Field of the Invention:

The present invention relates to a voice switching control for use in a bidirectional voice-communication system including loud-speaking telephone instruments or interphone instruments or the like, and more particularly to a control system for controlling audio signals on voice transmission lines between such instruments.

Background Art:

As is known in the art, a loud-speaking telephone instrument has a microphone for transmitting voice signals and a loud-speaker for receiving voice signals transmitted from the microphone of another loud-speaking telephone instrument.

The loud-speaker is capable of emitting a relatively large amount of sound energy, much more than the amount of sound energy emitted by an earphone of a conventional handheld telephone instrument. The loud-speaker and the microphone of a loud-speaking or hand-free telephone instrument are assembled in a relativly close relationship within the same casing or cabinet of the instrument. Inevitably, the loud- speaker is acoustically coupled with the microphone in a strong manner. An interphone instrument also has a pair of electric/acoustic and acoustic/electric converters which are acoustically coupled with each other.

The acoustic coupling between a pair of converters (loud-speaker and microphone) of hand-free telephone instrument and/or interphone apparatus is apt to produce a number of adverse effects on the bidirectional voice-communication system with a pair of hand-free telephone or interphone instruments.

For example, assuming that a first microphone of a first hand-free telephone instrument is connected via a voice transmission line or channel to a second loud-speaker of second hand-free telephone instrument and that a second mirco- phone of the second telephone instrument is connected via another voice transmission line to a first loud-speaker of the first telephone instrument, an acoustic energy inputted to the first microphone is fed back to itself, because a closed loop is formed owing to the acoustic coupling between the loud-speaker and the corresponding microphone. The level of fed-back signal often becomes high enough to produce a howling.

With a view that a relatively strong acoustic coupling between the loud-speaker and the microphone is inevitable, one and probably the only technique to completely eliminate the occurrence of howling is to provide control means for disconnecting or substantially disconnecting one or more voice transmission channels so that no closed loop is formed during operation-,. thus making a selected voice transmission channel which is required to transmit a voice message.

Such control means or system is called "voice switching control system" which is the subject matter of the present invention.

A typical prior voice switching control system includes a pair of voice switches or variolossers. One of the voice switches is disposed in a first voice transmission line or channel between a first microphone of the first telephone or interphone instrument or station and a second loud-speaker of the second station. The other of the voice switches is disposed in a second voice transmission line between a second microphone of the second station and a first loud-speaker of the first station. In operation, a pair of voice transmission lines is energized in a complementary manner by the switching of the two voice switches which introduces two different attenuation (loss) factors in voice signals from associated microphones. Operation mode ("ON or OFF state") of the voice switches is controlled by a control circuit which monitors a pair of outputs from first and second microphones, i.e. the voice signal conditions in the first and second voice transmission lines ahead of the voice switches. The two outputs are passed through wave shaping circuits to be compared by a comparator. Whenever the level of first microphone output voice signal in the first voice transmission line is greater than the level of second microphone output voice signal in the second voice transmission line, the comparator controls the first voice switch to remain "ON" and controls the second voice switch to remain "OFF", and vice versa. Accordingly, no closed loop is formed during operation. However, owing to the strong acoustic- loudspeaker/microphone-coupling and since it is necessary to reproduce a high level of sound at a loud-speaker, the level of output from the microphone of the station which is receiving a voice message from other station becomes comparative to and sometimes greater than the level of output from the other microphone of the other station. If this occurs, a message cannot be transmitted. Therefore, the control circuit usually includes a pair of auxiliary variolossers each of which is disposed in the connecting line between associated voice signal (output of microphone) pick-up point on the transmission line and associated wave shaping circuit. The comparator controls the auxiliary variolossers in a similar manner as it controls -associated main variolossers or voice switches on transmission lines thereby to eliminate malfunction as described.

However. the combination of main and auxiliary variolossers makes it difficult or impossible to interrupt a message from a first station and to insert a new message from a second station to be transmitted to the first station.

