A guy mucking around with an RTLSDR discovered something disturbing… a signal at 24MHz that turned out to be room audio from his office. No, this wasn’t an eavesdropping device. The “bug” seems to be the microphone preamp and ADC from his laptop soundcard — the ADC operates at 24MHz.
I’d say this is a fine example of why it’s worth getting the service manual (or iFixit guide) for your laptop. So that you can proceed to cleanly remove or at least unplug the camera and microphone and Bluetooth card and anything else you don’t need.
The vaguely ironic thing is right near 24MHz used to be a very common frequency (27MMHz) for baby monitors, ultra-cheap room bugs, remote control toys, etc. So if you were intentionally looking for bugs and found room audio on 24MHz, it would be logical to look for the re-purposed and probably modified baby monitor…
“I accidentally stumbled upon a signal in the 24MHz range, appearing to be 4 carriers. I tuned to it and heard silence, then someone came into my office and started talking and I could hear them speak. The signal appeared to be coming from my other laptop (not the one running the SDR) and was pretty weak (my antenna, the crappy one that comes with the dongle, stuck to a metal stapler was right next to the HP laptop). Here’s a picture
Both mics transmit independently, in the picture I rubbed one mic. The signal appears to be mirrored.
When I tap the microphone, or make a loud noise that would clip the preamp, the signal drifts off and then slowly comes back to its original frequency, as illustrated here (only one of the two mics drifted, if I hit it harder or clip both mics, both will drift).
I’m pretty sure that if I build a nice high-gain antenna optimized for 24Mhz I would be able to pick up the sound from some distance away. The laptop is an EliteBook 8460p. I have checked identical laptops and they do not transmit at this frequency. I didn’t have the time to scan the full spectrum though. I’m guessing the preamp is really crappy and somehow ends up transmitting FM at HF freqs.
Anyone has any ideas about this? I work in a high security setting and having laptops transmitting audio from everyone’s office/meeting room etc is a really big deal. I somewhat doubt it to be an intentional listening device due to the weird frequency drifting. For now I guess I’ll just disconnect the mic preamp pcb.”
“Looking at the first microphone ADC Google could find me, it has a rated clock frequency of around 24 MHz. Maybe the ADC is never turned off (power savings, anyone?) and leaks timing information through crappy filtering and shielding.[…]
it’s easy to accidentally create FM – all you need is something to be slightly non-linear and have an unexpected multiplicative effect on your signals. see, for a pathological example, http://en.wikipedia.org/wiki/Rusty_bolt_effect
i haven’t looked at EMC stuff for many years, but the ADC clock being accidentally coupled into a path which isn’t linear could absolutely cause this, and no preamp is completely linear 😉
the drift after clipping sounds like the automatic gain control on the preamp kicking in, but i couldn’t speculate as to why that would change the carrier frequency.
sounds like a bad case, but it could be as simple as dirt or damaged components…”