United States Patent (19)
`Hashemi et al.
`
`11)
`45
`
`4,139,731
`Feb. 13, 1979
`
`54) TELEPHONE CONFERENCESYSTEM WITH
`ACTIVE ANALOG CONFERENCE
`Mike A. Hashemi, Bolingbrook;
`(75. Inventors:
`Gregory P. Pucci, Westmont, both of
`1.
`73) Assignee: Wes.com, Inc., Downers Grove, Ill.
`21) Appl. No.: 832,788
`22 Filed:
`Sep. 12, 1977
`51) Int. C.’.............................................. HO4M 3/56
`52 U.S. C. ............................. 179/1 CN; 179/8 BC
`(58. Field of Search ............ 179/1 CN, 1 VC, 18 BC,
`179/1 HF, 170 D
`
`56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`3/1962 Hilliard et al. ...
`... 79/1 VC
`3,026,374
`3,060,265 10/1962 Duncan et al.
`... 179/1 CN
`3,135,829
`6/1964 Hultberg........
`... 79/1 CN
`3,730,995
`5/1973 Mathews ....
`... 179/1 VC
`3,824,344
`7/1974 James et al.
`... 179/1 CN
`3,891,801
`6/1975 Wang ................................ 79/1 CN
`Primary Examiner-Kathleen H. Claffy
`Assistant Examiner-Gerald L. Brigance
`Attorney, Agent, or Firm-Leydig, Voit, Osann, Mayer
`& Holt, Ltd.
`
`ABSTRACT
`57
`A telephone conference system includes a conference
`bridge for coupling multiple conference ports associ
`ated with corresponding conference stations. The
`bridge includes means for summing incoming signals
`from all the conference ports and supplying the summed
`signals to all the conference ports. A differential ampli
`fier is connected to each of the conference ports and
`differentially receives (1) incoming signals at the corre
`sponding conference port and (2) the output of the
`summing means so as to cancel from the output of the
`summing means any signal received by the summing
`means from the corresponding conference port. Thus,
`the differential amplifier passes only that portion of the
`output of the summing means that is received from
`conference ports other than the particular port to which
`that differential amplifier is connected. A voice actu
`ated switch is connected to each conference port for
`controlling the transmission of incoming audio signals
`to the respective ports, and the number of switches that
`can be closed at the same time is limited to a preselected
`maximum, while permitting the continuous transmission
`of outgoing audio signals from all the ports regardless of
`which switches are open or closed.
`9 Claims, 5 Drawing Figures
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`CSCO-1006
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`U.S. Patent
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`Feb. 13, 1979
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`Sheet
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`| of 4
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`4,139,731
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`CSCO-1006
`CISCO SYSTEMS, INC. / Page 2 of 10
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`CSCO-1006
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`U.S. Patent Feb. 13, 1979
`
`Sheet 2 of 4
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`4,139,731
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`sa2222222407 (24
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`CSCO-1006
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`U.S. Patent
`
`Feb. 13, 1979
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`Sheet 3 of 4
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`4,139,731
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`----
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`
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`LEHTZT, ZOE# – No.??H,
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`CSCO-1006
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`U.S. Patent
`
`Feb. 13, 1979
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`Sheet 4 of 4
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`4,139,731
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`22%-
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`CSCO-1006
`CISCO SYSTEMS, INC. / Page 5 of 10
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`

`

`1.
`
`15
`
`TELEPHONE CONFERENCE SYSTEM WITH
`ACTIVE ANALOG CONFERENCE
`DESCRIPTION OF THE INVENTION
`The present invention relates generally to telephone
`conference systems and, more particularly, to telephone
`conference systems that employ active analog confer
`ence bridges for coupling multiple conference stations
`10
`to each other.
`It is a primary object of the present invention to pro
`vide a telephone conference system that can accommo
`date any desired number of conference stations without
`degrading the quality or level of the transmitted signals
`and without loss of stability in the conference bridge. In
`this connection, one specific object of the invention is to
`provide such an improved telephone conference system
`which does not produce singing or other adverse effects
`that interfere with signal transmission.