Without the strong acoustic coupling between a loud- speaker and a microphone, there would be nothing to it. Therefore, the above disadvantage in the prior system is another consequence derived from the inevitable existance of a strong acoustic coupling.

Disclosed in the Japanese Patent Publication No. 22281/ 1971 is a voice switching control system which also controls a pair of voice switches in transmission lines by sensing the outputs of microphones as described. However, instead of using a pair of auxiliary variolossers as described above, the system employs four sensed signal amplifiers, two of which have different gains from the other two, disposed in sensed signal branches, and a pair of comparators outputs of which are connected to a second-stage comparator for controlling the voice switches, the second-stage comparator being comprised of a pair of cross-coupled NAND gates. This arrangement ensures that the first and second voice switches are operated in an exclusive 'OR' (complementary) manner. However, while a talker at a first station is "continuously or incessantly" speaking to the first microphone, it is very difficult for an other person at a second station to interrupt the speech and to insert a new message which he wishes to transmit to the first station. As a conclusion, this system also has a defect similar to that of the first-mentioned prior system.

Disclosed in a paper entitled "On the study of Voice Switching-Control" by J. Takahashi et al, in Proceedings of Acoustic Engineering Society of Japan, pages 485-486; Oct. 1973 is a voice switching control system which senses the levels of voice signals at four points, namely two sets of a pair of output and input, from and to the microphones and the loud- speakers of first and second stations.

The input to the loud-speaker and the output from the microphone of each station are compared by a comparator. The pair of output signals obtained from the two comparators is applied to a logic circuit which three-position controls a pair of variolossers. A first position defines an "ON or no attenuation state", third position, "OFF or full attenuation state" and second or intermediate position "some attenuation (half the full attenuation) state".

More specifically, the comparator produces a high level signal to the logic circuit only when the level of output from the microphone (first input to the comparator) is greater than the level of input to the loud-speaker (second input to the comparator) by a predetermined value, and otherwise produces a low level signal. While the first comparator for comparing the microphone output and loud-speaker input of the first station is producing a high level signal and the second comparator for comparing the microphone output and loud-speaker input of the second station is producing a low level signal (namely, while only the user at the first station speaks to the microphone of the first station), the logic circuit controls the first variolosser to be placed in 'first (no attenuation) position' to transmit a voice message from the first station to the second, and controls the second variolosser to be placed in 'third (full attenuation) position'. While both the first and second-comparators are producing high signals (namely, while both the users at first and second stations speak to the microphones at an appropriate sound level), the logic circuit controls both of the first and second variolossers to be placed in 'second (half the full attenuation) position'.

Such arrangement and control makes it easy to insert a new voice message by the user at one station while the other user at the-other station is transmitting a message because the comparator is arranged to compare the output from microphone with the input to the loud-speaker of the same station.

However, the inserted new voice message is transmitted to the other station with a relatively restricted level of sound during the transient, simultaneous or full duplex voice .communication mode of operation, because both of the variolossers introduce half the full attenuation in the voice signals in this mode. Further, a closed loop for voice signals is formed. In order to avoid howling, the overall gain for the closed loop must be less than the value of gain defining a critical oscillation condition. These restrictions on the' amplitude of voice signals during such transient, simultaneous voice communication make it difficult to obtain a sufficiently high level of sound at the output of loud-speaker. Consequently, the listener often cannot perceive or hear the voice information clearly.

A delay element comprising a wave shaping circuit is interposed between the voice signal pick-up point and the control input to the comparator. The delay element delays the switching of variolosser. Accordingly the beginning portion of voice message is not transmitted from one station to the other.

In general, it is desirable that voice switching control systems for controlling loud-speaking stations satisfy the following requirements:

• (1) no howling,
• (2) a high level of sound at the output of loud-speaker of station,
• (3) easy to interrupt the voice message from the other station and to insert and transmit a new message from one station to the other, and
• (4) little or no risk of malfunction due to ambient noises around the station.

