Abstract:
A key telephone system comprising a terminal device and a main apparatus which exchanges speech signals with the terminal device, the terminal device having a loudspeaking circuit for providing a predetermined amount of loss to either one of first speech signals selected to be transmitted to the main apparatus and second speech signals received from the main apparatus to prevent howling in a loudspeaking conversation mode, said system comprising: a first communication unit provided in the terminal device, for sending the first speech signals to the main apparatus through a first channel and receiving the second speech signals from the main apparatus through a second channel separated from the first channel; second communication unit provided in the main apparatus and connected to the first communication unit, for receiving the first speech signals from the first communication unit through the first channel and sending the second speech signals to the first communication unit through the second channel; in the first and second communication unit side tones between the first and second speech signals not being generated, whereby the predetermined amount of the loss caused by the loudspeaking circuit can be set to less than a loss which may be required for howling prevention if the side tones are generated.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to improvement of the loudspeaking function of key telephone system. 
     FIG. 1 shows configuration of a key telephone according to the prior art. A block designated reference number 1 is a main apparatus which is connected to a plurality of key telephones 2 1  - 2 n  through two-wire speech signal lines 3 1  -3 n  and through two-wire control signal lines 4 1  -4 n . In the main apparatus, the speech signal lines 3 1  -3 n  are connected through speech signal transformers 5 1  -5 n , two/four wire conversion circuits 6 1  -6 n  and CODECs 7 1  -7 n  to a time switch 8. Also, the control signal lines 4 1  -4 n  are connected through control signal transformers 9 1  -9 n  and two/four wire conversion circuits 10 1  -10 n  to a control circuit 11. The time switch 8 performs speech pat switching under control of the control circuit 11. 
     In the key telephone 2, the speech signal line 3 is connected through a speech signal transformer 12 and a two/four wire conversion circuit 13 to mode selector switches 14 and 15. The mode selector switches 14 and 15 select a handset conversation mode or a loudspeaking conversation mode. When contacts a of the switches are on, the handset conversation mode is selected. In the handset conversation mode, a handset 16 is connected to the two/four wire conversion circuit 13 through a received speech signal amplifier 17 and a transmitted speech signal amplifier 18. On the other hand, in the loudspeaking conversation mode in which contacts b are on, a loudspeaking circuit 19 is connected to the two/four wire conversion circuit 13. Received speech signals from the two/four wire conversion circuit 13 are sent through a received voice switching circuit 20 and a speaker amplifier 21 to a speaker 22, and transmitted speech signals from a microphone 23 are sent through a microphone amplifier 24 and a transmitted voice switching circuit 25 to the two/four wire conversion circuit 13. At the same time, the levels of the received speech signals and transmitted speech signals are detected with a received speech detector 26 and a transmitted speech detector 27, respectively, and then supplied to a comparator 28. The comparator 28 compares the received speech signal level, the transmitted speech signal level and a voltage level from a voltage setter 29, and according to the comparison results of the three levels, reciprocally changes losses of the received voice switching circuit 20 and the transmitted voice switching circuit 25. As a result, a loop gain of a closed loop formed by acoustic coupling between the speaker 22 and the microphone 23 is controlled to be less than 1, and howling (oscillation) can be prevented. 
     One of the problems in the prior-art key telephone system is that the losses of the received voice switching circuit 20 and the transmitted voice switching circuit 25 in the loudspeaking conversation mode must be set to be greater to prevent the howling because there are a plurality of closed loops which may generate howling. Assume that, for example, in FIG. 1, conversation is held between the key telephone 2 1  in the loudspeaking mode and another separate key telephone 2 n . When the separate telephone 2 n  is in the handset conversation mode, there are two closed loops which may generate howling. One of the loops is formed by the following processes: the transmitted speech signal from the microphone 23 1  is sent to the two/four wire conversion circuit 13 1  and a part of the sent signal is mixed as a side tone with the received speech signal, and this side tone is sent to the speaker 22 1 , then a part of the side tone is supplied again to the microphone 23 1  owing to acoustic coupling. The other loop is formed through the following processes: the transmitted speech signal from the microphone 23 1  is sent to the two/four wire conversion circuit 6 n  in the main apparatus and a part of the sent signal is mixed as a side tone with the transmitted speech signal from the telephone 2 n , and this side tone is returned to the telephone 2 n , and then a part of the side tone is fed from the speaker 22 1  to the microphone 23 1  again. When the accompanying telephone 2 n  is also in the loudspeaking conversation mode, further another closed loop is added. The loop is formed through the following processes: the transmitted speech signal from the microphone 23 1  is sent to the speaker 22 n  and a part of the sent signals is supplied to the microphone 23 n  owing to acoustic coupling; the signals supplied to the microphone 23 n  are sent to the speaker 22 1 , then a part of the sent signals is supplied again to the microphone 23 1 . 