`It is another object of this invention to provide an
`20
`improved telephone conference system of the foregoing
`type which includes an active analog conference bridge
`that introduces substantially zero loss into the confer
`ence system regardless of the number of conference
`ports connected to the system.
`25
`A further object of the invention is to provide such an
`improved telephone conference system that includes an
`active analog conference bridge that remains stable
`regardless of the transhybrid losses at the conference
`30
`ports.
`A still further object of the invention is to provide
`such an improved telephone conference system that
`permits multiple conference ports to be accessed to the
`bridge at the same time without producing objection
`able feedback through any given port.
`35
`Yet another object of this invention is to provide such
`an improved telephone conference system that can be
`efficiently manufactured and used in a wide variety of
`different applications at a relatively low cost.
`Other objects and advantages of the invention will be
`40
`apparent from the following detailed description and
`the accompanying drawings, in which:
`FIG. 1 is a block diagram of a telephone conferenc
`system embodying the invention;
`FIG. 2 is a circuit diagram of the conference bridge
`45
`included in the system of FIG. 1;
`FIG. 3 is a circuit diagram of one of the audio detec
`tors included in the system of FIG. 1; and
`FIGS. 4aa and 4b is a more detailed schematic dia
`gram of one exemplary embodiment of the conference
`50
`system illustrated in FIG. 1.
`While the invention will be described in connection
`with certain preferred embodiments, it will be under
`stood that it is not intended to limit the invention to
`these particular embodiments. On the contrary, it is
`55
`intended to cover all alternatives, modifications and
`equivalent arrangements as may be included within the
`spirit and scope of the invention as defined in the ap
`pended claims.
`Turning now to the drawings and referring first to
`60
`FIG. 1, there is illustrated a telephone conference sys
`tem having n conference stations 10 connected to a
`conference bridge 11 via in hybrids 12. In the illustrative
`system, the stations 10 are all two-wire stations, and
`thus the hybrids 12 are needed to interface these stations
`65
`with the four-wire ports P1, P2... Pn of the conference
`bridge 11. In a system where four-wire stations are used,
`the hybrids 12 can be eliminated and the stations con
`
`4,139,731
`2
`nected directly to the ports of the conference bridge.
`The bridge ports P1, P2. . . Pn receive signals from the
`transmit lines T1, T2 . . . Tn of the respective hybrids
`12, and return signals on the receive lines R1, R2... Rn
`of the respective hybrids. As will be apparent from the
`ensuing description, the illustrative conference system
`can be used with virtually any desired number of con
`ference stations and ports, although only three such
`stations and ports have been shown in the drawings in
`order to facilitate the description thereof.
`As is well known in the telephone system art, the
`basic purpose of a conference bridge is to couple an
`incoming signal at any conference port to all the other
`conference ports, but without feeding that signal back
`to the port at which it is received. These incoming
`signals, which of course originate at the stations 10, will
`be referred to herein as the "audio' signals, but it is to
`be understood that the illustrative conference system is
`also applicable to the transmission of data in digital
`form. Transmission of an audio signal from any given
`station 10 to its corresponding port P is controlled by a
`voice-actuated switch 13 (a "VOX') connected in the
`transmit line T leading from each hybrid 12 to the cor
`responding port P in the bridge. Thus the switches 13
`determine which of the stations 10 is "turned on,' i.e.,
`accessed to the bridge 11 in the transmit direction, at
`any given time. In the receive direction, all the stations
`10 continuously receive all the signals passing through
`the bridge.
