Oct. 23, 1962
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`R. H. DUNCAN ETAL
`CONFERENCE CALL CIRCUIT
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`3,060,265
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`Filed Jan. 26, 1960
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`CSCO-1005
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`Oct. 23, 1962
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`R. H. DUNCAN ETAL
`CONFERENCE CALL CIRCUIT
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`3,060,265
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`Fied Jan. 26, 1960
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`2 Sheets-Sheet 2
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`724 (4.6%72%? 4W2/7
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`United States Patent Office
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`3,060,265
`Patented Oct. 23, 1962
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`3,050,265
`CONFERENCE CALL CRCUIT
`Robert H. Duncan, Arlington Heights, Thomas. Paul
`Miller, Mouat Prospect, and Anthony W. Maciu is aid
`Soiy L. Fudaley, Chicago, B., assignors to Interia
`tional Telephone and Telegraph Corporation, New
`York, N.Y., a corporation of Maryland
`Filed Jan. 26, 1960, Ser. No. 4,789
`22 Claims. (C. 79-)
`This invention relates to telephone systems and more
`particularly to conference call circuits for use in Such
`systems.
`Typical conference call circuits provide for matching
`the impedances of all interconnected circuits with pro
`visions for equalizing the attenuation of signals by all
`stations and with provisions for negating signal attenua
`tion caused by the introduction of an excess number of
`stations. However, conference call circuits have not met
`all of these requirements primarily because the character
`istics of individual subscriber lines are not Sufficiently
`uniform; and because the strength of the voice signal at
`a subscriber's receiver is a function of the number of
`stations connected to the conference call circuit, i.e., the
`characteristic impedance of the conference circuit is low
`ered as stations are connected thereto and raised as sta
`tions are removed therefrom. The result is that confer
`ence circuits which have been used in the past were un
`stable, and provided inefficient power transfer, improper
`sidetone and combinations thereof.
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`An object of this invention is to provide new and im
`proved conference call circuits.
`A further object of this invention is to provide con
`ference call circuits which may simultaneously serve
`either a large number or a small number of stations, as
`required.
`Another object of this invention is to provide confer
`ence call circuits wherein each receiver and transmitter
`is terminated by its own characteristic impedance.
`Still another object of this invention is to provide tele
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`phone systems having controlled sidetone.
`Yet another object of this invention is to provide con
`ference call circuits that require no adjustment of signal
`amplitude as stations are added to or dropped from the
`conference.
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`Another object of this invention is to minimize in
`stability in conference call circuits.
`In accordance with this invention, a conference call
`circuit is provided with transmit and receive channels
`which are common to all conversing subscribers and
`which are interconnected by a high gain link amplifier
`that compensates for any attenuation. Each subscriber
`line which may be connected into the conference circuit
`is provided with individual transmit and receive chan
`nels each containing a device for individually isolating
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`the transmitter and the receiver from the common chan
`nel, thereby preventing changes in the characteristic im
`pedance of the common channel which might otherwise
`be caused when stations are added to or dropped from
`a conference call. The individual transmit channel and
`the individual receive channel of each subscriber line are
`interconnected so that signals originating in such indi
`vidual transmit channel are inverted and fed back into
`the individual receive channel associated therewith to
`control sidetone at the associated Subscriber station.
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`The above mentioned and other objects of this inven
`tion together with the manner of obtaining them will be
`come more apparent and the invention itself will be best
`understood by making reference to the following descrip
`tion of a preferred embodiment of the invention taken
`in conjunction with the accompanying drawings, in
`which:
`FIG. 1 is a block diagram which shows a portion of
`a conference call circuit; and
`FIG. 2 shows the details of the circuit which may be
`used to complete the hollow blocks of FIG. 1.
`Where possible, simple terms are used and specific
`items are described hereinafter to facilitate an under
`standing of the invention; however, it should be under
`stood that the use of such terms and references to Such
`items are not to act in any manner as a disclaimer of
`the full range of equivalents which is normally given by
`established rules of patent law. To illustrate, the at
`tached drawings show transistorized amplifiers; however,
`any suitable amplifying and isolating means such as
`vacuum tubes may be used. Still further the amplifiers
`are shown as two-stage amplifiers; whereas, any combina
`tion of stages that results in a proper signal phase shift
`may be utilized. Although only three subscriber lines
`are shown, any suitable number may be interconnected
`via the conference circuit. Quite obviously other exam
`ples could be selected to illustrate the manner in which
`specific terms that have been used and items have been
`described are entitled to a wide range of equivalents.
