupled to an ordinary analog telephone subscriber circuit of an analog telephone. The telephone hybrid interface circuit through a second telephone line is coupled to a wall jack to communicate with a central office. The telephone signal processing unit processes digital signals to remove line echo and acoustic echo. The full-duplex speakerphone adapter can be switched to a hands-free speakerphone mode or an ordinary analog telephone mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1A  shows a conventional speakerphone with a handset; 
         FIG. 1B  shows a conventional speakerphone without a handset; 
         FIG. 2  shows a full-duplex speakerphone adapter connected to an ordinary analog telephone according to an embodiment of the invention; 
         FIG. 3A  shows a block diagram of the full-duplex speakerphone as shown in  FIG. 2  according to another embodiment of the invention; 
         FIG. 3B  shows a block diagram of the effective data path inside the full-duplex speakerphone adapter when the switch switches to the position S for hands-free speakerphone mode according to another embodiment of the invention; 
         FIG. 3C  shows a block diagram of the effective data path inside the full-duplex speakerphone adapter when the switch switches to the position T for ordinary analog telephone mode according to another embodiment of the invention; and 
         FIG. 4  shows a simplified block diagram of the full-duplex processing unit of the full-duplex speakerphone adapter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 2  shows a full-duplex speakerphone adapter  300  connected to an ordinary analog telephone  102  according to an embodiment of the invention. As shown in  FIG. 2 , the ordinary analog telephone  102  does not directly connect to a wall jack  31 . The ordinary analog telephone  102  uses a telephone line  32  to plug into the telephone line jack  300   h  of the full-duplex speakerphone adapter  300 . The full-duplex speakerphone adapter  300  is connected to the wall jack  31  through another telephone line  32   b , and hence outwards to a telephone central office  30 . 
     The full-duplex speakerphone adapter  300  is responsible for providing the proper impedance on the telephone line  32   b , both during an on-hook condition and an off-hook condition. The full-duplex speakerphone adapter  300  is also responsible for providing the proper impedance and line voltages on the telephone line  32 , both during the on-hook condition and during the off-hook condition, such that the ordinary analog telephone  102  that plugs into the telephone line jack  300   h  performs as if it is connected to the telephone wall jack  31  interfacing with an ordinary analog telephone subscriber circuit. 
     During the normal telephone on-hook and idle condition, the full-duplex speakerphone adapter  300  interfaces with the telephone line  32   b  in an on-hook condition. 
     During an inbound call, the full-duplex speakerphone adapter  300  detects the ring condition the telephone line  32   b  by using a telephone hybrid interface circuit  130 , and, using the subscriber loop interface circuit  132  to provide the ring generation to the telephone line  32  to ring the ordinary analog telephone  102 . As the user takes the ordinary analog telephone  102  off hook, the full-duplex speakerphone adapter  300  establishes a relay data signal path between the telephone line  32   b  and the telephone line  32  such that voice data path is established between the ordinary analog telephone  102  and the wall jack  31 . The user can use the switch  270  to switch the full-duplex speakerphone adapter  300  to a hands-free speakerphone mode for hands-free conversation. 
     During an outbound call, the user takes the ordinary analog telephone  102  off hook. The subscriber loop interface circuit  132  on the full-duplex speakerphone adapter  300  detects the off-hook condition of the analog telephone  102  and puts the telephone line  32   b  which is coupled to the telephone hybrid interface circuit  130  to an off-hook condition, and then establishes a relay data signal path between the telephone line  32   b  and the telephone line  32  such that voice data path is established between the ordinary analog telephone  102  and the wall jack  31 . The dial tone from the central office  30  comes to the wall jack  31  and is relayed to the ordinary analog telephone  102 . The user can dial the outbound digits on the ordinary analog telephone  102 . Once the dialing is completed, the user can use the switch  270  to switch the full-duplex speakerphone adapter  300  to a hands-free speakerphone mode for hands-free conversation. 