`In accordance with one important aspect of the pres
`ent invention, the conference bridge 11 is an active
`analog bridge comprising means for summing signals
`from all the conference ports and supplying the summed
`signals to all the conference ports, and a plurality of
`differential amplifiers each of which is connected to one
`of the conference ports and differentially receives (1)
`incoming signals at the corresponding conference port
`and (2) the output of the summing means so as to cancel
`from the output of the summing means any signal re
`ceived by the summing means from the corresponding
`conference port, whereby each differential amplifier
`passes only that portion of the output of the summing
`means that is received from conference ports other than
`the particular port to which that differential amplifier is
`connected. Thus, referring to FIG. 2, incoming signals
`at each of the conference ports P1, P2... Pn are passed
`through a resistor R1 and an amplifier 20 to the invert
`ing input of a differential amplifier 21 whose output is
`connected to the receive line R of the corresponding
`hybrid 12. The principal purpose of the amplifier 20 at
`each port is to provide a signal source with a low output
`impedance. The gain of each amplifier 20 is controlled
`by a feedback resistor R2, which is typically selected to
`provide again of unity. The output of each amplifier 20
`is fed to the differential amplifier 21 through a resistor
`R3 so that the magnitude of the input signal to the in
`verting input of the amplifier 21 is less than the magni
`tude of the output from the amplifier 20 because of the
`drop across the resistor R3. For example, the magnitude
`of the input to the amplifier 21 from resistor R3 is typi
`cally about half of the magnitude of the output signal
`from the amplifier 20. The gain of the amplifier 21 is
`controlled by a feedback resistor R4, which is selected
`to provide a gain that produces a bridge output signal
`with the same amplitude as the corresponding input
`signal or signals.
`For the purpose of summing the incoming signals
`from all the conference ports P1, P2... Pn, the outputs
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`4,139,731
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`4.
`of all the amplifiers 20 are connected via resistors R5 to
`the hybrids 12 are sufficiently poor, the gain from the
`a summing amplifier 23. The gain of the summing ampli
`input terminal to the output terminal of a given port P
`fier 23 is controlled by a feedback resistor R6, and is
`could exceed the transhybrid loss of the hybrid 12 asso
`typically set to provide unity gain. From the summing
`ciated with that particular port and thereby render the
`amplifier 23, the summed audio signals are supplied via
`bridge unstable. Such instability would give rise to
`resistor R7 to the inverting input of an amplifier 24
`"singing' which would degrade the quality of the audio
`which adjusts the phase and amplitude of the summed
`signals passing through the bridge.
`signals to permit cancellation of individual components
`However, by limiting the number of conference sta
`thereof in the differential amplifier 21. More specifi
`tions that are permitted to transmit into the bridge A1 at
`cally, the amplifier 24 inverts the output of the summing
`10
`any given time, this instability can be avoided. For a
`amplifier 23 so that the output of the amplifier 24 is in
`typical worse case transhybrid loss of 8dB, it is pre
`phase with the output of the amplifier 20, which is in
`ferred to limit the maximum number of conference
`verted by the summing amplifier 23 before it reaches the
`stations that can simultaneously transmit into the bridge
`amplifier 24. The gain of the amplifier 24 is set by a
`to three or four so that the cumulative gain from the
`feedback resistor R8 so that the amplitude of each indi
`15
`input terminal to the output terminal of any given port
`vidual component of the composite sum signal at the
`P cannot exceed the transhybrid loss of the hybrid 12
`output of the amplifier 24 is the same as the amplitude of
`associated with that port. This allows the bridge to be
`that same individual signal as delivered to the inverting
`expanded to accommodate any desired number of con
`input of the differential amplifier 21 at the particular
`ference stations without endangering the stability of the
`port where that signal is received by the bridge. Conse
`20
`bridge, even though the output terminals of all the con
`quently, when the output of the amplifier 24 is delivered
`ference ports P remain active at all times.
`to the non-inverting inputs of all the differential ampli
`In the particular system illustrated in FIG. 1, the
`fier 21, the individual audio signal received by any
`voice-actuated switches 13 are turned on and off in
`given port is cancelled from the composite sum signal
`response to the presence and absence, respectively, of
`fed to the differential amplifier 21 at that particular port
`audio signals on the respective transmit lines T to which
`from the amplifier 24. Thus, the signal that is transmit
`the switches 13 are connected, as long as the number of
`ted back to each conference station 10 on its receive line
`switches closed at any given time is less than a prese
`Rincludes only the signals derived from the other con
`lected maximum. When the number of closed switches
`ference stations, so that a speaker at any conference
`reaches the preselected maximum, all the remaining
`station never hears his own voice being fed back to him.