`Brief Description
`Turning now to FIG. 1, a conference call circuit is
`shown in block diagram form. Each of the three sub
`scriber stations A, B and N has an individual transmit
`and receive channel. For example, subscriber station
`A has a transmitter , an isolation transmitter amplifier
`2, a receiver 5, and an isolation receiver amplifier 4.
`A first output of the transmitter amplifier 2 is connected
`to a first input of the receiver amplifier 4 by way of an
`individual circuit branch such as sidetone control cir
`cuit 3, so that a sample of all signals originating in trans
`mitter 1 may be inverted and fed back into receiver am
`plifier 4 to control the sidetone. Another output ter
`minal on transmitter amplifier 2 is connected by any
`suitable means such as plugs and jacks or automatic
`switches at point 30 to a common channel or transmis
`sion line such as transmit bus 21 which is terminated by
`a characteristic impedance 25. Another input terminal
`on receiver amplifier 4 is similarly connected to a com
`mon channel or transmission line such as receive bus 22
`which is also terminated by a characteristic impedance
`26. The common transmit and receive channels are, in
`turn, interconnected by a link amplifier 11 which provides
`a uniform amplification of all voice signals originating
`in the conference call system.
`It should be noted that the invention provides a tele
`phone system wherein all subscriber lines terminate in
`the uniform impedance of an individually and perma
`nently associated amplifier which is adjusted to compen
`sate for loss on the associated line. While the attached
`drawings and this description speak only of conference
`call circuits, it should be understood that the principles
`of the invention may be used any time that a uniformity
`of line impedance is desired.
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`Detailed Description
`For a more complete understanding of the invention,
`reference is made to the detailed circuit individually
`associated with subscriber station. A together with the
`common link equipment as shown in F.G. 2.
`More particularly, FIG. 2 relates to FIG. 1 in the fol
`lowing manner. Transmitter 1 of subscriber station A
`is shown near the upper left-hand corner of FIG. 2, re
`ceiver 5 of subscriber station A is shown near the lower
`left-hand corner, isolation transmitter amplifier 2 is shown
`in the upper third, isolation receiver amplifier 4 is shown
`in the lower third, and sidetone control feedback circuit
`3 is shown near the left-hand edge. The remainder of
`FIG. 2 is the common channel which serves all subscriber
`lines that participate in a conference call.
`It should be
`understood that the circuits which complete hollow boxes
`7, 12, 15 and 23 are identical to those shown in FIG. 2
`in connection with amplifiers 2 and 4.
`Next, it is assumed that a party at station A is talking
`after a conference call has been set up.
`Transmitter amplifier-Means are provided for obtain
`ing two output signals from transmitter amplifier 2, a
`first of such signals being used to provide the principal
`voice signal and a second of such signals being used to
`provide controlled sidetone. More particularly, transmit
`ter 1 is connected to transmitter amplifier 2 through trans
`former 26 and a network comprising capacitor 205, re
`sistors 202, 203 and 264, the purpose of which is to
`match the characteristic impedance of the transmitter.
`Signals from transmitter
`are applied through trans
`former 20i and coupling capacitor 205 into variable re
`sistor 268 and then to the base of transistor 210 where
`the signals are amplified. Capacitor 296 and resistor 207
`along with resistor 212 make up the bias and by-pass cir
`cuits for transistor 210. The amplified signal from tran
`sistor 210 is applied via capacitor 252 to the base of tran
`sistor 2-i6 which provides the two output signals.
`The principal voice signal is obtained from the emitter
`of transistor 26 while the sidetone control signal is ob
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`tained from the collector thereof. The voice signal is
`extended to the common link of the conference circuit
`over a path that can be traced from the emitter of tran
`sistor 216 through the primary winding of transformer
`222 and resistor 223 to ground. The output side of trans
`former 222 is connected via terminals 30 to the common
`transmit bus 2 which is terminated by a characteristic
`impedance 25. A high impedance is presented to bus
`21 by amplifier 2 to allow a great number of transmitting
`stations to be interconnected without substantially alter
`ing the characteristic impedance of bus 2i. The sidetone
`control signal travels over an individual circuit branch
`than can be traced from the collector of transistor 216
`through transformer 224 (which is tuned by capacitors
`221 and 225), coupling capacitor 226, potentiometer 227,
`and resistor 228 to sidetone control circuit 3 and a first
`input circuit of receiver amplifier 4. Potentiometer 227
`is adjusted to cause the amplitude and phase of the side
`tone control signal to be approximately 180° out of phase
`with the principal voice signal taken from the emitter of
`transistor 216.
