I'm a mechanical engineer myself. I've built microphones before in a college laboratory where all we used (proved) was "any object that vibrates from sound, which can be used to alter an electrical current, can be turned back into sound"
Even the most basic form of digital audio stored as PCM (redbook, etc) is actually nothing but a file where every data-point in the file is an integer representing the 'magnitude' of a disturbance during that short milli/microsecond of time. Any vibrating 'disturbance' is the very definition of "sound" itself too.
I'd consider myself an electrical engineer. How do you suppose you would transform and capture those vibrations to electrical signals with enough fidelity? I'm aware of techniques like perhaps laser diode self mixing. How will you do it in a way that cannot be detected even by experts?
What I'm getting at is: how do you capture those signals without the use of obvious silicon devices (so no MEMS or opamps or whatever)? I'd imagine you have to hide it in software somehow. Then how do you acquire that signal? A simple analog input pin to the ADC of a cheap microcontroller is not good enough.
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.
Even the most basic form of digital audio stored as PCM (redbook, etc) is actually nothing but a file where every data-point in the file is an integer representing the 'magnitude' of a disturbance during that short milli/microsecond of time. Any vibrating 'disturbance' is the very definition of "sound" itself too.