`open switches are prevented from closing. Thus, the
`It will be appreciated that the operation described
`normal opening and closing of the switches 13, when
`above is the same for all the conference ports, regard
`less than the preselected maximum number of switches
`less of how many ports are included in the system. That
`are closed, is controlled by digital signals derived from
`is, the incoming signals from all the ports P1, P2... Pn
`audio detectors 30 each of which is connected to one of
`are fed to a single summing amplifier 23, and each port
`35
`the transmit lines T1, T2 ... Tn. The digital output of
`has its own separate differential amplifier 21 for cancel
`these audio detectors 30 goes from low (representing a
`ling the incoming signal at that port from the outgoing
`binary "0") to high (representing a binary "1'), when
`sum of signals transmitted through its amplifier 21 back
`ever the audio signal on the corresponding transmit line
`to the corresponding conference station 10. The bridge
`T exceeds a predetermined threshold level set by the
`can be easily expanded to accommodate virtually any
`audio detector, and the detector output remains at the
`desired number of conference stations because the num
`"1" level as long as the corresponding audio signal
`ber of stations connected to the bridge does not affect
`remains above the threshold level. The outputs of all the
`the loss through the bridge. Indeed, as viewed from the
`audio detectors 30 are fed into a register 31 which is
`input and output terminals of each conference port P,
`clocked each time a new audio signal appears on one of
`the loss through the bridge is substantially zero, and this
`the transmit lines T, and each clocking of the register 31
`remains true regardless of the number of conference
`ports included in the bridge. Furthermore, there is sub
`transfers the new set of "1's and "O's from the detec
`stantially no degradation of the quality of the audio
`tors 30 to the corresponding voice-actuated switch 13.
`signals as they are passed through the bridge.
`Those switches 13 that receive "O's from the register
`In accordance with another aspect of the present
`31 are opened (or remain open), and those switches that
`receive '1's are closed (or remain closed).
`invention, the voice-actuated switches between the
`The locking pulses for the register 31 are generated
`conference stations and the conference ports in the
`bridge are controlled so that the number of stations that
`each time one of the outputs from the audio detectors 30
`can transmit audio signals to the bridge at the same time
`changes from a "O' to a '1', i.e., each time a new audio
`is limited to a preselected maximum. The major factor
`signal appears on one of the transmit lines T. Thus, the
`limiting stability of the conference bridge 11 is transhy
`output of each detector 30 is connected to a monostable
`multivibrator 32 which generates a pulse capable of
`brid loss (four-wire return loss) of the two-wire/four
`clocking the register 31 each time the detector output
`wire hybrids 12 used with two-wire conference stations,
`changes from "0" to "1". These pulses from the multivi
`or the amount of feedback between the receive and
`transmit lines in the case of four-wire stations. Thus, a
`brators 32 are supplied to the clock input of the register
`fraction of each outgoing signal from the output termi
`31 through an OR gate 33 and an AND gate 34. As long
`nal of a conference port will return to its input terminal
`as the number of closed switches 13 is less than the
`due to transhybrid loss or feedback. All of these return
`preselected maximum, the AND gate 34 is enabled by a
`signals are summed in the summing amplifier along with
`binary "1" signal from a comparator 35 so that the
`all other incoming signals, and supplied to the differen
`pulses from the multivibrators 32 are passed through the
`65
`tial amplifiers 21 of all the conference ports. Conse
`gate 34 to clock the register 31 each time a new audio
`quently, if a sufficient number of conference stations 10
`signal appears on one of the transmit lines T1, T2 . . .
`are connected to the bridge, and if transhybrid losses in
`Tn.
