Abstract:
A circuit arrangement to enable one party to have a two-way conversation simultaneously with an outside party and an internal party. The circuit arrangement includes circuitry so that the outside party cannot hear anything that is said by the internal party or vice versa. Thus, the one party has the ability to talk to both the outside and the internal party without them hearing each other while using one subset and not putting anyone on hold. The circuitry is also provided with a designated button, which when depressed places all three parties in a two-way conference. In addition, the circuitry includes a resistance network coupled between the transmitter and receiver of the one subset both in the outside circuit and the internal circuit associated therewith to reduce sidetones in the receiver of the one subset.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to telephone systems and more particularly to telephone circuits employing key telephone systems. 
     Existing key telephone systems having the capabilities for outside as well as internal communications require the use of two subsets or placing the outside call on hold while the internal call is conducted. In either case, this requires depressing many buttons or manipulating several subsets to perform the desired operation. 
     There are several shortcomings to the above operating procedures. If the two subset approach is used, there is always the inconvenience of holding the two subsets. If the single subset is used in conjunction with the hold button, the outside party who is calling for some type of assistance does not have the assurance that anything is being done to solve his problems since he does not hear any part of the conversation that transpires internally. 
     A solution to this problem is disclosed in my copending application Ser. No. 622,287, filed Oct. 10, 1975. A circuit arrangement is disclosed in this copending application that will enable one party to have a two-way conversation simultaneously with an outside party and an internal party without the outside party and inside party hearing each other. A button is provided to actuate circuitry in the circuit arrangement to place all three parties in a two-way conference. 
     One disadvantage with the circuit arrangement of this copending application is that the sidetones in the receiver of the subset of the one party is undesirably high. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a telephone circuit at the subset of the one party which results in reduced sidetones in the subset receiver relative to those sidetones present in the circuit arrangement of the above cited copending application. 
     As in the case of the circuit arrangement of the above cited copending application the new telephone circuit does not require the use of two subsets and it does not require the use of the hold button. When this new telephone circuit is used, one party using the new telephone circuit using one handset is able to have a two-way conversation with the outside party as well as with the internal party. At the same time, the outside party cannot hear anything that is said by the internal party or vice versa. In short, this gives the party employing the new telephone circuit the ability to talk to both parties without them hearing each other while using one handset and not putting anyone on hold. As an additional benefit, by depressing a designated button all three parties can be put into a two-way conference. A further benefit is that the sidetones in the receiver of the subset employing the new telephone circuit is reduced. 
     A feature of the present invention is the provision of a telephone circuit to eliminate use of a hold button comprising: a subset having a receiver and a transmitter for use by one party; a plurality of central office lines connected to outside parties; a plurality of internal lines connected to internal parties; first means to couple the receiver and the transmitter to a selected one of the central office lines to enable a first two-way conversation between the one party and a selected one of the outside parties, the first means including a first network coupled between the transmitter and the receiver to reduce sidetones in the receiver; second means to couple the receiver and the transmitter to a selected one of the internal lines to enable a second two-way conversation between the one party and a selected one of the internal parties, the second means including a second network coupled between the transmitter and the receiver to reduce sidetones in the receiver; and third means coupled between the input of the first and the input of the second means, the third means having a first state to enable the first and second conversations to be carried on simultaneously and to prevent the selected one of the outside parties and the selected one of the internal parties from hearing each other and a second state to selectively enable the one party, the selected one of the outside parties and the selected one of the internal parties to be placed in a two-way conference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 is a block diagram of the telephone circuit in accordance with the principles of the present invention; and 
     FIG. 2 is a schematic diagram partially in block form of the impedance matching networks, transmitter and receiver isolation amplifiers and patch unit of FIG. 1 in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, when a call comes in from outside on the CO (Central Office) lines, the signal is connected to an impedance matching network NTW1 by depressing the proper one of flashing buttons A-J of the CO line selector switch S2. Network NTW1 contains all of the components necessary to match the impedances of the transmitter and receiver to the selected CO line. The received signal at the receive part of network NTW1 passes through isolation amplifier A2 to the subset receiver REC. The signal from the subset transmitter TRANS passes through isolation amplifier A1 back to network NTW1, thus completing the connection to an outside or CO line. 
