Re: VLF: Some natural signals

[Andy Talbot <andy.g4jnt@gmail.com>]

I once made an overnight recording of (the carrier of) some HF broadcast signals around 3MHz arriving via a skywave path.   The recording was done using G3PLX's   Dopplergram software, and was made at a sample rate of around 8Hz saving the data in I/Q format

The original idea was just to view the ionospheric Doppler shifted pattern on a waterfall, watching how it moved, faded and split into several  parallel 'strands' from different ionospheric layers.

But by playiong back at 1000 times faster  (8kHz sampling) then frequency converting from zero centre up to a few kHz  some really wonderful 'natural' sounds were generated.     The 8 hour recording played back in 30 seconds, accompanied by some wonderful whoops and chirps, especially so at dusk and dawn.   Parts sounded a bit like whales or dolphins.   I really must try this again, but am havign trouble identifying any suitable target broadcast signals.     There is so much less HF broadcast than there was 20 years ago when we did this.

Andy  G4JNT

On 23 April 2017 at 16:38, <hvanesce@comcast.net> wrote:

Hello Claudio,

Yes I agree with your sentiment, I was thinking of listening to the time-domain sub-Hz content; mixing or demodulating to remove any carrier (using Spectrum Lab or Matlab for example) and playing back 100x or 1000x faster (using Matlab for example), to take advantage of the ability of the brain to discern things that come through the ear that the brain cannot discern when those same patterns come through the eye (through the eye as with a time domain plot, spectrogram or other). Basically speeding up the playback of the natural (sub-Hz in this case) modulation to put it in the ear's response band. An interesting example of aural-path value comes from noise cancellation and/or weak-signal extraction work, where listening to (audio) playback of raw ULF/VLF RF data can reveal signal and noise characteristics that may generally not be noticed in spectrograms, FFTs and time-domain plots. But mainly the hope that the ear's processing may help to appreciate and enjoy some of the elegance in slower natural signals, just by listening to their sub-Hertz content with faster playback; for patterns that are particularly enjoyable, memorable or meaningful via listening.

It's interesting that you mention tenths of Hz shortwave-RF broadcast signal fading; I have looked forward to some future opportunity to listen to slightly simpler signal-fading in ULF and VLF bands by methods such as the above; listening for characteristics of ionospheric motion and other processes. Some signatures of ULF and VLF fading processes can be seen in plots of signal phase and amplitude vs time, but I'm looking forward to hearing what the ear can do with the same information.

73,

Jim AA5BW



-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Claudio Pozzi
Sent: Sunday, April 23, 2017 10:15 AM
To: rsgb_lf_group@blacksheep.org
Subject: Re: VLF: Some natural signals

On Sunday 23 April 2017 06:34:07 you wrote:
> Thanks Paul,
>
> What a wonderful sound.
>
> Unfortunate that bird's magnetic sensors don’t have the
> sensitivity/bandwidth for what you captured; they could perhaps tune
> their nav systems for reduced sensitivity to the disturbance that
> chorus indicates.
>
> Do you know if anyone has made audio translations of the mHz to 1Hz
> ionospheric-TEC waves (which modulate signals in HF and other bands)?
> I'm referring to what some folks call ULF waves, where ULF refers to a
> geophysical-community convention of roughly 1 mHz to 1 Hz.
>
> Or if anyone has made audio translations of the mHz to Hz modulation
> that magnetic storms can impress on VLF signals?
>
> 73, Jim AA5BW
>

Hi Jim.

Interesting question,but how can I made this audio translation?

The 10 mHz signal period (T=1/F) is 100 seconds.

So if I modulate in frequency or in amplitude a 1000 Hz carrier what sound can I heard?
And the sound of the translated frequency like SSB USB (1000+0,01 Hz) is indistinguishable from a 1000 Hz sound.

A 10 mHz frequency signal must be at least 100 seconds long for one complete sinusoid.
If it's shorter how can I measure the frequency?
Can I assume that the missing part of signal is a sinusoid?

In my opinion VLF signals (lower than few Hz) are better analyzed in time domain, not in frequency domain.

If you listen a 3600 seconds recording of VLF sound reproduced high speed in
3,6 seconds the sound should be interesting. But each "frequency" is multiplied by 1000.

The short wave AM broadcasting fading is an "amplitude modulation" with few tenths of Hz frequency (and some group delay distortion).

73 de Claudio IK2PII




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