`
`30
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`50
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`current flowing from the base of the transistor Q1
`For the purpose of disabling the AND gate 34 when
`through a resistor R16 and a zener diode D3. The result
`the number of closed switches 13 reaches the prese
`ing current flow through the emitter-collector circuit of
`lected maximum, the comparator 35 receives input sig
`the transistor Q1 produces a voltage drop across a resis
`nals from a summing amplifier 36 and a reference signal
`tor R17 which is the binary "1" output signal, or "audio
`source 37. The summing amplifier 36 receives and sums
`flag,' from the audio detector. 30. It should be noted
`the output signals from all the audio detectors 30, via
`resistors R9, thereby producing an output signal which
`that the threshold at which the illustrative audio detec
`tor will present a binary "1" at its output is determined
`is proportional to the number of transmit lines that are
`by the gain of the amplifier 38 and the values of the
`active at any given time. This signal is supplied to the
`resistors R13 and R4 (which determine the level of the
`comparator 35 where it is compared with a reference
`reference signal supplied to the comparator 39).
`signal generated by the source 37 and representing the
`When the audio signal is removed from the input of
`preselected maximum number of voice-actuated
`the amplifier 38, the capacitor C1 discharges through a
`switches 13 that can be closed at any given time.
`resistor R12. As the voltage at the capacitor C1 be
`As long as the signal from the summing amplifier 36
`comes more negative than the reference voltage sup
`remains below the level of the reference signal from the
`plied to the comparator 39, the output of the compara
`source 37, the comparator 35 produces an enabling
`tor 39 goes high, and the capacitor C2 recharges slowly
`binary "1" signal to the AND gate 34. However, when
`through the resistor R15. When the voltage at the ca
`the output signal from the summing amplifier 36 ex
`pacitor C2 becomes sufficiently high to turn off the
`ceeds the reference signal, the comparator 35 produces
`transistor Q1, current flow through the emitter-collec
`a binary "0" signal which disables the AND gate 34 to
`20
`tor circuit of the transistor Q1 is terminated, and the
`prevent any further clocking of the register 31. As long
`output of the audio detector 30 changes from a binary
`as clocking of the register 31 is prevented, the status of
`'1' to '0'. the zener diode D3 ensures that the transis
`the switches 13 cannot be changed. Consequently, re
`tor Q1 is turned off when the output of the comparator
`gardless of how many additional audio signals are pro
`39 is high.
`duced at the stations 10, no further switches 13 can be
`25
`It can be seen that the illustrative audio detector
`closed until termination of one of the audio signals pass
`provides "fast attack' and "slow release'. That is, the
`ing through a previously closed switch.
`presence of an audio signal changes the output of the
`Upon termination of one of the audio signals passing
`audio detector from a binary "0" to “1” much faster
`through a closed switch 13, the output from the corre
`than the termination of the audio signal changes the
`sponding audio detector 30 changes to a binary "0". At
`30
`output from a "1" to a "0", because the capacitor C2
`the same time the output signal of the summing ampli
`discharges rapidly through the diode D2 but charges
`fier 36 drops below the level of the reference signal, and
`more slowly through the resistor R15. This fast
`the comparator 35 again produces an enabling binary
`attack/slow-release feature is desirable to avoid chop
`“1” signal to the AND gate 34. Then when a new audio
`ping off a portion of the audio signal at either the begin
`signal appears on any of the transmit lines T1, T2 . . .
`35
`ning or the end of the signal.
`Tn, the output of the corresponding detector 30
`One of the particularly useful advantages of the con
`changes from a "0" to "1" to trigger the corresponding
`ference system provided by this invention is the ease
`multivibrator 32, and the resulting pulse is fed through
`with which it can be expanded to accommodate addi
`the OR gate 33 and the AND gate 34 to clock the regis
`tional conference stations. This advantage is illustrated
`ter 31. This opens the switch 13 in the line in which the
`by the system shown in FIG. 4, which also includes a
`audio signal is terminated, and closes the switch 13 in
`more detailed circuit diagram of the inputs to the com
`the line carrying the new audio signal hy transferring
`parator 35.
`the respective "1's and "O's from the detectors 30
`The system of FIG. 4 includes a “master' set of con
`through the register 31 to the corresponding switches
`ference stations 10 coupled through a bridge 11 in the
`45
`13.