However, each of the above reqirements conflicts with one or more of the others to some extent as discussed. All the prior systems represent a compromise between these rather conflicting requirements, and leave room for improvement.

It is highly desirable that the voice switching control system be applicable to a telephone conference communication system including at least three loud-speaking stations by which a conference is held.

Disclosure of the Invention:

With this background of the invention in mind it is therefore an object of the present invention to provide an improved voice switching control system which satisfies the requirements as described above to a maximum degree.

It is another object of the present invention to provide a voice switching control system which is applicable to a telephone conference communication system including at least three stations for loud-speaking by which a conference is held.

It is still another object of the present invention to provide a voice switching control system wherein a message from another station can be surely interrupted by the insertion of a new message from one station and the newly inserted message is transmitted to the other station (or the other stations) without attenuation or loss of the message signal, and without introducing the switching delay of voice switch.

In accordance with the present invention, a voice switching control system for controlling voice signals for transmission between stations each including a microphone and a loud-speaker comprises: at least two voice switches disposed in different voice signal transmission lines or channels respectively; each of the transmission lines being operatively connected between the microphone of one of the stations and the loud-speakers of the other stations; means for sensing each output voice signal of the microphones and each input of the loud-speakers of the stations; comparators each for comparing the output of the microphone of one of the stations with the input voice signal of the loud-speaker of the same station; each one thereof operatively producing a first signal only when the output voice signal of the associated microphone is greater in amplitude than the input voice signal of the associated loud-speaker and operatively producing a second signal in the other voice signal condition; logic circuits each for controlling the associated voice switch at a selected one of two positions, a first position defining an ON state of the voice switch and a second position defining an OFF state of the voice switch; each one of the logic circuits having a first control input for receiving the output of the associated comparator and a second control input for receiving the outputs of the other comparators; and, each one thereof being .set to control the associated voice switch at its first position upon the receipt, by the first control input, of the first signal from the associated comparator and the receipt, by the second control input, of the second signal from any one of the other comparators, each one of the logic circuits being reset to control the associated voice switch at its second position upon the receipt, by the first control input, of the second signal from the associated comparator and the receipt, by the second control input, of the first signal from any one of the other comparators, each one of the logic circuits being inverted to control the associated voice switch so as to invert the position thereof upon the receipt, by the first control input, of the first signal from the associated comparator and the receipt, by the second control input, of the first signal from any one of the other comparators, whereby a voice message newly inserted into a microphone of any one of the stations serves to interrupt another voice message which has been transmitted to the loud-speaker associated with the microphone, and the newly inserted message is transmitted to any other station without attenuation of voice signal introduced by the associated voice signal switch.

Preferably, each one of the logic circuits may comprise a conventional J-K Flip-Flop. The low cost and simple design of the Flip-Flop helps to lower the total cost and to simplify the construction of the voice switching system.

For a voice switching control system for controlling a pair of voice signals transmitted between a pair of the stations, a single (and common) J-K Flip-Flop for controlling a pair of the voice switches may preferably be used instead of two J-K Flip-Flops for controlling one and the other of the two voice switches, respectively. More specifically, the first control input (J input) of the single J-K Flip-Flop is connected to the first comparator for comparing the input of the".first station with its output while the second control input (K input) is connected to the second comparator for comparing the input and output of the second station. Q output of the J-K Flip-Flop is connected to the first voice switch while Q output of the J-K Flip-Flop is connected to the second voice switch.

The sensing means or voice signal pick-up means includes amplifying means for amplifying voice signals and rectifying means for rectifying the amplified signals.

Thus, each output voice signal of each microphone and each input voice signal of each loudspeaker is amplified and rectified. Each rectified signal representing the instantaneous amplitude of input voice signal to the loudspeaker is further smoothed by a smoothing circuit in order to reduce the risk of malfunction of the comparator due to the temporary increase or decrease in amplitude of the input voice signal to the loud-speaker. However, each rectified signal representing the amplitude of output voice signal of the microphone is directly connected to the comparator portion in order to rapidly activate the comparator to turn on the associated voice switch. With such asymmetric arrangement, the beginning portion of the voice message is clearly transferred from the station into which the voice message is inserted to the other stations. The beginning or head portion of voice message is regarded as the most important segment of information because, it usually is a critical factor for listeners in determining their comprehension of the voice message.