`It should be noted that the impedance of transmit bus
`21 remains uniform regardiess of the number of stations
`that are connected into the conference call circuit. More
`particularly, transmitter always sees only its own char
`acteristic impedance looking into transformer 20, while
`transmit bus 21 sees only an extremely high impedance
`which has little or no effect on its characteristic impe
`dance when looking back into transmitter amplifier 2.
`Therefore, the number of transmit channels that are con
`nected in the conference call circuit, does not effect the
`impedance of bus 2.
`Link amplifiers.-Means are provided for uniformly
`amplifying all voice signals originating in the conference
`call circuit. That is, the signal on transmit bus 2 is
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`applied to link amplifier 11 through transformer 231 and
`then applied to transistor 233 which is a grounded emitter
`type amplifier. Transistor 233 is properly biased from
`the power supply through resistor 234, resistor 232 and
`also through one side of transformer 235. Transformer
`23E is used to match the impedance of link amplifier 11
`to the characteristic impedance of transmit bus 2 where
`by maximum power transfer is achieved.
`After amplification in transistor 233, the voice signal
`is applied through transformer 235 to the base of tran
`Sistor 237 where there is further amplification. Transistor
`237 is properly biased from the power supply through
`resistors 239 and 236. The emitter of transistor 237 is
`connected to ground through a winding of transformer
`238 which is used to match the impedance of link ampli
`fier ii to the characteristic impedance of receive bus
`22 whereby maximum power transfer is achieved. Thus,
`it is seen that all voice signals are amplified by link am
`plifier it which is a high gain, two stage, transistorized
`amplifier that utilizes inductive coupling exclusively rather
`than capacitive couping.
`Receiver amplifier.-Means are provided in the form
`of receiver amplifier 4 for isolating the receive channels
`of individual Subscriber stations and for providing con
`trolled sidetone under the influence of signals received
`Over conductors 3. In greater detail, the amplified voice
`signals appearing on receive bus 22 are applied through
`transformer 256 to the base of transistor 258 which is also
`biased to ground through resistor 255. Transformer 250
`presents a high impedance to receive bus 22; therefore,
`the impedance of the bus is not materially affected by the
`addition or removal of stations to the conference circuit.
`To control sidetone in receiver amplifier 4, the base of
`transistor 259 is directly coupled to transmitter amplifier
`2 through resistor 252. The amplitude of the sidetone
`control Signal appearing on conductors 3 can be adjusted
`by potentiometer 253. Resistors 254 and 256 form a volt
`age divider for applying a proper bias to the base of tran
`sistor 259. The emitter of transistor 259 is connected
`to ground through resistor 260. Both the voice signal
`applied through transformer 250 from receive bus 22,
`and the Sidetone control signal received over conductors
`3 are vectorially summed in the connection between the
`collector of transistor 259 and the emitter of transistor
`258. As explained above, the signal appearing on con
`ductors 3 is approximately 180 out of phase with that
`applied from receive bus 22 via transformer 250; there
`fore, the voice and sidetone control signals in combining
`buck and the talking subscriber at station. A hears only
`a controlled amount of sidetone. Although, the potenti
`ometers 227 and 253 could be adjusted to completely can
`cel all signals originating at transmitter , they are pref
`erably adjusted to provide the sidetone which occurs dur
`ing normal or non-conference calls.
`To energize receiver 5, the vectorially summed signal
`appearing at the collector of transistor 258 is applied
`through coupling capacitor 26, resistor 262 and ampli
`tude adjusting potentiometer 265 to the base of transis
`tor 267. The base of transistor 267 is properly biased
`through a voltage divider including potentiometer 265
`and resistors 264 and 263. The collector of transistor
`267 is also biased from the B supply through resistor
`263 and one winding of transformer 268. Capacitor 266
`acts as a filter from the collector of transistor 267 to
`ground. The output signal from the collector of transis
`tor 267 is applied through transformer 268 to receiver 5.
`Thus, it is seen that the receiver looking back into trans
`former 268 sees an impedance that matches its own im
`pedance regardless of how many subscriber stations are
`conferenced in the circuit.
