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Audio Integration with Doorbell Intercom

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Hi All,

I am trying to integrate SBC (RPi) to a specific door bell intercom system. I have opened and tried to understand how the intercom works and can share the following circuit information:
The indoor unit (that contains the headset) is powered by AC which is stepped down and rectified to supply the primary 7809 linear Voltage regulator. At the heart of this circuit is a KA2201 Audio Amp IC.

Now comes the part I found interesting.

The Outdoor unit (containing the bell button, mic and speaker) is connected to the indoor unit via only Two wires. On inspection and probing I found that these wires are Gnd and the 9V output of the 7809. o_O It seams the system is modulating the audio onto the Vcc line that also powers the outdoor unit. talk about cost saving measures.

I am trying to design a circuit that is able to inject and read audio from this line using Texas Instruments - ADC (PCM1861) and DAC (PCM5102) which should allow me to read and write audio via the I2S interface. This circuit is powered by the same RPi PSU and is will only require the audio signal from the intercom line.

I was able to test audio insertion over the line using only a coupling capacitor and an aux cable connected to me cell phone, however using only a capacitor this does not feel like a good solution.

I would like to know, what circuit/protections could I use to correctly integrate onto this pwr/audio line while protecting my down stream components? Should I use an audio transformer to isolate? if so would you be able to advise on a potential circuit as I have zero experience with these?

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