The comparator portion of the voice switching control system includes a substracting element for substracting the amplified, rectified and smoothed signal Vi (input signal to the loud-speaker) from the amplified and rectified but not smoothed signal Vo (output signal from the microphone of the same station). The substracted signal (Vo-Vi) is then compared with a preselected reference voltage signal by a comparating circuit which produces a logic '1' signal when the substracted signal is greater than the reference or threshold voltage and produces a logic 'O' signal when the substracted voltage is less than the threshold voltage.

Brief Description of the Drawings:

The above-mentioned and other features of the present invention will become more apparent by reference to the following description together with the accompanying drawings, in which:

• Fig. 1 is a block diagram of a voice switching control system for controlling two voice switches in accordance with the present invention;
• Fig. 2 is a detailed diagram of a preferred embodiment of a voice switching control system for controlling three voice switches;
• Fig. 3 is a detailed diagram of a preferred embodiment of a voice switching control system for controlling two voice signals.

Description of the Preferred Embodiments:

Referring now to Fig. 1, a voice switching control system in accordance with the present invention is shown and generally designated by VSC. In this embodiment, the control system comprises two voice switching control units VSC-1 and VSC-2 and is arranged to control two voice switches SW-1 and SW-2 in the voice transmission channels between two stations ST-1 and ST-2. However, it is noted that the number "two" is only exemplary and that the present invention is equally and easily applicable to a voice switching system for controlling three or more different voice signals between three or more stations, which will be apparent hereinafter.

Each station has a microphone (e.g. MC-1) for transmission of voice signals and a loud-speaker (e.g. SP-1) for receiving voice signals from another station. As described above, the acoustic output of the loud-speaker is relatively great during normal loud-speaking communication. Due to the closely spaced relationship of the loud-speaker and the microphone, some of a relatively large amount of acoustic energy from the loud-speaker is coupled with the microphone as shown by the dotted line a-1 or a-2.

A voice switch (e.g. SW-1) is disposed in the voice signal transmission line or channel between the microphone (e.g. MC-1) of one of the stations and the loud-speaker (e.g. SP-2) of the other station and is operated to selectively activate the voice transmission line to transmit voice signals from one station to the other.

Each voice switching control unit (e.g. VSC-1) senses or picks up the voice signal output and input of the associated station (e.g. ST-1) at a point (0-1) ahead of the associated voice switch (SW-1) and at a point (I-1) on an input line to the loud-speaker of the station, respectively.

A comparator (eg. C-1) compares a pair of sensed signals. When the output of the microphone is greater in amplitude than the input to the loud-speaker of the same station, the comparator produces a first signal (e.g. 'High' level signal) and when the onput is less than the input, the comparator produces a second signal (e.g. 'Low' level signal). The output of the comparator (C-l) is connected to a first control input (J) of associated logic circuit (L-l) and is connected to a second control input (K) of the other logic circuit (L-2). When applied to a control system for controlling more than two voice switches, the output of the comparator is connected to the other logic circuits. For such multi-control system the number of logic circuits is equal to the number of voice switches. However, for a two-voice-switches control system, a single logic element may be used. Such preferred embodiment will be described in more detail hereinafter.

Each logic circuit (e.g. L-1) controls its associated voice switch (SW-1) at a selected one of the two positions, a first position defining an effectivly or practically 'ON' state of the voice switch and a second position defining an effectively 'OFF' state of the voice switch.

The logic circuit (L-l) is operated as follows:

• (1) it is set to control associated voice switch (SW-1) to turn on upon the receipt of first (logic '1') signal from associated comparator (C-l) and the receipt of second (logic 'O') signal from the other comparator (C-2); it is reset to control the voice switch to turn off upon the receipt of second signal from associated comparator and the receipt of first signal from the other comparator; and it is inverted to change the position or state of the voice switch upon the receipt of first signals from both the associated comparator and the other comparator.

