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Show HN: Formally verified FPGA watchdog for AM broadcast in unmanned tunnels

github.com

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

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

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

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

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?

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.

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 :)

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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