`When a subscriber at a station other than station A is
`talking, no signal is received at receiver amplifier 4
`Over conductors 3 from individually associated trans
`mitter amplifier 2; therefore, the only signal appearing at
`receiver 5 is that applied from receive bus 22, through
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`transformer 250, transistor 258, transistor 267 and trans
`former 268. Since no sidetone control signal is re
`ceived over conductors 3, there is no signal combining
`with the signal through transistor 259. Therefore, the
`signal driving receiver 5 when a subscriber at station B
`or N is talking is made up solely of the signal from re
`ceiving bus 22. The high impedance that receive bus 22
`sees when looking into receiver amplifier 4 through trans
`former 250 results in no material alteration of the char
`acteristic impedance of bus 22. Therefore, any reason
`able number of receive channels can be interconnected in
`the conference circuit without creating instability.
`The power transfer loss resulting from the lack of
`impedance matching between transmit bus 2i and trans
`mitter amplifiers 2, 7 and 15; and between receive bus 22
`and receiver amplifiers 4, 12 and 23 is compensated for
`by link amplifier i. The power loss problem is not as
`serious as the instability problems that would result if
`there were no isolation amplifiers and the impedance of
`transmit bus 2 and of receive bus 22 were allowed to
`vary as a function of the number of stations that are
`connected thereto.
`While the principles of the invention have been shown
`and described 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 on the scope
`of the invention.
`What is claimed is:
`1. A four wire conference call system comprising a
`plurality of Subscriber stations, each having a transmitter
`and receiver associated therewith, first means associated
`with each of said stations for isolating the transmitter
`associated therewith, second means associated wtih each
`of said stations for isolating the receiver associated there
`with, common amplifier means, means for selectively
`connecting said first and second means of any calling
`station through said common amplifier, means for selec
`tively connecting said first and said second means of any
`desired called stations to said common amplifier for inter
`connecting each of said transmitters to all of said re
`ceivers, individual means for linking the isolating means
`associated with the same subscriber stations, and means
`including Said last named means for automatically con
`trolling sidetone responsive to actuations of said trans
`mitter.
`2. In a four wire conference call system, a plurality of
`Subscriber stations, each comprising transmit means and
`related receive means, first isolating means individually
`associated with each of said transmit means, second iso
`lating means individually associated with each of said
`receive means, separate means for interconnecting the
`transmit and related receive isolating means associated
`with the same subscriber station, first common bus means
`linking each of said transmit isolating means, second
`common bus means linking said receive isolating means,
`means including associated isolating means and common
`link means for selectively connecting any calling sub
`Scriber station with a desired number of called stations,
`and means associated with each of said isolating means
`responsive to signals imparted over said interconnecting
`means for controlling the amplitude of at least some of
`Said signals in said related receive means.
`3. The communication system of claim 2 wherein said
`isolating means associated with each of said transmit
`means comprises an amplifier, means within said amplifier
`for splitting signals appearing in said associated transmit
`means, and means for inverting a portion of said split
`signal.
`4. In the communication system of claim 3 an imped
`ance matching means for interconnecting each of said
`transmit means and the amplifier individually associated
`therewith whereby maximum power is transferred from
`said transmit means into an associated one of said ampli
`fiers, means for applying a portion of said split signal to
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`all of said isolating means that are associated with said
`receive means, and means for applying another portion of
`said split signal to the particular receive isolating means
`that is associated with the station at which said split
`signal originates.
`5. The communication system of claim 2 wherein
`said isolating means associated with each of said trans
`mit means comprises means for splitting said signals re
`ceived from said associated transmit means, means for
`phase inverting a portion of said split signal, means for
`controlling the phase and amplitude of said inverted por
`tion of said split signal, means for applying said con
`trolled signal directly to the isolating means associated
`with the receive means of the station where said con
`trolled signal originated, and means responsive to said
`last named means for controlling sidetone in said last
`named station.
`6. In a communication system, a conference call circuit
`comprising a plurality of subscriber stations each having
`electrically isolated transmitter and receiver means as
`sociated therewith, means for interconnecting said iso
`lated transmitters, means for interconnecting said isolated
`receivers, means for linking said interconnected trans
`mitters and receivers thereby providing conferencing
`means, means individually associated with each of said
`stations for independently transferring signals from said
`isolated transmitter of said associated station to the iso
`lated receiver of the same station, and means responsive
`to receipt of said independently transferred signals for
`controlling sidetone at said same station.