A typical operation of the system is as follows:

• Assuming that while no effective level voice signal exists on the transmission lines, and either of the comparators is producing a second signal, and each logic circuit is receiving the second signal at either of its control inputs, the logic circuit L-1 is applying an 'OFF' signal to the voice switch SW-1 and logic circuit L-2 is applying an 'ON' signal to the voice switch SW-2, then the switch SW-1 is 'ON' but the switch SW-2 is 'OFF', however, the initial condition of the voice switches is not important.

For simplification and clarification of description, the term 'first signal' produced by the comparator will be hereinafter referred to as a logic '1', and the term 'second signal' as a logic 'O'.

Under such condition, i.e. SW-1 is OFF and SW-2 is ON, when a user at one of the stations, for example, the station ST-1 starts to insert a voice message into the microphone MC-1, the output of the microphone increases to become greater than the input to the loud-speaker SP-1. Then, the comparator C-1 produces a logic '1' signal which is applied to the logic circuit L-1 which is, in turn, set to turn on the voice switch SW-1 thereby to transmit the voice message from the station ST-1 to the other station ST-2. At the same time, the logic circuit ST-2 receives logic '0' and '1' signals at its first and second control inputs, respectively, and is responsively reset to turn off the switch SW-2 thereby to prevent the formation of a closed loop for voice signals.

Under such condition, when a second user at the station ST-2 starts to insert a new message into the microphone MC-2, the output of the microphone increases in addition to the output produced by the acoustic coupling with the loud-speaker SP-2 which has been receiving the voice message from the station ST-1. When the output of the microphone MC-2 becomes greater than the input to the loud-speaker SP-2, the comparator _C-2 produces and applies a logic '1' signal to J input of the logic circuit L-2 and K input of the logic circuit L-1. Responsively, the logic circuit L-1 is inverted to change the position of switch SW-1, that is, turn off the switch SW-1, thereby to interrupt the voice message transmission from the station ST-1 to the station ST-2. Simultaneously, the logic circuit L-2 is also inverted to change the position of switch SW-2. However, this inverting turns on the switch SW-1 because this switch has previously been in 'OFF' condition. Accordingly, the newly inserted voice message is transmitted from the station ST-2 to the station ST-1 while interrupting the message transmission from the station ST-1 to the station ST-2.

It is noted here that the principle which is recognized from the above basic voice switching control system as shown in Fig. 1 is easily applied or expanded to a multi voice switching control system for controlling three or more different voice signals.

Such multi voice switching control system needs a number of N(N>3) voice switching control units (first through Nth units) each of which is similar to a unit USC-1 or USC-2 as shown in Fig. 1. The output of each voice switch SW-i is connected to all the loud-speakers but the loud-speaker SP-i of the station ST-i (a group of these connected loud-speakers may be regarded as complementary loud-speakers SP-i). The output of each comparator C-i is connected to J input of a logic circuit L-i and connected to K input of the other logic circuits L-i.

Such complementary connection in cooperation with the simple design of each voice switching control unit permits the two way voice communication system to serve as a conference communication system by which a conference is held. This feature is_highly advantageous.

Referring now to Fig. 2, a detailed block diagram of a preferred embodiment of a voice switching control system for controlling voice signals between three stations is shown.

In the illustrative embodiment, the control system comprises three identical voice switching control units 1 through 3. Unit 1 is assigned to a first station including a microphone 101 and loud-speaker 102. Similarly unit 2 is assigned to a second station including a microphone 201 and a loud-speaker 202, and unit 3 is assigned to third station including a microphone 301 and a loud-speaker 302.

Since each unit is identical, the following description refers only to unit 1 except when a reference to the entire voice switching control system or to relationship between units is needed.