     If at this same time it is desired to also talk to an internal party, the desired internal line is selected by depressing the appropriate one of buttons A-I on internal line selector switch S1. The transmitter signal of the subset goes through the isolation amplifier A4 to an impedance matching network NTW2 and then through internal line selector switch S1 to the selected internal line. Network NTW2 contains all the components necessary to match the impedances of the subset transmitter and receiver to the selected internal line. The received signal comes in through select switch S1 through network NTW2 and through isolation amplifier A3 to the subset receiver REC. In this condition, the party employing the subset can hold a two-way conversation with both the external and internal parties, but the internal party cannot hear the outside party and vice versa. 
     If it is desired to patch all three parties together for a conference, this can be done by depressing the patch button on selector switch S1 and all parties are connected together via the patch unit 9, impedance matching network NTW1 and isolation amplifiers A1 and A2. 
     Referring to FIG. 2, the impedance matching network NTW1 and NTW2 will be described. Transformer T3 and resistors R7 and R9 provide the proper match and isolation to the incoming external telephone circuit from switch S2 including tip (T) and ring (R) conductors. The varistor VI changes resistance inversely to the current flowing through it, thereby helping to keep the looses over a long and short loop the same. Resistor R8 and capacitor C6 provide a filter network to suppress high frequency signal components of the dial pulses. Resistor R6 through switch S6 provides a load for network NTW1 when there is no CO line connected thereto by selector switch S2. Switch S6 is coupled to the pushbutton switches of selector switch S2 and is open when one pushbutton switch is depressed to couple a selected CO line to network NTW1 and is closed when none of the pushbutton switches are depressed. 
     Transformer T2, resistors R31, R32, R33 and R34, capacitor C13, varistor V2 and switch S7 perform the identical functions as above for the internal telephone circuit. 
     The transmit circuit will now be described with reference to FIG. 2. Resistors R5 and R18 provide DC (direct current) current to operate the transmitter TRAN. Capacitor C9 couples the signal from transmitter TRAN into operational amplifiers 10 and 11. Operational amplifiers 10 and 11 are operated with differential inputs in order to reduce the amplification of a common mode signal. Resistors R17 and R19 and resistors R20 and R22 determine the gain of amplifiers 10 and 11, respectively. Capacitor C10 shunting resistor R22 helps to reduce the high frequency response on the internal circuit. Resistors R16 and R21 provide loads on amplifiers 10 and 11, respectively. Resistors R14 and R15 and resistors R23 and R26 in conjunction with transformers T3 and T2 provide the proper impedance load for the transmit operational amplifiers 10 and 11, respectively. Resistors R14 and R26 along with the transformers T3 and T2 help to reduce the available signal going into receive amplifiers A2 and A3 from the transmit amplifiers 10 and 11, respectively, thereby reducing side-tones in the receiver REC. 
     The receive circuit will now be described with reference to FIG. 2. The receive signals pass through transformers T3 and T2 and then through capacitors C7 and C14 to their respective receive amplifiers A2 and A3. Amplifiers A2 and A3 include operational amplifiers 12 and 13, respectively. Operational amplifiers 12 and 13 are operated with differential inputs in order to reduce the amplification of a common mode signal. Resistors R13 and R27 maintain the necessary load on the input transformers T3 and T2, respectively. Resistors R10 and R12 determine the gain of the external receive operational amplifier 12. Resistors R29, R30 and R35 and capacitor C15 determine the gain of the internal receive operational amplifier 13 and also provide some phase shift so that the external and internal signals are in phase. Resistors R11 and R28 and capacitors C8 and C16 provide impedance match and DC isolation for receiver REC which is connected across the output of operational amplifiers 12 and 13. 
     Resistors R24 and R25 and capacitors C11 and C12 provide a filtered plus and minus 12 volts to power operational amplifiers 10 - 13 and also provides a +12 volts reference potential which is coupled to operational amplifiers 10- 13 and the resistor circuits associated therewith. 
