

In a telephone system both the mic and the speaker signal are multiplexed into one signal. To be able to connect your phone to you mic-in and line-out of your pc you have to de-multiplex these signals.

The solution of Chris was to extract the mic an speaker signal before it is multiplexed inside the phone.

But this can also be done by a transformer (which is also used to prevent the 9V DC from going into you soundcard). The kind of transformer used for this application is a so called secondary centre tapped transformer. Meaning that it has 2 connections at its primary side (where the telephone will be connected) and 3 connections at its secondary side. The middle connection is physically connected to the middle of the secondary coil of the transformer. This middle connector is used as a shared ground for both the mic and the line-out.

Another issue is the input impedance of a phone line. When a phone line doesnâ€™t see the right input impedance reflections will occur, resulting in echoes or even in disabling the line. A telephone line has a input impedance of 600 Ohms, so the transformer has to be a 600 Ohm transformer. At the secondary side of the transformer a 150 Ohm resistor has to be placed at the middle connection to make the secondary input impedance 600 Ohm as well, resulting in a balanced transformer. The second part is the tricky part. A normal telephone system uses only two wires to send both the microphone and the speaker signal. From basic electronics you might know that you need 2 wires to send a signal, and at least 3 to send 2 signals, because one of the wires is acting as a reference (usually called ground).In a telephone system both the mic and the speaker signal are multiplexed into one signal. To be able to connect your phone to you mic-in and line-out of your pc you have to de-multiplex these signals.The solution of Chris was to extract the mic an speaker signal before it is multiplexed inside the phone.But this can also be done by a transformer (which is also used to prevent the 9V DC from going into you soundcard). The kind of transformer used for this application is a so called secondary centre tapped transformer. Meaning that it has 2 connections at its primary side (where the telephone will be connected) and 3 connections at its secondary side. The middle connection is physically connected to the middle of the secondary coil of the transformer. This middle connector is used as a shared ground for both the mic and the line-out. Another issue is the input impedance of a phone line. When a phone line doesnâ€™t see the right input impedance reflections will occur, resulting in echoes or even in disabling the line. A telephone line has a input impedance of 600 Ohms, so the transformer has to be a 600 Ohm transformer. At the secondary side of the transformer a 150 Ohm resistor has to be placed at the middle connection to make the secondary input impedance 600 Ohm as well, resulting in a balanced transformer. This all might seem complicated but as can be seen from this figure, the circuit is pretty simple and small.