Audio Design Question

[lovis]

I’m currently working on a project to create an adapter that connects a mobile network to a landline. I have an old rotary phone that I want to use on my desk. However, I don’t want to modify or destroy the internals of the phone. To achieve this, I’m using an interface board from Silvertel (AG1171).

The Question:
The mobile modem datasheet includes an audio reference design. From what I understand, the microphone and speaker signals are not referenced to ground. On the other hand, the AG1171 board requires a signal that is referenced to ground.

Can somebody maby point me in the direction what i have search to find more information about something like this. Sorry if this is a really stupid question. Thanks
I have some images of the reference designs

[SteveThackery]

My background is in telecommunications, but I don't fully understand what you wish to achieve.

Why would you want to connect a landline to the mobile network? And what do you mean by "connect"?  Or do you mean you want to use an old rotary phone to make and receive calls on your mobile network?  (In which case no landline will be involved.)

[Roehrenonkel]

Hi lovis,

welcome to the forum. :-)

If you have a 611 or 612 (most common) it has probably a "Kohlemikrofon" (coal-corn-mic)
and needs higher bias than the electret-condenser-mike in the app-note.
To connect asymmetric source to the interface just ground the *_N-input.
Use your search-engine for "Hybrid-transformer". ;-)

Would be cool to have such a phone in the car, with the Original bell-ring. :-)))
But the ringger needs higher voltage (AC) than 5 Volts.
Be sure to report about you progress.

Good luck