Mystery Signal from a Helicopter (2014)

Sox is amazing.<p>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.<p>It was great for a bunch of reasons:<p>- it could so a frequency analysis to show me which frequencies were present in the audio<p>- I figured out there was the frequency of the sound when the ball left the barrel but also a frequency for the gun&#x27;s firing mechanism<p>- I could then do a bandpass on just the frequency of the ball leaving the barrel<p>- It then let me convert the audio to a numerical format that made analyzing the peaks and valleys much easier in Perl<p>- The end result was I could accurately calculate shots per second given just a raw recording and some processing time

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.<p>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.

Source: Worked in TV news my whole life, with lots of time in news helicopters.<p>The audio signal is split left and right with the announcer on one and the telemetry on the other, it&#x27;s easy to have some telemetry bleed over into the voice channel.<p>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.<p>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:<p><a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=7160242" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=7160242</a> (82 comments)<p>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?<p>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 imagine this was shared thanks to the Malaysian flight discussion, specifically this thread: <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=34510103" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=34510103</a>

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&#x27;s. I&#x27;d heard it before in earlier contexts but don&#x27;t remember any details.<p>Anyone know what that is?