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Mystery Signal from a Helicopter (2014) (windytan.com)
177 points by tomduncalf 7 days ago | hide | past | favorite | 28 comments

Sox is amazing.

I used it about 10 years ago to take raw audio of paintball guns shooting and help pre-process said audio for analysis of shots per second.

It was great for a bunch of reasons:

- it could so a frequency analysis to show me which frequencies were present in the audio

- I figured out there was the frequency of the sound when the ball left the barrel but also a frequency for the gun's firing mechanism

- I could then do a bandpass on just the frequency of the ball leaving the barrel

- It then let me convert the audio to a numerical format that made analyzing the peaks and valleys much easier in Perl

- The end result was I could accurately calculate shots per second given just a raw recording and some processing time

I used it to diagnose a funny noise on my car, by recording the ambient sound in the cabin with my phone - velcroed to the dashboard with its microphone pointing down at the gearbox tunnel - and announcing my speed when the noise was particularly pronounced.

Then I used sox to extract loud but low frequencies, and since I could hear it correlated with road speed I was able to verify that it was a particular bearing in the front propshaft by working out the relationship of frequency and gear ratio.

All without crawling underneath and getting oily!

The amusing thing about that for me is that, as someone with perfect pitch, I've used the pitches made by failing components in my car for diagnostic purposes without even thinking about the fact that most people can't do that without technological help. It's just... you know what pitch it is and therefore how fast it's moving/rotating/oscillating. Everyone knows that about everything they hear. Right?

Makes me wonder how many lived experiences I take for granted that are totally different for others, and likewise the lenses through which others see the world that I would never imagine.

As a musician, I'm glad I don't have "perfect pitch". It would be impossible to play with any other musicians!

I would love to see a video of how exactly you did this. Nice work

It's sad that if we have one option we're comfortable with we might never find another that might serve us better. I'm a coder and a command line fiend but I've been using Audacity all these years and somehow managed to never stumble across Sox.

Completely aside, I think what you describe is going to soon be considered an arcane art - I fear "machine learning" is going to be the catch-all to describe what is really just pure math.

Machine Learning = we learned how to make the machine do what we wanted

I'll have to check it out!

In the past for commercial audio analysis, I've used ArtemiS Suite from Head Acoustics for similar requirements. It would trivially do all of those things, including the Perl (well, what the Perl did).

I think it's really fascinating from a UI perspective, the multi-column separation of the project, audio files, analyses (chained back to other analyses where appropriate), graphics, and report generation makes it really flexible. Rather than pointing and clicking through a wizard each time you want to accomplish something, or up-arrowing in the command line to a previous entry, you set up a pipeline to accomplish that thing and then iterate the pipeline intake over all your inputs and it spews the outputs you need. And all that from an intuitive GUI with lots of graphical feedback, rather than a command line with more difficult discovery of features.

It changes your activity from executing commands one after another to engineering a script to execute those commands automatically, you almost can't help but make something reusable.

Tragically, it's locked behind a 4- or 5-figure license cost. Not a big deal when you're using it for testing automotive products, but not exactly accessible for paintball hobbyists.

I don’t know how to express the level of “I don’t know anything” I feel when I read posts like these and the writer seems to just pull knowledge from their pores. “I looked at the wave and I knew exactly what it was and what to do”. I find myself more interested in how long it took them to figure that out and how they got to it than the content itself.

Same for any topic in HN that people manage to have very intense and niche knowledge for. Maybe I have the same odd knowledge about some other thing, but I don’t find fancy lock washers as brain teasing as figuring out a helicopter route from a YouTube video’a left audio channel.

I don't have the skills of the person who decoded the signal... however, if you get your ham radio license and start playing with the various digital modes, you'll start to recognize the various sounds of different modes. An APRS data packet [1] has a very unique sound. Same with with RTTY [2]. Our brains are really good at pattern matching if they're exposed to a pattern enough times. Obviously, you'd still need a computer to decode those signals, but the "sound" of the signals can give you a very good headstart.

[1]: https://youtu.be/XEbjNDEnjMc

[2]: https://youtu.be/wzkAeopX7P0

In the TV series "Burn Notice" there are APRS packets used as "walkie talkie sound effects" while they're doing some sort of heist.