       FIG. 3A  shows a block diagram of the full-duplex speakerphone  300  as shown in  FIG. 2  according to another embodiment of the invention. The full-duplex speakerphone adapter  300  comprises a telephone hybrid interface circuit  130  through the telephone line  32   b  to connect to the wall jack  31 . The telephone hybrid interface circuit  130  can be controlled by the telephone control logic unit  350  to switch to an off-hook condition or to an on-hook condition. The telephone hybrid interface circuit  130  can also detect the ring phone condition generated by the central office  30  and notify the telephone control logic unit  350 . 
     The full-duplex speakerphone adapter  300  also comprises a subscriber loop interface circuit  132  which provides the proper impedance and line voltage on the telephone line  32  to the ordinary analog telephone  102 , both during the on-hook condition and during the off-hook condition. The ordinary analog telephone  102  that plugs into the telephone line jack  300   h  performs as if it is connected onto the telephone wall jack  31  interfacing with an ordinary analog telephone subscriber circuit (not shown in  FIG. 3A ). The subscriber loop interface circuit  132  can generate proper ring voltage and frequency and is controlled by the telephone control logic unit  350  to ring the ordinary analog telephone  102 . The subscriber loop interface circuit  132  also comprises the on-hook and off-hook detection circuit (not shown in  FIG. 3A ) to detect the hook condition of the ordinary analog telephone  102  through the telephone line  32 . 
     During an inbound call, the telephone hybrid interface circuit  130  detects the ring condition on the telephone line  32   b , and notifies the telephone control logic unit  350 , which in turn controls the ring generation on the subscriber loop interface circuit  132  to ring the ordinary analog telephone  102  through the telephone line  32 . As the user takes the ordinary analog telephone  102  off hook, the subscriber loop interface circuit  132  detects the off-hook condition and notifies the telephone control logic unit  350 . The telephone control logic unit  350  then establishes a relay data signal path between the telephone line  32   b  and the telephone line  32  such that the ordinary analog telephone  102  is virtually connected to the wall jack  31 . Then, users can use the switch  270  to switch the full-duplex speakerphone adapter  300  to the hands-free speakerphone mode for hands-free conversation. 
     During an outbound call, the user takes the ordinary analog telephone  102  off hook. The hybrid interface circuit  130  detects the off-hook condition of the analog telephone  102  and notifies the telephone control logic unit  350 , which in turn controls the hybrid interface circuit  130  to enter into an off-hook condition on the telephone line  32   b . The telephone control logic unit  350  then establishes a relay data signal path between the telephone line  32   b  and the telephone line  32  such that the ordinary analog telephone  102  is virtually connected to the wall jack  31 . The dial tone from the central office  30  comes to the wall jack  31  and is relayed to the ordinary analog telephone  102 . Users can dial the outbound digits on the ordinary analog telephone  102 . Once dialing is completed, users can use the switch  270  to switch the full-duplex speakerphone adapter  300  to the hands-free speakerphone mode for hands-free conversation. The switch  270  comprises two switching positions S and T respectively for a hands-free speakerphone mode and an ordinary analog telephone mode. 
     As shown in  FIG. 3A , the telephone signal processing unit  370  comprises a full-duplex processing unit  170 , a telephone control logic unit  350 , a line echo canceller  165  and the switch  270 . The telephone signal processing unit  370  can process signal data in digital domain. The telephone signal processing unit  370  can be implemented entirely in hardware, but is typically implemented as algorithms and control software running on a digital programmable processor. The programmable processor can also execute additional telephone signal processing algorithms or user interface controls. The processor itself could be a digital signal processor (DSP) device, a reduced instruction set computer (RISC) device, or any other programmable processing device capable of handling the tasks. 