`same manner described above in connection with
`In FIG. 3 there is shown an exemplary circuit com
`FIGS. 1 and 2. This "master' set of conference stations
`prising one of the audio detectors 30. An audio signal
`is interconnected with a similar set of "slave' confer
`from one of the transmit lines T1, T2 . . . Tn is fed
`ence stations 10' which are coupled through a confer
`through a resistor R10 to the inverting input of an oper
`ence bridge 11 in the same manner as the "master' set of
`ational amplifier 38 whose gain is determined by a feed
`50
`stations except for certain modifications in the control
`back resistor R11. The output of the amplifier 38 is
`circuit. More specifically, the output of the summing
`passed through an integrator formed by a diode D1 and
`amplifier 36' associated with the "slave' stations is not
`a capacitor C1 to generate a signal whose voltage level
`connected to a comparator, but rather is connected to .
`increases with increasing amplitude of the audio input
`the input of the master summing amplifier 36 through an
`signal. More specifically, the diode D1 rectifies the
`55
`amplifier 40. This amplifier 40 adjusts the voltage level
`output of the amplifier 38, and charges the capacitor C1
`of the output from the slave summing amplifier 36 so
`to a predetermined voltage level. This integrated signal
`that it corresponds to the level of an identical number of
`is applied to the inverting input of a comparator 39
`input signals to the master summing amplifier 36. That
`which receives a reference signal at its non-inverting
`is, the level of the output from the slave summing ampli
`input from a voltage divider comprising resistors R13
`fier 36' is increased so that the signals from the audio
`and R14.
`detectors 30' in the slave system have exactly the same
`When the voltage across the capacitor C1 becomes
`effect on the master summing amplifier 36 as the outputs
`more positive than the reference voltage supplied to the
`from the audio detectors 30 in the master system. Con
`comparator 39, the output of the comparator goes from
`sequently, the output of the master summing amplifier
`high to low. This quickly discharges a capacitor C2
`65
`36 represents the total number of audio signals being
`(which has been previously charged by the high output
`transmitted in both the master and slave systems. This
`of the comparator 39) through a diode D2. As the ca
`output signal from the master summing amplifier 36 is
`pacitor C2 discharges, it turns on a transistor Qi with
`
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`supplied via resistor R18 to the comparator 35, where it
`is compared with a reference signal derived from a
`reference voltage source Vr via a voltage divider com
`prising resistors R19 and R20.
`As explained previously, the output of the compara
`tor 35 indicates whether the number of audio signals
`being transmitted is above or below the preselected
`maximum at any given time. In the particular system
`illustrated in FIG. 4, the output of the comparator 35
`indicates whether the total number of audio signals
`10
`being transmitted in both the master and slave sets of
`conference stations is above or below a preselected
`maximum for the entire combined system. It is neces
`sary that this single limit be set for the entire system
`because the two conference bridges 11 and 11" must nbe
`interconnected so that audio signals generated at the
`stations in the master set are transmitted to the stations
`in the slave set, and vice versa. Thus, the "limiting”
`output from the comparator 35 is supplied to both the
`AND gates 34 and 34 which control the clocking of the
`20
`registers 31 and 31' in the control systems for the two
`sets of conference stations. In order to clock both these
`registers 31 and 31' in response to a clocking pulse from
`either AND gate 34 or 34, the outputs of the two AND
`gates are supplied to an OR gate 41 which has its output
`25
`connected to the clock inputs of both registers 31 and
`31'. Consequently, the appearance of a new audio signal
`from either set of conference stations 10 or 10' (while
`the number of closed switches 13 and 13' is still below
`the preselected maximum) clocks both registers 31 and
`30
`31'.
`To interconnect the two bridges 11 and 11", a pair of
`switches S1 and S2 (see FIG. 2) in the slave bridge 11"
`are opened. The opening of switch S1 connects the
`output of the summing amplifier 23 in the slave bridge
`11' to the input of the amplifier 24 in the master bridge
`11 via line 50, thereby summing the outputs of the sum
`ming amplifiers 23 in the two bridges. Opening switch
`S2 in the slave bridge 11' connects the output of the
`amplifier 24 in the master bridge 11 to the voltage-fol
`lower amplifier 25 in the slave bridge 11' via line 51, so
`that the sum of all the incoming signals from both brid
`ges is supplied to all the differential amplifiers 21 in both
`bridges. It will be understood that the lines 50 and 51
`from the two bridges must be connected to each other.
