Mastodawn

I'm finding the recent Veritasium videos are a bit weird... they mention a recent "thing 1", then do a super-deep historical dive into a somewhat related "thing 2" (presenting it as "the origins of thing 1").

This one [1] is "The Secret Spy Tech Inside Every Credit Card", where they briefly poke at NFC and then wind back to talk about the Soviet's listening device, gifted to the US Ambassador in 1945 [2].

They explain in some detail how the device works, showing it as what many woud recognise as Frequency Modulation, before calling it Amplitude Modulation(?!)

... the Wiki page states "the membrane and the post formed a variable capacitor acting as a condenser microphone and providing amplitude modulation (AM), with parasitic frequency modulation (FM) for the re-radiated signal", which is... I mean... okay, I guess... but isn't any FM broadcast actually AM too by this definition?

1: https://www.youtube.com/watch?v=YSJY3DvnybE

2: https://en.wikipedia.org/wiki/The_Thing_(listening_device)

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Michael Ossmann

@attie You piqued my interest, so I just watched the video, and I think they (appropriately) described the signal reflecting from The Thing purely as AM. The mechanism for producing AM is that the device's resonant frequency changes with air pressure, but the effect this has on the radio wave is AM.

I believe that the "parasitic frequency modulation" mentioned in Wikipedia is inconsequential. I don't see a citation.

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Attie Grande 5d ago

@mossmann

> You piqued my interest
My apologies 😂

If I understand things correctly, an exciting signal was aimed at The Thing on a given frequency, causing it to resonate.

The Thing's tuning varied with the air pressure, which meant that as it went in/out of resonance with the exciting signal, the resulting re-radiated signal would have changed in amplitude more signficantly, and probably frequency more subtly.

So yes, the resulting output signal would be largely AM... but my point was more that this isn't what they showed in the bottom-left graph - it wiggles up and down in frequency with consistent amplitude, which suggests FM.

Side note: based on the positioning of that bottom left graph, I feel it would also be reasonable to expect that this is the exciting signal... but it isn't.

Maybe I'm on my "communicate clearly" hobby horse again, but the more recent videos have the feel of "trying to communicate a complex topic without fully understanding it", and "trying to spin a tale into something more substantial than it needs to be".

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Michael Ossmann 5d ago

@attie Apology accepted!

My interpretation of the bottom-left graph is that it is a plot of The Thing's resonant frequency. I agree that it could be clearer.

Yeah, the way they connect the dots from The Thing to contactless credit cards is questionable, but The Thing is certainly the earliest example of an intentional RF retroreflector.