`7. The conference call circuit of claim 6 wherein said
`linking means includes a multistage amplifier means,
`means for coupling said amplifier stages to provide a high
`gain and minimum distortion, and means also associated
`With Said amplifier for matching impedances to maximize
`power transfer between said interconnected transmitters,
`said amplifier and said interconnected receivers.
`8. A conference call circuit including a plurality of
`Subscriber stations each having individually associated
`transmit and receive channels, a plurality of amplifiers,
`means for individually associating one of said amplifiers
`With each of said channels, each of said amplifiers asso
`ciated with said transmit channels having one input and
`two outputs, each of said amplifiers associated with said
`receive channels having two inputs and one output, first
`means for connecting one of said outputs of each ampli
`fier in said transmit channels to one of said inputs of
`the amplifier in an associated receive channel, second
`means including a common link for inter-coupling said
`other of Said outputs of each amplifier in said transmit
`channels and the other of said inputs of each amplifier
`in said receive channels, and means for bucking signals
`Supplied via said first and said second means for con
`trolling sidetone at said subscriber stations.
`9. The conference call circuit of claim 8 and means
`for controlling the gain of each of said channel ampli
`fiers to compensate for losses in said individually asso
`ciated channel, thereby providing substantial uniformity
`of signal strength regardless of the variations of loss in
`Said channels.
`10. The conference call circuit of claim 9, wherein said
`means for connecting one of said outputs of each of said
`amplifiers in said transmit channels to one of said inputs
`of each of said amplifiers in said receive channels com
`prises a common receive bus, a transmit bus, and means
`for coupling the transmit bus to said receive bus.
`11. In the conference call circuit of claim 10, means
`including said amplifiers in said transmit channels for
`providing a constant terminating impedance to said trans
`mit channel, and means including said amplifier in said
`receive channels for providing a constant driving imped
`ance to said receive channel.
`12. The conference call circuit of claim 8, wherein said
`said one output of each of said amplifiers of said transmit
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`channels is phase inverted from said other output of each
`of said amplifiers of said transmit channels.
`13. The conference call circuit of claim 8, wherein Said
`means for bucking said signals comprises a pair of tran
`sistors each having at least control, input, and an output
`electrodes, means connecting said first means to said con
`trol electrode of one of said transistors, means for con
`necting said second means to the control electrode of the
`other of said transistors, means for interconnecting the
`output electrode of a first of said transistors and for con
`necting the input electrode of a second of said transisters,
`and means including said last named means for deriving
`a signal which is the vectorial sum of signals applied to
`said control electrodes.
`14. A conference call system comprising a plurality
`of telephone stations each having a transmitter and a re
`ceiver, a transmission line, a plurality of high impedance
`circuits associated with each of said transmitters and re
`ceivers, means for selectively connecting the transmitter
`and receiver of any calling station through its associated
`high impedance circuits to said transmission line and for
`selectively connecting the receiver of any desired called
`station through its associated high impedance circuit to
`said line, means for transmitting signals with a first phase
`and first amplitude over said transmission line to the re
`ceivers of said calling and said called stations, individual
`circuit branches interconnecting the transmitter of each
`station with the receiver of such station, means for trans
`mitting signals with a second amplitude and a second
`phase over the corresponding individual circuit branch to
`the receiver of said calling station, and means for pro
`ducing a side-tone signal independent of the number of
`stations connected to said line by combining said signals
`of said first and second amplitudes and phases.
`15. The system of claim 14 wherein said high impedance
`means comprises high impedance transformer circuits.
`16. A four-wire conference call system comprising a
`plurality of telephone stations each having a transmitter
`and a receiver, means associated with each of said trans
`mitters for producing signals with a first phase and a first
`amplitude and signals with a second phase and a second
`amplitude, a first transmission line common to each of
`Said transmitters, a second transmission line common to
`each of said receivers, a plurality of high impedance cir
`cuits individually associated with each of said transmitters
`and receivers, means for selectively connecting the trans
`mitter and receiver of any calling station through its asso
`ciated high impedance circuits to said common transmis
`sion lines and for selectively connecting the receiver of
`any desired called station through its associated high im
`pedance circuit to said second transmission line, amplifier
`means for connecting said first and second transmission
`lines for transmitting said signals with a first phase and
`a first amplitude from the calling station to the connected
`receivers, individual circuit branches directly intercon
`necting the transmitter of each station with the receiver
`of the same station for transmitting said signals with a
`second amplitude and a second phase from the transmitter
`of said calling station to the receiver of said calling sta
`tion, and means for combining the signals of first and sec
`ond amplitudes and phases to produce side-tone signals in
`dependent of the number of stations connected to said
`lines.