Unit 1 includes a voice switch 103, the input of which is connected to the output of the microphone 101 of first station through a voice signal transmission line 104 via an amplifier 105, and the output of which is connected to the inputs of the loud-speakers 202 and 302 of second and third stations through a line 104' to voice signal adder elements 206 and 306 to a line 104" to voice signal amplifiers 207 and 307. It is noted that each of voice switches (103, 203 and 303) is disposed in a 'different' transmission line or channel established between the microphone of one of the stations and the loud-speakers of the remaining stations.

Unit 1 includes a pair of first and second sensing circuits for sensing a voice signal output from the microphone 101 of the first station at a point 108 on the line 104 and a voice signal input to the loud-speaker 102 of the same station at a point 108' on the line 204" (304") after the voice signal adder element 106, respectively. The sensing circuits include a first amplifier 109 in the first sensing line and a second amplifier 109' in the second sensing line. Gains of these amplifiers are selected so that a comparator (which will be described after this) does not malfunction due to an acoustic coupling between the loud-speaker 102 and the microphone 101. Preferably, the filtering function of the amplifier 109 is to provide compensation to the frequency characteristics of the acoustic coupling between the loud- speaker and the microphone. The filtering characteristic of the amplifier 109 is determined taking into consideration the sound structure of the language spoken by the users of the system, particularly the frequency distribution of the voice with which the speech is started. For example, where the language to be used is Japanese, the amplifier 109 preferably has a high-pass filtering function for passing signal frequencies higher than, for example, ten and several hundred Hertz. The first amplifier 109 is connected to a first rectifying circuit 110 which rectifies a first amplified, sensed, voice signal from the microphone 101. Similarly, the second amplifier 109' is connected to a second rectifying circuit 110' for rectifying a second, amplified, sensed, voice signal of the loud-speaker 102. The second sensing circuit further includes a smoothing circuit 111 for smoothing the second rectified signal. The smoothing circuit 111 produces an output which follows the input signal without delay while the input signal is increasing in level. However, after the input signal has reached the peak value (and while signals less than the peak value are being received), the output is maintained at a constant value for a relatively long time contact (e.g. 0.1 second). This rapid following characteristic for increasing input signals and maintaining characteristic for maintaining the peak value for a long time serves to invert the comparator without delay and stabilize the inverted condition thereof in order to stabilize the inverted condition of J-K Flip-Flops which will be described.

In contrast with the second sensing circuit which has a smoothing circuit as described, the first sensing circuit does not include any smoothing circuit which effectively introduces a delay in the turning-on of the associated voice switch.

Unit 1 includes a comparator comprising a subtracting element 112 and a comparating circuit 113. The subtracting element 112 produces a subtracted signal between the non- smoothed signal V₁ from the first sensing circuit and the smoothed signal V₂ from the second sensing circuit by subtracting either the signal V₂ from the signal V_l or subtracting the signal V_- from the signal V₂. The subtracted signal (V₁-V₂) or (V2-V1) is compared with a reference or threshold voltage signal Vth by the comparating circuit 113. The threshold voltage Vth has either an appropriate positive or negative value depending upon whether the subtracted signal represents (V₁-V₂) or (V₂-V₁), respectively.

The comparating circuit 113 outputs a logic '1' signal when the (V₁-V₂) signal is greater than a positive threshold voltage Vth (or the (V₂-V₁) signal < a negative Vth), and outputs a logic '0' signal when the (V₁-V₂) signal is less than the positive threshold voltage (or (V₂-V₁)>the negative Vth). The absolute value of the threshold voltage |Vth| is determined by the consideration that larger values advantageously increase the operational stability but disadvantageously impair the response to a new insertion of voice signals into the microphone while smaller values lower the stability but establish a positive response.

In order to simplify the following description, the limitation that the subtraction element produces a (V₁-V₂) signal and thus the comparating circuit compares the (Vl-VZ) signal with a positive threshold voltage Vth will be assumed.

