Reading online VLF

For the core of the VLF handling section of the neural nets, my current idea couldn’t be much more straightforward. Take periodic spectrograms of the signal(s) and use them as input to a CNN-based visual recognition system. There are loads of setups for these available online. The ‘labeling’ part will (somehow) come from the seismic data handling section (probably based around an RNN). This is the kind of pattern that hopefully the network will be able to recognise (the blobby bits around 5kHz):

Screenshot from 2017-07-01 18-07-52

“Spectrogramme of the signal recorded on September 10, 2003 and concerning the earthquake with magnitude 5.2 that occurred in the Tosco Emiliano Apennines, at a distance of about 270 km from the station, on September 14, 2003.” . From Nardi & Caputo, A perspective electric earthquake precursor observed in the Apennines

It’ll be a while yet before I’ll have my own VLF receiver set up, but in the meantime various VLF receiver stations have live data online, available through vlf.it. This can be listened to in a browser, e.g. Renato Romero’s feed from near Turin at http://78.46.38.217:80/vlf15 (have a listen!).

So how to receive the data and generate spectrograms? Like a fool I jumped right in without reading around enough. I wasted a lot of time taking the data over HTTP from the link above into Python and trying to get it into a usable form from there. That data is transmitted using Icecast, specifically using an Ogg Vorbis stream. But the docs are thin on the ground so decoding the stream became an issue. It appears that an Ogg header is sent once, then a continuous stream. But there I got stuck, couldn’t make sense of the encoding, leading me to look back at the docs around how the transmission was done. Ouch! I really had made a rod for my own back.

Reading around Paul Nicholson’s pages on the server setup, it turns out that the data is much more readily available with the aid of Paul’s VLF Receiver Software Toolkit. This is a bunch of Unixy modules. I’ve still a way to go in putting together suitable shell scripts, definitely not my forte. But it shouldn’t be too difficult, within half an hour I was able to get the following image:

img

First I installed vlfrx-tools, (a straightforward source configure/make install, though note that in latest Ubuntu in the prerequisites it’s libpng-dev not libpng12-dev). Then ran the following:

vtvorbis -dn 78.46.38.217,4415 @vlf15

– this takes Renato’s stream and decodes it into buffer @vlf15.

With that running, in another terminal ran:

vtcat -E30 @vlf15 | vtsgram -p200 -b300 -s '-z60 -Z-30' > img.png

– which pulls out 30 seconds from the buffer and pipes it to a script wrapping the Sox audio utility to generate the spectrogram.

 

 

 

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Author: Danny Ayers

Web research and development, music geek, woodcarver. Originally from rural northern England, now based in rural northern Italy.

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