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Update: One of the places that had this ultrasonic beep from their PA system has now stopped using it!
There's now a much quieter tone at a much higher frequency, outside anyone's hearing range for sure, which only shortly appears every now and then. Maybe they upgraded their sound system?
It's amazing how noise resistant LoRa is. It took me a while to realize that there can be plenty of perfectly decodable LoRa traffic even though the recording spectrograms look empty.
Here's a complete Meshtastic GPS position message (~50 chirp symbols each 250 kHz wide) buried in noise and some kind of smart meter traffic. I got a correct CRC after just 1 parity flip!
Whenever I run sine beeps through a home-made spectrum visualizer, and I use the square window or the Tukey window, I get these weird-looking vortices...
They stay the same size and shape regardless of FFT size, but they get bigger with bigger window overlap.
It might be a bug because I don't see it in other software. But it's pretty
Redsea v1.2 can now receive little pictures sent via RDS2 by some radio stations - usually these would be station logos meant for car radios. For now, this feature has very much an "engineer's UI" :)
Here's some logos our contributors have seen so far...
Pretty interesting: The 'whoosh' also happens with the sound of thunder, like at 0:20 in this video, possibly getting echoed from the clear ocean surface :) https://www.youtube.com/watch?v=Twvwtjefyak
Here's the autocepstrum showing a scale converted to millimetres echo delay in sound speed. Again it probably graphs the angle of the incoming sound.
This time though, nothing is moving. Maybe the angle sweep comes from the finite speed of sound.
Did you know that old-time TV scanlines aren't horizontal but slightly diagonal, continuously descending? Somehow I just learned this in some forum thread and it makes a lot of sense - both the vertical and horizontal position are just independent, smoothly increasing scans.
If you try to make a simple 'TV emulator' in a pixel grid it might end up looking like this (experience: mine)
Autocorrelation plot of an ECG recording beautifully visualizes the interbeat interval, or instantaneous BPM measurement, for each heartbeat :)
This ECG sample is from the Fantasia Database - recorded from people lying down watching the Disney movie Fantasia (DOI: 10.1152/ajpregu.1996.271.4.R1078). I also used Apple vDSP and some home-made GUI art.
Here's the autocorrelation for a long one. The whoosh even goes up in the beginning. The maximum lag here is 7.6ms which would be 2.6 meters for an echo. Which is almost the height of a person and back...
I took it from this video https://www.youtube.com/watch?v=oWfVSDK3CWg#t=2m10s at 2m 10s. No I'm not in a Youtube wormhole
