Bilateral Current Source for a Multi-Terminal Intercom

This invention relates to audio circuits for an intercom system, and more particularly, to a bilateral current source which operates as a two-wire to four-wire converter.

In an intercom system which is designed for two-way communication between a plurality of stations, it is standard practice to connect all the stations by a single wire pair. Since each station both transmits and receives signals on this same common pair of wires, some local means is necessary to isolate the receiving circuit or load from the transmitting circuit or local source. This has been accomplished in various ways, such as by use of the transformer hybrid circuit commonly used in telephone circuits and by sidetone suppressor circuits which cancel out the local voice signal from the speaker output.

The present invention is an improvement in that it provides a simpler and more reliable circuit which functions as a bilateral current source for both driving all the other receiving stations on the line from the local source (microphone or the like) and also providing an output to the local load (speaker, earphone, or the like) from the signal received on the line from other stations. It presents a high input impedance to the line, allowing a number of stations to be connected to the line. At the same time the circuit provides the two-wire to four-wire conversion by suppressing sidetones in the output to the receiver or local load.

These and other advantages of the present invention are achieved by a circuit comprising a differential type operational amplifier having the local voice signal connected to the negative input through a first resistor, the input being also connected to the output terminal of the amplifier by a second resistor. A potentiometer is connected in shunt across the two resistors, the tap on the potentiometer providing the output to the local receiver. The communication line is connected to the positive input of the amplifier by a third resistor and to the output terminal by a fourth resistor in series with the third resistor and that is small in value compared to the other resistors. The positive input is also connected to a reference potential source through fifth resistor. The amplifier acts as a high impedance current source for transmitting voice signals to the line and acts as a buffer for signals received from the line. Sidetones from the local voice signal are canceled out of the signal to the receiver by the potentiometer.

For a better understanding of the invention reference should be made to the accompanying drawings, wherein:

FIG. 1 is a block digram of one station in an intercom system; and

FIG. 2 is a schematic circuit diagram of the two-wire to four-wire converter of the present invention.

Referring to the drawings in detail, FIG. 1 shows one station of a multi-station intercom system in which all of the stations are connected by a two-wire communication line 10. Each station includes a transmitter or local audio signal source, such as a microphone 12 and preamplifier 14. Each station further has a receiver or local audio load including an amplifier 16 and a speaker, earphone or other audio transducer 18. A two-wire to four-wire converter circuit 20 operates to connect the voice signal from the transmitter to the line 10 for transmission to other stations. The converter 20 also couples signals received on the line from other stations to the receiver. The converter provides isolation of the local receiver from the voice signal output of the local transmitter. It also provides a high impedance voltage controlled current source connected to the line 10 so as to minimize the loading effect of multiple stations on the line.

Referring to FIG. 2, the two-wire to four-wire converter circuit 20 is shown in detail. The voice signal from the preamplifier 14 is connected to the positive input of a differential type operational amplifier 22. The negative input of the differential amplifier 22 is also connected to the output terminal through a feedback resistor 24. Amplifier 22 functions only as a buffer, having substantial unity gain. The voice signal at the output of the amplifier 22 typically has a voltage of the order of 3 volts peak to peak. The buffer circuit operates as a very low impedance source of the voice signal.

The output is coupled through an isolation capacitor 26 to the negative input of a second differential type operational amplifier 28 through a series resistor 30. A feedback resistor 32 connects the output terminal of the amplifier 28 to the negative input terminal. The output terminal of the amplifier is connected through a relatively small resistor 34 to the line 10 through another isolation capcitor 36. The junction between the resistor 34 and the capacitor 36 is connected to the positive terminal of the differential amplifier 28 through a resistor 38. Also the positive input terminal of the amplifier 28 is connected to a reference voltage source through a resistor 40. The resistors 30 and 40 are preferably of equal value as are the resistors 32 and 38. While isolation capacitors 26, 36, and 44 are shown, it will be appreciated that direct coupling may be used if the d.c. reference voltages are set at the proper levels for the various solid state elements in the amplifiers connected by the common line. The capacitors merely provide coupling of the a.c. audio signal while allowing the coupled circuit elements to operate at whatever d.c. levels are available.

As thus far described, the circuit operates as a high impedance voltage controlled current source for transmitting a voice signal on the common line 10. The transconductance of the circuit is given by the following expressions

Thus for the values shown, the transconductance is equal to

The output impedance is given by the following expression ##EQU1## For one percent resistors, the minimum output impedance, ##EQU2## Assuming a 1.5 volt input signal, the circuit therefore provides a driving current of 5 milliamps at a source impedance in excess of 10,000 ohms. The resistor 34 is made small, preferably less than one percent of the resistor 38, to get the desired transconductance.

Signals received from other stations over the line 10 are applied to the positive input terminal of the differential amplifier 28. In the receiving mode, the amplifier 28 acts as a buffer amplifier in the same manner as the amplifier 22. The signal received from the line 10 is applied to the receiver amplifier 16 through a potentiometer 42, capacitor 44 and volume control potentiometer 46. To prevent any sidetones resulting from the local voice signal being coupled to the receiving amplifier 16, the other end of the potentiometer 42 is connected to the output terminal of the buffer amplifier 22. Because the differential amplifier 28 inverts the input signal, the signals at opposite ends of the potentiometer resulting from the local voice signal are of opposite phase. Therefore, by adjusting the tap on the potentiometer 42, the potentiometer can be set at a null point where the two signals exactly balance each other out. The tap on the potentiometer will be centered if e.sub.o =-1. e.sub.i where e.sub.o is the voltage at the output of the amplifier 28. This condition will apply if R32/R30=R.sub.L /R.sub.L + R34, where R.sub.L is the impedance of the line 10, which typically is 200 ohms. As a result, the local voice signal does not produce any sidetone on the local receiver.

From the above description it will be seen that the present invention provides a bilateral current source for putting a voice signal on the line or driving the receiver from a voice signal received over the line. The circuit functions as a two-wire to four-wire converter since the circuit isolates the voice signal source from the local receiver by virtue of the null balance effect of the potentiometer 42. The circuit acts like a relatively high impedance converted to the common line and therefore a large number of terminals can be connected to the line without loading the line. The circuit does not function to terminate the line and therefore does not have to match the impedance of the line. While the circuit has been described in its preferred embodiment for coupling audio signals to or from a common line in an intercom system, the circuit is not limited to only audio signals, but is applicable to use with a digital bus in a data processing system, for example.