I've forgotten now which ep it was but I always fancied trying to pick them off the audio and decode them to see if there are any clues in it.

SSTV is another distinctive one, and the Aphex Twin track "Bonus High Frequency Sounds" is a pic of Richard James in his studio. Often wondered about the SSTV-like feedback in that Radiohead track, too...

The 90’s movie Sneakers with Robert Redford comes to mind. It’s a heist movie set in the California Bay Area and without revealing spoilers there’s a fantastic whodunnit involving the decoding of audio signals.

It's funny, heist movies are basically public relations for thieves.

"Burn Notice" had an Irish character in it who dropped the accent after a couple of episodes, but her back story was that she was and explosives expert and a "former IRA operative, but not a terrorist".


That did not play particularly well in the UK, to put it mildly. Not sure if it was meant to be public relations for terrorist bombers...

Indeed some are very recognisable once you start to hear them often. I can recommend [1] if you want to explore more.

[1] https://www.sigidwiki.com/wiki/Signal_Identification_Guide

Source: Worked in TV news my whole life, with lots of time in news helicopters.

The audio signal is split left and right with the announcer on one and the telemetry on the other, it's easy to have some telemetry bleed over into the voice channel.

The antenna on the helicopter is programmed to always point at any one receive antenna and the receive antenna uses that data to track the helicopter to maintain line-of-sight for the video feed.

I believe many use cases of this have migrated to fully digital signal chains so the days of the weird audio may be ending.

Previously discussed back in 2014:

https://news.ycombinator.com/item?id=7160242 (82 comments)

I thought it seemed familiar, though the details were evicted from my LRU brain long ago. Cheers.

That’s really interesting. I guess it must be coming through as interference, which would indicate, maybe, that something wasn’t well shielded in the audio circuit?

A kind of funny thought — maybe something that was supposed to be grounded ended up floating, because the reporter was, uh, not in contact with the ground.

I've always wondered, is Earth's electrical ground significantly different from that of a random planet hundreds of light years away?

If two spaceships from different parts of the galaxy were ever able to meet, would they experience the biggest static electricity shock in history?

Apparently this is not an issue with terrestrial space stations, because spacecraft only accumulate something like an 80V potential as they travel through the atmosphere. But what about another star system that has sailed through who knows how many charged nebulae?

Well birds regularly charge themselves up to 100s of thousands of volts, then merrily fly back to earth with no significant effect. I think the point is the total stored charge is very small, so not much energy is transferred.

Heh never thought about this. Mind blowing.

It sounds like its working as designed. It just uses the audio as a side channel to transmit telemetry data. Unless you ran the resulting video / audio through something specifically meant to remove this then even a recording is going to still contain the data.

I imagine this was shared thanks to the Malaysian flight discussion, specifically this thread: https://news.ycombinator.com/item?id=34510103

Exactly, I thought it was interesting enough (for an audio geek like me at least haha) to warrant its own post!

I think recognizing there is a signal at all is the most difficult part of the task here. Once you know there is something there, the process of decoding becomes more straight forward using common methodology.

There is this very faint (presumed modem) signal I started hearing in cars back in the early 90's. I'd heard it before in earlier contexts but don't remember any details.

Anyone know what that is?

APRS: Automatic Packet Reporting System

Ham radio operators use it for rovers and balloons, among other things. News choppers use it to report their position. If you have a radio, you can hear amateur APRS on 144.39 in the United States and 144.80 in Europe. There is even an APRS digipeater on the ISS at 145.825 MHz FM. You will hear a few APRS stations being repeated on most passes. The whole APRS network is actually pretty cool. If you have an SDR, decoding is pretty straight forward: https://www.w8wjb.com/wp/2021/03/21/decoding-aprs-packets-wi...

I believe the news helis use the same protocol but with obviously more frequent updates, but I've never tried to intercept it.

In the 90s and early 2000s, it was not uncommon for me to hear some modem-like sounds from various devices with speakers (PC speakers, audio system speakers, car speakers). The sounds were basically interference from nearby (as in, mere feet away at most) cellphones of the era. I assume since then that cellphones have become less leaky and/or consumer electronics have become better at filtering out such interference, because I don't really notice this phenomenon anymore.

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