Trying to interface with intercom / door phone system

Hi

I have a project idea where I want to interface with my intercom / door phone system. Its a proprietary system developed by SSS Siedle, and there is next to no documentation or research on how these devices work.

I have hooked the incoming wires onto my scope and found that the device likely uses a combination of analogue and digital signals for its operation.

From my understanding the system operates as a serial bus where each flat is connected to each-other, and not directly to the central unit.

When idle the bus sits at 30V. Whenever I submit a command (Unlock front door, or start audio/vide) the signal/bus/incoming lead drops to 0Vmin and creates a square wave signal with 9Vmax. Once "one" the signal returns to 30V (Slightly above 30v the first few ms).

I believe I should be able to interface quite simply with this by using an optocoupler or simply just some resistors as a voltage divider.

The problem however is the audio/video part. From my scoping this seems to be modulated into the 30V (carrier?) signal. Whenever there's video transmitted over the bus I can see a 20MHz sine wave that with about 100mV variation between Vmin and Vmax.

How would I best go forth and read this signal? I do not necessarily need to process it on the Arduino (or ESP* or similar), but I would use the Arduino (Or ESP* or similar) to intercept the signal before passing it on to a computer or wirelessly or something.

Would I need an external 200+MHz ADC (ie. an ST ADC120?) coupled together with a resistor voltage divider or some other smart voltage divider?
Is this at all feasible?

PS: Im no EE and I am likely not familiar with anything that go beyond EE basics.

What do you want to interface with?

Do you want to be able to open the door with your Arduino?

Do you want to be able to view the video?

// Per.