Right, the fidelity is not that good, when just sampling arbitrary vibrating objects. For example a simple accelerometer attached to the center of a lamp shade might create an intelligible signal, but one attached to a heavy table might not. Remember, AI can build speech from a very noisy signal, however.
To pick up a better signal you need either a large flat or concave object (like even a computer case), or else something small and flimsy. You may know that even shining a laser pointer at the window of a building can pick up sound from a mile away simply by using a telescope to capture video of it and then let video processing detect the miniscule jostle in position due to the vibrating window. That trick has been used for decades, and works well.
But yes recording stuff secretly from an electrical device means you need either computer microcode or software secretly onboard that is recording signal variations that emerge from these physical vibrations, but there is almost no way, even after full disassembly, that even an electronics expert can necessarily detect this setup.
Why not? You can't hide microcode or MEMS IMU elements or analog circuitry in a way that decapping and examination by experts cannot uncover. This very article shows acid etching the epoxy packages to inspect the die. There are experts like Ken Shirriff (kens on HN) who reverse engineer silicon devices at the transistor level.
How do you arrive at the conclusion that microcode or software cannot be detected even by experts?
Sure given enough time to study chips and microcode it can be reverse engineered. Despite my degree in M.E. I've been a software developer for 30 yrs. :) So I know you're right.
I was just saying it's a practical impossibility for even an electronics expert and even after disassembling something, to detect if a device had been recording or not, just by examining the device physically.
Once you start reverse engineering the code itself you're likely to find any hidden surveillance, but even then not guaranteed, because sufficiently obfuscated code can camouflage what it's true purpose was.
To pick up a better signal you need either a large flat or concave object (like even a computer case), or else something small and flimsy. You may know that even shining a laser pointer at the window of a building can pick up sound from a mile away simply by using a telescope to capture video of it and then let video processing detect the miniscule jostle in position due to the vibrating window. That trick has been used for decades, and works well.
But yes recording stuff secretly from an electrical device means you need either computer microcode or software secretly onboard that is recording signal variations that emerge from these physical vibrations, but there is almost no way, even after full disassembly, that even an electronics expert can necessarily detect this setup.