     Because there may be two or more such closed loops as described above, the losses of the voice switching circuits 20 and 25 must be set so large that each loop gain does not exceed 1. As a result, a high voice level is required for switching between the speech signal transmission state (where the loss of the received voice switching circuit 20 is large and the loss of the transmitted voice switching circuit 25 is small) and the speech signal reception state (where the loss of the received voice switching circuit 20 is small and the loss of the transmitted voice switching circuit 25 is large), thus causing the problem of difficulty in transmitting voices mutually. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to enable loudspeaking conversation similar to natural conversation by reducing losses of the voice switching circuits. 
     The invention provides a key telephone system comprising a terminal device and a main apparatus which exchanges speech signals with the terminal device, the terminal device having a loudspeaking circuit for providing a predetermined amount of loss to one of first speech signals selected to be transmitted to the main apparatus and second speech signals received from the main apparatus to prevent howling in a loudspeaking conversation mode, said system comprising: 
     a first communication means provided in the terminal device, for sending the first speech signals to the main apparatus through a first channel and receiving the second speech signals from the main apparatus through a second channel separated from the first channel; 
     second communication means provided in the main apparatus and connected to the first communication means, for receiving the first speech signals from the first communication means through the first channel and sending the second speech signals to the first communication means through the second channel; 
     in the first and second communication means side tones between the first and second speech signals not being generated, 
     whereby the predetermined amount of the loss caused by the loudspeaking circuit can be set to less than a loss which may be required for howling prevention if the side tones are generated. 
     Between the main apparatus and the terminal device, the first speech signals from the terminal device and the second speech signals from the main apparatus are transmitted through channels separated from each other, thereby eliminating the two/four wire conversion circuit as in prior-art system which generates a side tone, without the closed loop which may cause howling due to the side tone. Therefore, the losses for the speech signals to prevent howling in the loudspeaking conversation mode can be set relatively small. Setting relatively small losses means that switching between the speech signal transmission state and the speech signal reception state can be performed at relatively low speech signal levels. Consequently, more natural conversation, that is, loudspeaking conversation close to simultaneous two-way conversation is realized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of configuration of the prior art, 
     FIG. 2 is a block diagram of a configuration showing a preferred embodiment of the key telephone system according to the invention, and 
     FIG. 3 shows comparison of switching characteristics of the speech signal transmission state and the speech signal reception state of the embodiment shown in FIG. 2 and the example of prior art in FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 2 shows a block diagram of configuration of a preferred embodiment according to the invention. In this figure, a block 41 is a main apparatus which is connected to a plurality of key telephones 42 1  -42 n  through two-wire digital transmission lines 43 1  -43 n . 
     In the main apparatus, the transmission lines 43 1  -43 n  are connected through digital signal transformers 44 1  -44 n  to digital communication circuit 45 1  -45 n , respectively. The digital communication circuit 45 1  -45 n  are connected to a time switch 46 which performs speech path switching and a control circuit 47 which controls the speech path switching, and exchange digital speech signals with the time switch 46 and digital control signals with the control circuit 47. 
     In a key telephone 42 1 , by way of example, the transmission line 43 1  is connected through a digital signal transformer 48 1  to a digital communication circuit 49 1 . The digital communication circuit 49 1  sends to and receives from the digital communication circuit 45 1  in the main apparatus, using a kind of half-duplex communication system called &#34;Ping-Pong transmission&#34;, with the digital speech signals and the digital control signals being incorporated into bursts in the form of B+D (B is a speech signal channel and D a control signal channel). In other words, between the key telephone 42 1  and the main apparatus 41, the transmitted speech signal (the speech signal transmitted from the key telephone 42 1  to the main apparatus 41) and the received speech signal(the signal transmitted from the main apparatus 41 to the key telephone 42 1 ) are transmitted over time-separated channels. The digital communication circuit 49 1  is connected to a CODEC 50 1  and a control circuit 51 1 , and exchanges digital speech signals with the CODEC 50 1  and the digital control signals with the control circuit 51 1 . 
     The CODEC 50 1  is connected to mode selector switches 52 1  and 53 1 , receives the digital speech signals from the digital communication circuit 49 1 , converts the digital signals to analog speech signals and sends the analog signals to the mode selector switch 52 1 , and receives analog speech signals from the mode selector switch 53 1 , converts the analog signals to digital speech signals and sends the digital signals to the digital communication circuit 49 1 . 
