Comment by reaperducer

Did you explore using frequencies outside the range of human hearing?

Amazon had modems very much like this in its little buttons that you could stick to your refrigerator and automatically order different items. When setting up the device, you could only hear the little clicks as it turned on and off.

I loved the technology. Hated that the prices changed all the time and you never knew what price you were going to pay ahead of time.