Blaupunkt (in the early 80s) had a similar system which was a lot more reliable also based on ultrasound, they used an encoding that was a bit less likely to show up in normal environmental sounds.
Doesn't / didn't Blaupunkt make hi-fidelity audio equipment? I would think the range of sounds output by its speakers would make finding distinct sounds difficult. Maybe their audio expertise came in handy.
Yes, they did, but this was on a TV set, a - for the time - large color TV. It was a pretty weird little box, I have been searching but can't find an image for it.
