5 ms·

Mystery Signal from a Helicopter (2014)

177 points by tomduncalf 4y ago

metadat 4y ago
Previously discussed back in 2014: https://news.ycombinator.com/item?id=7160242 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.
bee_rider 4y ago
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.
- reeckoh 4y ago
  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?
  - hgomersall 4y ago
    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.
    - psychphysic 4y ago
      Heh never thought about this. Mind blowing.
- aarong11 4y ago
  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.
macintux 4y ago
I imagine this was shared thanks to the Malaysian flight discussion, specifically this thread: https://news.ycombinator.com/item?id=34510103 https://news.ycombinator.com/item?id=34510103
- tomduncalf 4y ago
  Exactly, I thought it was interesting enough (for an audio geek like me at least haha) to warrant its own post!
alexpotato 4y ago
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
- ComputerGuru 4y ago
  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.
  - dylan604 4y ago
    Machine Learning = we learned how to make the machine do what we wanted
- Gordonjcp 4y ago
  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!
  - 4y ago
graupel 4y ago
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.
culopatin 4y ago
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.
- hugs 4y ago
  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 https://youtu.be/XEbjNDEnjMc [2]: https://youtu.be/wzkAeopX7P0 https://youtu.be/wzkAeopX7P0
  - Gordonjcp 4y ago
    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...
    - hbarka 4y ago
      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.
xwdv 4y ago
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.
hindsightbias 4y ago
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?
- ff317 4y ago
  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.
- PM_me_your_math 4y ago
  [dead]
deleted 4y ago
[deleted]

177 points by tomduncalf 4y ago

Previously discussed back in 2014: https://news.ycombinator.com/item?id=7160242 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.

bee_rider 4y ago

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.

reeckoh 4y ago
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?
- hgomersall 4y ago
  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.
  - psychphysic 4y ago
    Heh never thought about this. Mind blowing.
aarong11 4y ago
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.

macintux 4y ago

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

tomduncalf 4y ago
Exactly, I thought it was interesting enough (for an audio geek like me at least haha) to warrant its own post!

alexpotato 4y ago

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

ComputerGuru 4y ago
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.
- dylan604 4y ago
  Machine Learning = we learned how to make the machine do what we wanted
Gordonjcp 4y ago
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!
- 4y ago

graupel 4y ago

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.

culopatin 4y ago

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.

hugs 4y ago
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 https://youtu.be/XEbjNDEnjMc [2]: https://youtu.be/wzkAeopX7P0 https://youtu.be/wzkAeopX7P0
- Gordonjcp 4y ago
  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...
  - hbarka 4y ago
    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.

xwdv 4y ago

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.

hindsightbias 4y ago

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?

ff317 4y ago
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.
PM_me_your_math 4y ago
[dead]

deleted 4y ago

[deleted]