Autobalance hybrid circuit

An autobalance hybrid circuit for connecting a 4-wire telephone line having separate receive and transmit pairs to a 2-wire line, comprising:

• a balancing network having an impedance that is adjustable in response to a control signal,
• a bridge formed of (a) a 2-wire telephone line coupling transformer connected to said 2-wire line, (b) a balancing network coupling transformer connected to said network, and (c) the two sides of the center-tapped primary of an output transformer, the secondary of said output transformer being connected to the 4-wire line transmit pair,
• the 4-wire line receive pair being connected across two nodes of said bridge, between the center tap of said output transformer primary and the junction connecting said coupling transformers, and
• comparator means, connected to the other two nodes of said bridge at the junctions connecting the ends of said center-tapped primary with the respective coupling transformers, for comparing the envelope voltages developed across said two coupling transformers in the bridge, and for providing a control signal to said balancing network which causes adjustment thereof so as to equalize said voltages, said hybrid circuit in this manner compensating for changes in the impedance of said 2-wire telephone line and preventing any audio input from said 4-wire line receive pair from being returned via said output transformer onto said 4-wire line transmit pair, said comparator means comprising:
• a first rectifier circuit having a low threshold, said first rectifier circuit being connected across the secondary of said 2-wire telephone line coupling transformer and developing a first voltage signal indicative of the magnitude of the envelope of an audio signal appearing across that 2-wire telephone line coupling transformer,
• a second rectifier circuit having a low threshold, said second rectifier circuit being connected across the secondary of said balancing network coupling transformer and developing a second voltage signal indicative of the magnitude of the envelope of an audio signal appearing across that balancing network coupling transformer, and
• a differential amplifier having two comparison inputs connected respectively to said first and second rectifier circuits, said differential amplifier comparing said first and second voltage signals and producing a control signal indicative of the difference therebetween, the output of said differential amplifier being connected to said balancing network so as to provide said control signal thereto, and wherein each rectifier circuit comprises:
• a first operational amplifier having a non-inverting input connected directly to the secondary of the respective 2-wire telephone line coupling transformer of balancing network coupling transformer,
• a pair of diodes, one polarity terminal of one diode and the opposite polarity terminal of the other diode being connected to the output of said first operational amplifier, and
• a second operational amplifier having its inverting and non-inverting inputs connected respectively to the other terminals of said pair of diodes so as to provide an output voltage signal indicative of the difference between the voltages developed across said pair of diodes.

An autobalance hybrid circuit for connecting a 4-wire telephone line having separate receive and transmit pairs to a 2-wire line, comprising:

• a balancing network having an impedance that is adjustable in response to a control signal,
• a bridge formed of (a) a 2-wire telephone line coupling transformer connected to said 2-wire line, (b) a balancing network coupling transformer connected to said network, and (c) the two sides of the center-tapped primary of an output transformer, the secondary of said output transformer being connected to the 4-wire line transmit pair,
• the 4-wire line receive pair being connected across two nodes of said bridge, between the center tap of said output transformer primary and the junction connecting said coupling transformers, and
• comparator means, connected to the other two nodes of said bridge at the junctions connecting the ends of said center-tapped primary with the respective coupling transformers, for comparing the envelope voltages developed across said two coupling transformers in the bridge, and for providing a control signal to said balancing network which causes adjustment thereof so as to equalize said voltages, said hybrid circuit in this manner compensating for changes in the impedance of said 2-wire telephone line and preventing any audio input from said 4-wire line receive pair from being returned via said output transformer onto said 4-wire line transmit pair, said comparator means comprising:
• a first rectifier circuit having a low threshold, said first rectifier circuit being connected across the secondary of said 2-wire telephone line coupling transformer and developing a first voltage signal indicative of the magnitude of the envelope of an audio signal appearing across that 2-wire telephone line coupling transformer,
• a second rectifier circuit having a low threshold, said second rectifier circuit being connected across the secondary of said balancing network coupling transformer and developing a second voltage signal indicative of the magnitude of the envelope of an audio signal appearing across that balancing network coupling transformer, and
• a differential amplifier having two comparison inputs connected respectively to said first and second rectifier circuits, said differential amplifier comparing said first and second voltage signals and producing a control signal indicative of the difference therebetween, the output of said differential amplifier being connected to said balancing network so as to provide said control signal therto, and wherein said balancing network comprises:
• a capacitor and a light-dependent resistor connected as an impedance network across the primary of said balancing network coupling transformer,
• a lamp situated to illuminate said light-dependent resistor, and
• a lamp supply circuit connected to said lamp and connected to said comparator means to receive said control signal therefrom, said supply circuit providing to said lamp an amount of power established by the magnitude of said control signal.

An autobalance hybrid circuit for connecting a 4-wire telephone line having separate receive and transmit pairs to a 2-wire duplex telephone line, comprising:

• a balancing network having an impedance that is adjustable in response to a control signal,
• a first coupling transformer having its primary winding connected to said 2-wire telephone line,
• a second coupling transformer having its primary winding connected across said balancing network, the primary windings of said first and second coupling transformers being connected in series,
• a third coupling transformer having its secondary winding connected across the series-connected primary windings of said first and second coupling transformers, the primary of said third coupling transformer being connected to the received pair of said 4-wire telephone line,
• first and second rectifier circuits connected respectively to the secondary windings of said first and second coupling transformers for deriving therefrom a pair of envelope voltages,
• a comparator connected to the outputs of said rectifier circuits for comparing the envelope voltages developed thereby and for producing a control signal indicative of the difference therebetween, said control signal being supplied to said balancing network and causing adjustment of the impedance thereof so as to equalize said envelope voltages,
• an operational amplifier having an input summing point, the secondaries of said first and second coupling transformers both being connected to said summing point with respective polarities such that when said envelope voltages are equal, the portion of the audio incoming from said 4-wire receive line and coupled to said summing point via said first and third coupling transformers will cancel the portion of said incoming audio supplied to said summing point via said second and third coupling transformers, the output of said operational amplifier being connected to the transmit pair of said 4-wire telephone line.