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At first I thought the "unmanned tunnels" description was just a way to avoid broadcast regulator scrutiny, but it does look like it's genuinely designed to be used underground as part of an emergency alert system. That led me to "leaky feeders", a type of broadcast antenna used in mines and tunnels.

https://en.wikipedia.org/wiki/Leaky_feeder

I'm curious about challenges (what's bad with AM broadcast in an unmanned tunnel?) and why the formally verified killswitch was necessary?

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I did a more aggressive internet search. This seems not possible given physics, as well as not documented (at least in the US) in the CDC Mine Accidents Database [0], which has been recording mine accidents since before the discovery + invention of AM radio.

Edit: The physics

- (lambda) = c / 1,620,000 Hz = 185 m :: 1.62 MHz is what I derived as a near max possible accidental frequency able to be produced by AM equipment

- 185 m / 2 = 92.6 m :: this is half a wave length

In order to resonate (let alone have enough power to "cook", which I didn't even look at because the wave can't even resonate), a tunnel must be at 92.6 m (fundamental) or 185 m wide or tall (2nd harmonic). Most tunnels are ~5m/3m wide/tall at most.

Dusted off my physics from my minor in college so someone feel free to correct me.

[0]: https://wwwn.cdc.gov/NIOSH-Mining/MMWC/MineDisasters/Table

And it would need to resonate magnetically.

Bollocks the wavelengths are on the order of hundreds of meters, there is no way you get microwave like heating out of that. Even at 30 MHz you're still looking at 10 meters wavelength, 3 meters at 100 MHz.

This system operates according to TFA up to the end of the AM band at roughly 1600 KHz, so 180 meters and change.

The danger is more likely there because someone might enter the tunnel and hit the feeder, which depending on the design can carry considerable power.

Fascinating. Any references? A cursory web search reveals nothing.

This is basically hilarious. Leaky Feeders are a few watts. and even a high powered multi-kW AM radio with 200m wavelength wouldn't resonate much in a rough walled tunnel multiple sq meters in cross section. It's both too large for there to be significant power density, and much less than a wavelength in diameter except in length where the tunnel passively attenuates the signal.

Fascinatingly false.

That is hilarious. You win the Internet for February 18, 2006.

I must be missing something ... Why broadcast speech over AM in an 'unmanned' tunnel ?? who's gonna hear/receive it ?? I wish the repo had some sort of use case summary or something...

Edit: I'm also sorta puzzled by the choice of AM in any sort of 'alert' context...Do people still listen to/use AM radios?

Updated the README with a use case section - vehicles with AM radios transit during construction/maintenance. Leaky feeder distributes the signal. Thanks for the feedback :)

AM radios are extremely simple and utterly fool proof, they can be made with only a handful of simple parts. Even so, these systems are archaic and these days you can make much more complex radio systems using a SDR which effectively reduces the whole radio to a pre-amp an ADC, an embedded CPU and some software.

AM is a good modulation choice for low signal environments. It is used in the airbands for this reason.

Precisely, it degrades very gracefully, unlike FM or digital.

I've also seen these used to add audio to art installations in commuter tunnels.

Thank you, I too was confused at the purpose of this

Could obtain better quality at the higher channel counts by phase shifting the audio for each channel such that the modulation peaks do not exactly align for each (as they do now). Even inverting the audio for half the channels would help.

>NCO: 12 Numerically Controlled Oscillators generate carrier frequencies (505–1605 kHz)

This seems like a crude way to do it.. why not provide all 110 carrier frequencies by using a polyphase channelizer? The bandwidth of the entire AM broadcast band is pretty low..

Yes, that's very true. It would make much more sense to hit all channels, at the same time, maybe they want to use some of the channels for comms even during an emergency?

> because automatically restarting a transmitter in an unmanned tunnel is not an acceptable failure mode

Why?

Also, surely they don't mean unmanned, who is listening to the AM emergency broadcast then?

I would assume because restarting would be saying "it's ok if the control system in our emergency transmitter sometimes fails, so let's just pretend it didn't happen and ignore it! yolo!".

I think a reasonable approach would be to have a redundant system, that gets activated if failure occurs in the first, and blasts a full alarm message to "abandon the tunnels, emergency system has failed!" type message.

If the emergency broadcast system fails, perhaps they need all the radio channels they use not to be jammed?

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someone's moltbot is lost

Nope, that's a human summary that took a few minutes.

Is there something in that comment that you didn't understand?

Here's another copy:

I happened to find a video about how the Osireon in Egypt is similar to structures constructed within Longyou caves. They are historically contemporary and they are separated by a multiple of 72 degrees. Angkor Wat and (TODO a number of other ancient facilities) perhaps coincidentally are also on a geographic pentagon at 72° degrees. Unfortunately, due to TODO some sort of procession you would expect such a LF/VLF comm net to be off by about 1° after 72 years though? Perhaps this is part of why they built new temples etc. adjacent to older sites. The Temple of Seti is built adjacent to the Osireon, for example.

Anyways, the scalloped marks on the walls of Longyou caves very likely operate as LF/VLF diffusers for local and far away signals that propagate TTE Through-the-Earth and also through water and hydrogen.

At 21cm, the northern Star Shaft in the Great Pyramid waveguides 1420 MHz. 1420 MHz is the fundamental frequency of Hydrogen.

The star shaft contain 0.25" copper rods that would have been deoxidized by hydrogen. The star shafts were enclosed. People have speculated that they were for ventilation, but the star shafts were found sealed with the oxidized copper rods inside.

That copper could have been from the Great Lakes back then.

Why would you add copper rods to the tallest structure around in the desert, in shafts that would accumulate hydrogen if the structure was filled with water that foams as it drains out?

Are those antennae and/or ionospheric and/or lightning charge collectors to charge the granite and discharge to make plasma in a 6,000 ton pressure vessel?

Is that an above ground cave? An above ground cave that makes: (0) ionized Water sanitized with hydrogen plasma and solar sanitized water, (1) Gold and Neodymium from SPP Surface Plasmon Polaritons in quartzite sand in electrified hydrogen plasma, (2) Doped A-CNT Carbon Nanotubes that glow blue in the presence of hydrogen and Diamond (3), and RF and EMF? Wouldn't it have been a beacon on multiple frequencies?

Anyways,

The pressure vessels found in Egypt - for example with a pyramid built around it at Saqqara - very likely resonate tuned LF resonance.

The pressure vessels may resonate a source signal that would stabilize hydrogen being slowly emitted to cool a quantum locked floating barge that could have made it easier to transport and place 100 ton granite blocks from a quarry miles away.

Also though, there are a number of hypotheses about pyramid construction that do not involve levitation or quantum locking of a depressurizing hydrogen vessel over neodymium - and maybe gold for corrosion - tracks or rails. Stephen Myers, for example, describes placing stones with barge cranes floating in a pool bounded by casing stones first. The Grand gallery (todo) does appear to be shaped like a water lock and there was no ladder or railing. And there are redundant portcullis doors. But the math to move a 100 ton stone without crushing wooden barges.

...

LoRA will work better than AM in a cave.

There are AM dead zones in caves. Scalloped walls that diffuse like Longyou caves can probably reduce those dead zones.

It looks like modern TTE systems are also built on VLF/LF.

Receiver cost for AM vs LoRa; but range? And then why not LF/VLF?

What

Is that a question?