Each output from the comparator is connected to J input of the associated J_-K Flip-Flop and to K inputs of the remaining J-K Flip-Flops. For example, the output of the comparator 113 of unit 1 is connected to J input of a J-K Flip-Flop 114 of unit 1 and to K inputs of J-K Flip-Flops 214 and 314 via associated OR elements 215 and 315.

Each J-K Flip-Flop operates as a conventioal J-K Flip-Flop does, and follows the Truth Table below.

Each J-K Flip-Flop is driven by clock pulses generated by a suitable pulse generator 16. A relatively high repetition rate of clock pulses is selected to speed up the response of the J-K Flip-Flop. The practical repetition rate may preferably be several tens kilo-Hertz or higher.

The Q output of each J-K Flip-Flop is connected to the control electrode or gate of the associated voice switch.

For example, the voice switch 103 of unit 1 is connected to Q output of J-K Flip-Flop 114 of unit 1.

The voice switch 103 is triggered into conduction and remains in 'ON' state when and whenever Q output of J-K Flip-Flop 114 becomes and remains logic '1' (e.g. 'High' level), and is turned off and remains in 'OFF' state when and whenever Q output of J-K Flip-Flop 114 becomes and remains logic 'O' (e.g. 'Low' level). The voice switch may comprise a controllable semiconductor switch such as a field effect transistor.

A typical operation of the voice signal control system shown in Fig. 3 is as follows:

• Initially, it is assumed that no effective voice signal exists in any one of the voice signal transmission channels between the three stations, thus causing either of the comparators to apply a logic 'O' signal to J and K inputs of all the J-K Flip-Flops, and that only the Flip-Flop 214 outputs a logic '1' signal, thus controlling the voice switch 203 at 'ON' position while the Flip-Flops 114 and 314 outputs a logic '0' signal, thus controlling the voice switches 103 and 303 at 'OFF' position.

Under such condition, when a user at the first station starts to insert a voice message into the microphone 101, the insertion causes (V₁-V₂) signal defining a control input to the comparator 113 to increase to a level higher than the level of the threshold voltage Vth. Responsively, the output of the comparator 113 turns into a logic '1' signal which is applied to J input of the Flip-Flop 114 and to K inputs of the Flip-Flops_214 and 314. As a result, the first Flip-Flop 114 receives J=1 and K=0 inputs, and the second and third Flip-Flops 214 and 314 receives J=0 and K=l inputs. Referring to the Truth Table of J-K Flip-Flop, upon the receipt of a subsequent first clock pulse the first J-K Flip-Flop is set to produce a logic '1' signal at its Q output to turn on the first voice switch 103, thus enabling the inserted voice message to be transmitted to the second and third loud-speakers 202 and 302. At the same time, the second J-K Flip-Flop is reset to turn off the second voice switch 203. During this period, the third J-_K Flip-Flop 314 remains reset to control the third voice switch 303 at 'OFF' position.

The 'OFF' state of the voice switches 203 and 303 prevents the system from forming any closed loop for voice signals.

Under such condition, when a second user at the second station inserts a new voice message into the microphone 201, the output of the microphone 201 increases in addition to the output produced by the acoustic coupling with the loud-speaker 202 (as shown in a dotted line 217) which has been receiving the voice message from the first station. Thus, (V₁-V₂) input signal to the second comparator 213 becomes greater than the threshold voltage Vth thereby to cause the comparator 213 to output a logic '1' signal. Responsively, J-K input condition of the second Flip-Flop 214 is changed into J=l and K=1 from J=0 and K=l. At the same time, J-K input condition of the first Flip-Flop 114 is changed into J=l and K=l from J=l and K=0. Referring again to the truth table of J-K Flip-Flop, after a subsequent first clock pulse, the second Flip-Flop 214 is inverted to produce a logic '1' signal at its Q output thereby to turn on the second voice switch 203 while the first Flip-Flop 114 is also inverted, which inverting, however, causes its Q output to produce a logic 'O' signal thereby to turn off the first voice switch 103.