     The control circuit 51 1  performs not only switching of the mode selector switches 52 1  and 53 1  in response to the control signals from the digital communication circuit 49 1 , hook signals from a hook switch 54 1 , pressed-key information from a key pad 55 1 , etc., but also performs various control operations, including control of output signal levels of a comparator 68 1 , which is described later. 
     The mode selector switches 52 1  and 53 1  selects the handset conversation mode and the loudspeaking conversation mode. FIG. 2 shows the handset conversation mode. In the handset conversation mode, a handset 56 1  is connected through a received speech signal amplifier 57 1  and a transmitted speech signal amplifier 58 1  to the CODEC 50 1 . On the other hand, in the loudspeaking conversation mode, in which the selector switches 52 1  and 53 1  are positioned reversely to the figure, a loudspeaking conversation circuit 59 1  is connected to the CODEC 50 1 . In this mode, received speech signals from the CODEC 50 1  are fed through a received voice switching circuit 60 1  and a speaker amplifier 61 1  to a speaker, and transmitted speech signals from a microphone 63 1  are fed through a microphone amplifier 64 1  and a transmitted voice switching circuit 65 1  to the CODEC 50 1 . At the same time, voice levels of the received speech signals and the transmitted speech signals are detected by a received speech signal detector 66 1  and a transmitted speech signal detector 67 1 , respectively, and fed to the comparator 68 1 . The comparator 68 1  compares the received speech signal level, the transmitted speech signal level and a voltage level out of a voltage setting circuit 69, and according to the comparison results of the three levels, supplies signals of logical value 1 or 0. The output signals reciprocally control losses of the received voice switching circuit 60 1  and transmitted voice switching circuit 65 1 . In other words, when the logical value of the output signal described above is 1, for example, a speech signal reception state is set so that the loss of the received voice switching circuit 60 1  is small and that the loss of the transmitted voice switching circuit 65 1  is large, and on the contrary, when the above logical value is 0, a speech signal transmission state is set so that the loss of the received voice switching circuit 60 1  is large and that the loss of the transmitted voice switching circuit 65 1  is small. Actual losses in the speech signal reception state and the speech signal transmission state depend on the actual levels of output signals corresponding to the logical values 0 and 1 from the comparator. The actual signal levels of the logical values 0 and 1 are controlled by the control circuit 51 1 . 
     The control of the losses will be described in detail below. Assume that conversation is held between the key telephone 42 1  in the loudspeaking conversation mode and another separate telephone 42 n . When the separate key telephone 42 n  is in the handset conversation mode, a hook signal specifying the handset conversation mode is fed from a hook switch 54 n  to a control circuit 51 n , which then sends out a digital control signal specifying the handset conversation mode to the digital communication circuit 49 n . The digital control signal is fed from the digital communication circuit 49 n  through the digital communication circuit 45 n , the control circuit 47 and the digital communication circuit 45 1  in the main apparatus to the digital communication circuit 49 1  in the key telephone 42 1 , and then received by the control circuit 51 1 . When the control circuit 51 1  receives the control signal specifying the handset conversation mode of the other telephone, the control circuit 51 1  controls levels of output signals of the comparator 68 1  so that the loss of the received voice switching circuit 60 1  in the speech signal transmission state and the loss of the transmitted voice switching circuit 65 1  in the speech signal reception state become predetermined first values. 
     On the other hand, when the other key telephone 42 n  is also in the loudspeaking conversation mode, the digital control signal specifying the loudspeaking conversation mode is fed from the control circuit 51 n  in the key telephone 42 n  through the same path described above to the control circuit 51 1  in the key telephone 42 1 . When the control circuit 51 1  receives the control signal specifying the loudspeaking conversation mode, the control circuit 51 1  controls levels of output signals of the comparator 68 1  so that the loss of the received-voice switching circuit 60 1  in the speech signal transmission state and the loss of the transmitted-voice switching circuit 65 1  in the speech signal reception state become the predetermined second values. 
