175
One of our Walmarts has anti-theft wheels
(lemmy.world)
None of the others in town have these, thought it was unusual enough to share
some models use a wire in the ground that emits a low frequency radio signal... which can be also be transmitted by the speaker in a phone by simply playing these mp3 files: https://www.tmplab.org/2008/06/18/consumer-b-gone/ (!)
What the fuck am I reading?
I’m not an expert in electricity and magnetism by any stretch of the imagination, but the way that I understand it is with any electrical current, there is an induced magnetic field, and vice versa. So the little parasites the article is referring to are the magnetic fields induced by the current to play the audio in the speaker. That magnetic field is the signal that triggers the antitheft device.
I think they mean that the electromagnetic field generated by sending an alternating current through a coil (or just a wire) induces a current and electrical field on the conductor. I've heard the term "parasitic losses" caused by reactance but I've never heard parasite or parasitic related to generation of EM radiation.
Given the current behavior of autocorrect, I’m assuming that’s not the author’s fault. My brain has reached the point that it skips over that and just reads “currents.” I don’t know how you get from a typo for currents to become parasites, but I’ve seen even worse corrections in my writing.
My guess would be "interference" that got autocorrected.
It's absolutely insane that a speaker coil works as an antenna in this case, but perhaps even more insane that the signal survives mp3 compression.
It has to cover a parking lot full of radio signals from cars. They're probably just listening for "close enough".
It's more likely a buried loop antenna, like an invisible fence.
Why is that insane? The entire point of an mp3 file is to be able to reproduce signals with reasonable accuracy. Seems like the signal has a frequency of around 8khz, which is very much in the range of human hearing and should be preserved by an mp3.
No, the point of MP3 is to compress audio in a lossy manner while minimizing the introduction of artifacts detectable by human hearing using psychoacoustic analysis. The coincidence that the necessary parasitic EM signal induced by speaker drivers happens to be created by a signal that doesn't suffer degradation by a relatively specific lossy compression method is remarkable.
Right, but artifacts in the ~8khz range will be detectable by human hearing. mp3s are going to be perfectly acceptable for many sounds in that frequency range... The fact that this works is evidence of that.
Plus, you know what else is lossy? Radio. If the signal is that fragile there's a good chance the locking mechanism wouldn't work in the first place.
Go actually read about MP3 compression before you continue misusing the term "lossy."
It's just going to be pulses of an 8khz signal. Why would an MP3 not encode this just fine?
Like I said before, the coincidence is what's remarkable.
It’s a bit of a coincidence that the signal is in the audible range, for sure! I’m not too surprised that an MP3 can reproduce a digitally modulated sin wave in the audible spectrum, but I can see how it’s surprising to people that a sound card is basically a dinky low frequency SDR. Van Eck Phreaking is another good example of this kind of stuff. CRTs in particular produce very obvious emissions which match what’s being displayed.
Can you imagine what would happen if someone went into a crowded store with a device playing this. A short loop through the isles and til queues would wreak havoc.
Sounds like a basis for a fantastic prank.