It is now understood that the inverting of first J-K Flip-Flop 114 serves to turn off the first voice switch 103 and that the inverting of second J-K Flip-Flop 214 serves to turn on the second voice switch 203.

The 'OFF' state of the first voice switch 103 and the 'ON' state of the second voice switch 203 must be retained or stabilized thereby to ensure the interruption of the first voice message from the first microphone 101 and to establish the transmission of the second voice message newly inserted into the second microphone 201.

Such stabilization is achieved by inverting the first comparator 113, thus applying J=0 and K=l inputs to the first J-K Flip-Flop 114 and applying J=l and K=0 inputs to the second J-K Flip-Flop. More specifically, the sensing circuit of unit 1 senses a voice signal increase at the point 108' on the input line to the first loud-speaker 102 immediately after the turning-on of the second voice switch 203. The rapid increase in the sensed voltage V₂ opposes the sensed voltage V₁ from the first microphone 101, thus lowering the (V_l-V₂) signal to less than the threshold voltage Vth, which controls the first comparator 113 to output a logic 'O' signal. Responsively, the first J-K Flip-Flop 114 receives J=0 input under K=l input and the second J-K Flip-Flop 214 receives K=0 input under J=l input. Upon receipt of a subsequent clock pulse, the first Flip-Flop is reset to establish the 'OFF' state of the first voice switch 103 and the second Flip-Flop is set to establish the 'ON' state of the second voice switch 203.

During the period of the above operation, the third Flip-Flop 314 remains reset to control the third voice switch 303 at 'OFF' state under J=0 and K=1 input condition.

In this manner, the new voice message inserted into the second microphone 201 is positively transmitted to the first and third loud-speakers 102 and 302 while interrupting the transmission of the first voice message which has been inserted in the first microphone 101. In the illustrative system, the time required to establish or stabilize OFF condition of switch 103 and ON condition of switch 203 by the inverting of the first comparator 113 into a logic "0" output from a logic "1" after the second comparator 213 is inverted to output a logic "1" from logic "0" by the insertion of new message may be negligible (the entire time including the time required for amplifier 207 to establish an amplified voice signal is about one milli-second).

Referring now to Fig. 3, a preferred embodiment of a voice switching control system for controlling voice signals between a pair of stations is shown in a block diagram. With respect to'the same circuit element, the same reference numeral is used in Fig. 3 and Fig. 2. However, it is noted that in the embodiment of Fig. 3 a common, single J-K Flip-Flop C14 is advantageously used to control either of first and second voice switches 103 and 203. The common J-K Flip-Flop C14 has J and K inputs connected to the first comparator 113 and the second comparator 213, respectively and Q and Q outputs connected to the first and second voice switches 103 and 203, respectively.

Such arrangement A of the single J-K Flip-Flop is logically equivalent to an arrangement A' comprising a sub-arrangement A₁ of a first J-K Flip-Flop and a sub-arrangement A₂ of a second J-K Flip-Flop in which J input, K input and Q output of the first J-K Flip-Flop are connected to the first comparator, the second comparator and the first voice switch, respectively while J input, K input and Q output of the second J-K Flip-Flop are connected to the first comparator, the second comparator and the second voice switch, respectively. Further, the sub-arrangement A₂ of the second Flip-Flop is operationally equal to an arrangement A'₂ wherein J input, K input and Q output of the second J-K Flip-Flop are connected to the second comparator, first comparator and the second voice switch, respectively.

Therefore, the arrangement A of the single Flip-Flop C14 is operationally equal to the arrangement A" comprising the sub-arrangement A₁ of the first J-K Flip-Flop and the sub-arrangement A'₂ of the second J-K Flip-Flop. The arrangement A" is identical with the arrangement of a pair of J-K Flip-Flops as shown in Fig. 1.

Therefore, the voice switching control system employing the common and single Flip-Flop, as arranged in Fig. 3 is operated in a similar manner as discussed with reference to Fig. 1 while preserving various advantages given by the embodiment of Fig. 2.