     The first and second values of the losses are such values that, in each case where the other key telephone 42 n  with which the loudspeaking is held is in the handset or loudspeaking conversation mode, loop gains of all closed loops which may generate howling are set to less than 1. This embodiment uses, as a communication between the main apparatus 41 and the key telephones 42, half-duplex communication in which the transmitted speech signal channel and the received speech signal channel are clearly separated, eliminates the two/four wire conversion circuits, which have been required in the prior-art system, and has no closed loop attributed to side tones by the two/four wire conversion circuits. Consequently, when the other telephone 42 n  is in the handset conversation mode, there is merely a closed loop attributed to the acoustic coupling between the speaker 62 1  and microphone 63 1  or of the handset 56 n . This loop enables the loop gain to be less than 1 with very small losses because the acoustic coupling of the handset 56 n  is very modest. Also, when the other telephone 42 n  is in the loudspeaking conversation mode, there is a closed loop attributed to the acoustic coupling between the speaker 62 1  and the microphone 63 1  and between the speaker 62 n  and the microphone 63.sub. n. However, there are additional two closed loops attributed to the side tones in the two/four wire conversion circuits like the prior-art system. Therefore, the first value of the loss can be set to much smaller than that in the prior-art system, and the second value of the loss can be set greater than the first value but considerably smaller than that in the prior-art system. 
     Thus, when the losses of the received-voice switching circuit 60 1  and the transmitted-voice switching circuit 65 1  are set to relatively small values, the voice levels required for switching of the speech signal reception state and the speech signal transmission state are lowered, and conversation similar to simultaneous two-way conversation is realized. This will be described in details with reference to FIG. 3. FIG. 3 shows switching characteristics of the speech signal reception state and the speech signal transmission state of the embodiment in FIG. 2 in comparison with those of the prior-art system in FIG. 1. The horizontal axis represents levels of the received speech signals fed to the received-voice switching circuits and the vertical axis levels of the transmitted speech signals fed from the microphones. 
     Lines L 1  and L 2  are, respectively, a switching threshold line from the speech signal reception state to the speech signal transmission state and a switching threshold line from the speech signal transmission state to the speech signal reception state in the prior-art system. In the prior-art system, if the transmitted speech signal level is X and the received speech signal level is 0 (point A), then the speech signal transmission state is set. That is, the loss of the received-voice switching circuit is relatively large (for example, Z) and the loss of the transmitted-voice switching circuit is relatively small (for example O). In this state, when the received speech signal level increases and reaches a level X+Z (point B), which equals to the loss Z of the received-voice switching circuit plus the transmitted speech signal level X, the speech signal transmission state turns into the speech signal reception state, and the loss of the received-voice switching circuit becomes relatively small (for example, O) and the loss of the transmitted-voice switching circuit becomes relatively large (for example, Z). After that, when the received speech signal level decreases and equals to the level X (point C), the speech signal reception state returns to the speech signal transmission state again. In this way, the switching from the speech signal transmission state to the speech signal reception state does not occur unless the received speech signal level is considerably larger than the transmitted speech signal level, that is, unless the other conversation party speaks very loudly. In contrast, in the embodiment, the threshold line of switching from the speech signal transmission state to the speech signal reception state is L 3  when the loss is set to a first value (for example, y 1 ) which is considerably smaller than that of the prior-art system, and the line is L 4  when the loss is set to a second value (for example, y 2 ). Consequently, assuming the initial condition described above (point A), the switching from the speech signal transmission state to the speech signal reception state is performed when the received speech signal level increases and reaches a level (point D or E) which equals the relatively small loss y 1  or y 2  plus the transmitted speech signal level X. 
     In other words, the switching from the speech signal transmission state to the speech signal reception state is performed with the received speech signal level which is the decrease of loss Z-y 1  or Z-y 2  lower than that of the prior-art system. 
     The operation described above is a case where the characteristics of the comparator are specified so that the speech signal transmission state is set in silent state in absence of the received speech signals and the transmitted speech signals. When, on the contrary, the speech signal reception state is set in the silent state, the low transmitted speech signal level which is reduced by a decrease of the loss turns the speech signal reception state into the speech signal transmission state. Consequently, the switching between the speech signal transmission state and the speech signal reception state is smoothly performed in loudspeaking conversation without particularly loud voices, and conversation similar to simultaneous two-way conversation is enabled. 
     The invention is not limited to the embodiment described above. For example, the communication system between the main apparatus and the key telephones can use another separate communication systems such as full duplex communications (channels are separated spatially) so long as channels of the received speech signals and the transmitted speech signals are separated. However, the embodiment using half duplex communications is more practical because half duplex communication can use two-wire transmission lines, and that communications between the main apparatus and the key telephones do not require such a high transmission rate that full duplex communication may provide. Though speech signals and control signals can also be transmitted through separate lines, such a system as described in the embodiment in which the speech signals and the control signals are incorporated into a single burst and transmitted has the advantage of a single transmission line in communications between a main apparatus and key telephones, in which such a high transmission rate is not required.