     Patch unit 9 will now be discussed with reference to FIG. 2. Patch unit 9 includes a CMOS (clad metal oxide semiconductor) D-type flip flop IC3 which is used to convert the momentary action conference switch S5 located in selector switch S1 (FIG. 1) into an alternate action switch to control the patch relay RL1. Resistors R3 and capacitor C5 provide an RC (resistor capacitor) circuit to debounce momentary conference switch S5. Resistor R4 and zener diode Z1 provide a regulated 10 volts DC for flip flop IC3 from the 28 volts DC supply. Any time a positive going pulse is present at pin 3 of flip flop IC3, the output at pins 2 and 5 of flip flop IC3 will change state. Resistor R36 and capacitor C4 provide an RC time constant to debounce the reset switches S3 and S4 located on the ends of line selector switches S1 and S2 (FIG. 1). Any time a positive pulse is present at pin 6 of flip flop IC3, flip flop IC3 is reset and patch unit 9 is turned off. The positive pulse at pin 6 of flip flop IC3 is present when both reset switches S3 and S4 are closed. Diode D4 suppresses any spikes present going into the switching transistor Q1. Resistor R1 limits current into transistor Q1. Transistor Q1 switches relay RL1 and the patch lamp 14 on when the output of flip flop IC3 is high (binary 1) and off when the output of flip flop IC3 is low (binary 0). Diode D5 provides spike protection for relay RL1. Resistor R2 reduces the +28 volts DC to 10 volts DC to illuminate lamp 14. The contacts of relay RL1 when relay RL1 is inoperative, which is the first state of patch unit 9, connects the internal tip (T) and ring (R) conductors directly to network NTW2. The contacts of relay RL1 when relay RL1 is actuated, which is the second state of patch unit 9, connects the internal tip (T) and ring (R) conductors to the outside of external tip (T) and ring (R) conductors through capacitors C18 and C19, respectively, for DC isolation and also disables NTW2. Resistor R37 provides a load for the internal lines when patch unit 9 is in its second state. When patch unit 9 is in its second state a two-way conference for the three parties is possible since the external lines and internal lines are connected together and also to the input of network NTW1 which is still operative when patch unit 9 is in its second state. 
     The values and types of the components employed in FIG. 2 of a successful reduction to practice are set forth hereinbelow. 
     
         ______________________________________R1               4.7K 1/4 watt resistorR2               390 ohm 1/2 watt resistorR3, R16, R21     1K 1/4 watt resistorR4               2.2K 1/2 watt resistorR5, R18          560 ohms 1 watt resistorR6               330 ohms 1/2 watt resistorR7, R9, R31, R34 150 ohms 1/2 watt resistorR8, R32          180 ohms 1/2 watt resistorR10, R22         33K 1/4 watt resistorR11, R14, R28    100 ohms 1/4 watt resistorR12              47K 1/4 watt resistorR13, R15, R23, R27            390 ohms 1/4 watt resistorR17              68K 1/4 watt resistorR19              3.9K 1/4 watt resistorR20              3.3K 1/4 watt resistorR24, R25         1.5K 1/2 watt resistorR26              120 ohms 1/4 watt resistorR29, R30         100K 1/4 watt resistorR33              680 ohms 1/4 watt resistorR35, R36         10K 1/4 watt resistorR37              1K 1/2 watt resistorC4, C7, C14      6.4 mfd 40V capacitorC5               1.0 mfd 50V capacitorC6, C9, C13      .12 mfd 200V capacitorC8, C11, C12, C16            50 mfd 35V capacitorC10, C15         .001 mfd 1KV capacitorC18, C19         10 ufd, N.P. capacitorD4, D5           GR22 diodeZ1               10V 1 watt zener diodeV1, V2           VaristorIC1, IC2         LM 1458 integrated circuitIC3              CA4013 dual D flip flopQ1               2N3568 transistorRL1              4PDT relayT2, T3           Transformer______________________________________ 
    
     While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.