       FIG. 3B  shows a block diagram of the effective data path inside the full-duplex speakerphone adapter  300  when the switch  270  switches to the position S for a hands-free speakerphone mode according to another embodiment of the invention. The voice data flows between the telephone hybrid interface circuit  130  and the microphone  50  and loudspeaker  51 . The user speech signal is picked up by the microphone  50  and then amplified by a pre-amplifier  180  before going into an analog input of the CODEC  141 . The CODEC  141  outputs audio signals from its analog output to the amplifier  181  that drives the loudspeaker  51  to produce a loud enough audio for the hands-free speakerphone mode. When the switch  270  switches to the hands-free speakerphone mode, the full-duplex speakerphone adapter  300  functions as a full-duplex speakerphone connected through the telephone line  32   b  to the wall jack  31 . 
     Noted that the digital signal path is established inside of the full-duplex processing unit  170  and the analog signal path is established outside of the full-duplex processing unit  170 . The full-duplex processing unit  170  comprises a line echo canceller  160  and an acoustic echo canceller  150 . The acoustic echo canceller  150  receives/transmits digital signals from/to the CODEC  141  and the line echo canceller  160 . The line echo canceller  160  also receives/transmits digital signals from/to the acoustic echo canceller  150  and the CODEC  140 . The line echo canceller  160  can remove line echo from the telephone hybrid interface circuit  130 . The acoustic echo canceller  150  removes acoustic echo from a loop through a loudspeaker  51  and a microphone  50 . 
       FIG. 3C  shows a block diagram of the effective data path inside the full-duplex speakerphone adapter  300  when the switch  270  switches to the position T for ordinary analog telephone mode according to another embodiment of the invention. The voice data flows between the telephone hybrid interface circuit  130  and the subscriber loop interface circuit  132 . The full-duplex speakerphone adapter  300  acts as a data path relay between the ordinary analog telephone  102  to the wall jack  31 . 
     The line echo canceller  160  removes line echo at the telephone hybrid interface circuit  130  and is coupled to the line echo canceller  165 . The line echo canceller  165  can attenuate line echo return signals generated at the subscriber loop interface circuit  132  which is coupled to the ordinary analog telephone  102  such that the line echo can not be transmitted to and heard by the far-end talker. 
       FIG. 4  shows a simplified block diagram of the full-duplex processing unit  170  of the full-duplex speakerphone adapter  300 . The line echo canceller  160  cancels the line echo signal E 1  caused by the telephone hybrid interface circuit  130  and the acoustic echo canceller  150  cancels the acoustic echo signal E 2  coupled from the loudspeaker  51  output to the microphone  50 . The feedback loop in the hands-free speakerphone mode is a signal loop path formed by the residual acoustic echo signal E A , the residual line echo signal E L , the line echo signal E 1  and the acoustic echo signal E 2 . Because the microphone  50  and the loudspeaker  51  are placed in the proximity of each other, when the microphone pre-amplifier  180  and the loudspeaker amplifier  181  provide sufficient gain for facilitating the hands-free speakerphone mode, the total feedback loop gain is likely to exceed the stability threshold and cause howling or singing. The feedback loop stability can be achieved by both the acoustic echo cancellation and the line echo cancellation working in conjunction with each other. 
     Both the acoustic echo canceller  150  and line echo canceller  160  have linear adaptive filters that adapt to the echo path impulse response such that, after adaptation, each will become effective in injecting attenuation against the echo signal at their respective echo removal node  151  and node  161 . If the residual line echo signal E L  after the line echo cancellation echo removal node  161  is small compared to that before entering node  161 , signal attenuation is effectively injected in the feedback loop by the line echo canceller  160 . Similarly, if the residual acoustic echo signal E A  after the acoustic echo cancellation node  151  is small compared to that before entering node  151 , signal attenuation is effectively injected in the feedback loop by the acoustic echo canceller  150 . In this manner, the feedback loop stability is achieved by the sum of the feedback loop signal attenuation achieved by the acoustic echo cancellation and the line echo cancellation in the full duplex processing module  170 . 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited to thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.