`17. The system of claim 16 wherein said high imped
`ance means comprises high impedance transformer cir
`cuits.
`18. A conference call system comprising a plurality
`of telephone stations each having a transmitter and a
`receiver, a transmission line, a plurality of high im
`pedance circuits individually associated with each of said
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`transmitters and receivers, unidirectional first amplifier
`means permanently connected to each of said transmitters
`and a second unidirectional amplifier means permanently
`connected to each of said receivers, means for Selectively
`connecting the transmitter and receiver of any calling
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`station through its associated amplifiers and high im
`pedance circuits to said transmission line and for selec
`tively connecting the receiver of any desired called sta
`tion through its associated amplifier and high impedance
`circuit to said line, means for transmitting signals with
`a first phase and first amplitude over said transmission
`line to the receivers of said calling and said called sta
`tions, individual circuit branches interconnecting the first
`amplifier associated with the transmitter of each station
`with the second amplifier associated with the receiver
`of such stations, means for transmitting signals with a
`second amplitude and a second phase over the corre
`sponding individual circuit branch to the receiver of said
`calling station, means for transmitting voice communica
`tion through said system with an amplitude that is inde
`pendent of the number of stations connected to said line
`and means for producing a side-tone signal by combining
`the signals of first and second amplitudes and phases,
`also independently of the number of stations connected
`to said line.
`19. A four-wire conference call system comprising a
`plurality of telephone stations each having a transmitter
`and a receiver, a plurality of unidirectional amplifiers,
`transmitter impedance matching means in the input of
`said first amplifier, high impedance means at the output
`of said first amplifiers, means for connecting each of said
`transmitters respectively to the input of one of said first
`amplifiers, a plurality of second unidirectional amplifiers,
`high impedance means at the input of each of said sec
`ond amplifiers, receiver impedance matching means at
`the output of said amplifiers, means for connecting each
`of said receivers respectively to one of said second am
`plifiers, a transmit channel common to the output of each
`of said first amplifiers, a receive channel common to the
`input of each of said second amplifiers, means for selec
`tively connection the first amplifiers and second ampli- .
`fiers of any calling station to said common transmit and
`receive channels and for selectively connecting the sec
`ond amplifier of any desired called station through its
`associated second amplifier to said common receive chan
`nel, a high gain amplifier means for connecting said trans
`mit channels to said common receive channel, means for
`transmitting signals of a first phase and a first amplitude
`from the calling station to the connected receivers, in
`dividual circuit branches directly interconnecting the first
`amplifier of each station with the second amplifier of the
`same station, means for transmitting signals of a second
`amplitude and second phase over the corresponding in
`dividual circuit branch, and means within said second
`amplifier for combining the signals of first and second
`amplitudes and phases to produce proper side-tone signals
`independently of the number of stations connected to said
`lines.
`20. The system of claim 19 wherein each of the said
`first amplifiers comprises means for transmitting said
`Signals received from the associated transmitter with the
`said first phase and first amplitude, and means for in
`verting the signals received from said transmitter and
`feeding said inverted signal to the associated receiver to
`provide signals of a second amplitude and a second phase.
`21. The system of claim 19 wherein the means for
`combining the signals of first and second amplitudes and
`phases comprises a first and a second transistor in each
`of Said second amplifiers, means for connecting said sec
`ond transistor so that its output biases said first transistor,
`means responsive to the output of said first transistor
`for energizing the receiver connected to the amplifier,
`and means for applying the signals with a second ampli
`tude and a second phase to the input of the second
`transistor to control the bias on said first transistor and
`thereby to control the output of the second amplifier
`and hence, the side-tone in the receiver connected thereto.
`22. The conference call circuit of claim. 19, wherein
`said means for combining said first and second signals
`comprises a pair of transistors each having at least con
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`trol, input, and output electrodes, means for connecting
`the associated one of said individual circuit branches to
`said control electrode of one of said transistors, means for
`connecting said common receive channel to the control
`electrode of the one of said transistors and the input
`electrode of the other of said transistors, and means in
`cluding said last named means for deriving a