Electronic – Phone line to 3.5mm audio line

analogaudiotelephone

I need to splice out the audio feed from the hand set line (4P4C jack) to a 3.5mm audio port on a computer to interoperate the call via speech recognition. I do not meddle around too much with analog signals, nor have I messed around with phone lines. I have found several pinout guides for the 4P4C connector, but not the technicalities I need to abide by. The primary guide I was referring to was:
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My overall plan was to cut a pre-existing handset coord and splice out the green and red lines. The mic lines black and yellow are not needed and will be left alone. The red wire will be tied to a potentiometer for varying volume and to satisfy the schematic (a \$500 \mbox{ } k\Omega\$ one is laying around). The audio will then be supplied to a 3.5mm jack red on the center ring with the green on the ground ring (the ring furthest from the tip). This will then be feed into the light blue jack on a computer.

Any advice? I would not like to plug this into my sound card and trash my audio in, or the whole sound card.

Best Answer

If you intend to use the mic input mic input to your sound card, it will have a current source or pullup resistor to supply a bias to the microphone to which it was intended.

It might not be good for the speaker to have DC flowing through it.

If your phone is connected to the telco, be very careful. The speaker may run at quite a CM voltage and the DC offset might be -48V (depending which way round it is wired).

Telephones wires are very good at attracting lightning. During a storm high common mode voltages may be on the earpiece.

Stricly speaking you should use an audio transformer to couple the earpiece signal into the sound card, this provides DC isolation and some protection against lightning. Adding a TVS across the output of that transformer would be a good idea too.

The ac signal on the earpiece might be quite small. Only a few hundred mV. For a mic input, this will be fine (and can be potted down at the output of your transformer) but for line levels, which are usually 1Vrms it might be very quiet.

Not all phones work the same way, so measure the voltage first using a scope and judge the divsor as required.

The audio transformer should also be protected against DC usign a DC blocking (aka coupling) capacitor.

This circuit protects against discharge and DC currents, though use with caution as a mistake will cost you a PC!
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