How does a video entry system send audio and video, as well as ring/unlock over 4 wires?

Question

I’ve been trying to understand how my door entry system works.

Between the monitor/base station and the bell/camera/speaker there’s only 4 wires:

• 12V DC
• GND
• Voice (2 way)
• Video (1 way)

I opened it up to find a few ICs. One in the base station seemed to be an ASIC specifically for this kind of device, but the datasheet didn’t say anything about ringing/sending unlock command.

Another IC turned out to be a “voice-switched speakerphone”: https://www.profusionplc.com/parts/mc34118g-s28-r

I’m guessing the ‘dial tone’ detector may be something to do with how the base station activates when someone presses the bell button?

I have no idea though how the relay at the remote end can be controlled by the base? There’s a simple two pin header that completes a circuit with the strike, but it’s not clear to me how that gets activated.

So far as I can tell the video line is just composite - though I’ve read some data can be stuffed in a not visible part. I doubt that would be in the base -> remote direction though.

Answer 1

I think I finally found the answer: a “stone transmission bridge”. https://www.instructables.com/id/2-Phone-Intercom-with-Buzzers/

Only difference I guess with an intercom is one side rings, but the other side’s ring is used for a door strike instead.