`45
`As can be clearly seen from the combined system of
`FIG.4, a few simple modifications and interconnections
`between the control systems for two different sets of
`conference stations permits those stations to be easily
`integrated into a single conference system. Further
`50
`more, this expansion of the number of conference sta
`tions, and corresponding conference ports in the bridge,
`can be continued indefinitely without altering the sub
`stantially zero loss across the input and output terminals
`of each and every conference port, and without produc
`55
`ing singing or otherwise degrading the quality of the
`signals being transmitted through the conference
`bridge.
`We claim as our invention:
`1. A telephone conference system comprising
`a telephone conference bridge for coupling multiple
`conference ports, said bridge comprising
`means for summing incoming signals from all the
`conference ports and supplying the summed
`signals to all the conference ports, and
`65
`a plurality of differential amplifiers each of which
`is connected to one of the conference ports and
`differentially receives (1) incoming signals at the
`
`8
`corresponding conference port and (2) the out
`put of said summing means so as to cancel from
`the output of said summing means any signal
`received by said summing means from said cor
`responding conference port, whereby each dif.
`ferential amplifier passes only that portion of the
`output of said summing means that is received
`from conference ports other than the particular
`conference port to which that differential ampli
`fier is connected,
`a plurality of voice-actuated switches each of which
`is connected to one of said conference ports for
`controlling the transmission of incoming audio
`signals to the respective ports, and
`limiting means connected to said switches and re
`sponsive to the audio signals transmitted to all the
`conference ports for limiting the number of said
`switches that can be closed at the same time to a
`preselected maximum, while permitting the contin
`uous transmission of outgoing audio signals from
`all said ports.
`2. A telephone conference system comprising a con
`ference bridge as set forth in claim 1,
`a plurality of conference stations,
`a plurality of voice-actuated switching means each of
`which is connected between one of said conference
`stations and the input terminal of the correspond
`ing conference port for controlling the transmis
`sion of incoming audio signals to the respective
`ports,
`means for closing each of said switching means in
`response to the commencement of an audio signal
`from the conference station connected thereto, and
`means for opening the closed switching means in
`response to commencement of an audio signal from
`another conference station in the absence of an
`audio signal from the conference station connected
`to the closed switching means.
`3. A telephone conference system as set forth in claim
`2 which includes limiting means responsive to audio
`signals from all the conference stations for disabling said
`closing means for all open switching means when audio
`signals are being transmitted from a preselected maxi
`mum number of the conference stations.
`4. A telephone conference system as set forth in claim
`2 which includes a first set of transmit lines associates
`with a first set of switch closing means, and a second set
`of transmit lines associated with a second set of switch
`closing means,
`audio detecting means connected to each transmit
`line for generating a digital signal in response to the
`presence of an audio signal on the corresponding
`transmit line,
`first and second summing means for summing the
`digital signals from the converters connected to the
`two sets of transmit lines to produce signals respre
`senting the number of transmit lines on which
`audio signals are present,
`a reference signal generator for generating a refer
`ence signal representing a preselected maximum
`number of transmit lines on which audio signals are
`permitted to be present at the same time,
`first and second gates for transmitting said digital
`signals from the converters connected to the two
`sets of transmit lines to the two sets of switch clos
`ing means, and
`limiting means responsive to the output signals from
`both summing means and to said reference signal
`
`CSCO-1006
`CISCO SYSTEMS, INC. / Page 9 of 10
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`5
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`10
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`15
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`4,139,731
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`7. A telephone conference system as set forth in claim
`for generating a master limit signal for disabling
`6 which includes
`both said first and second gates when the total
`means for opening the closed switching means in
`number of transmit lines on which audio signals are
`response to commencement of an audio signal from
`present exceeds said preselected maximum.
`another conference station in th