Well, after a little bit of mucking about, I was able to down-shift the segment I needed into a range that is audible to the human ear. Here's a snippet:
http://www.sbrac.org/files/vlf.mp3 (I have it in FLAC and OGG and WAV as well, just figured most folks would have a MP3 player :-) ). It's about 40 seconds of a segment of the VLF band, between 23KHz and 28KHz, downshifted so that 23KHz is at DC. The dominant "tone" is the U.S. Navy VLF station called "NAA", located in Cutler, Maine, about 850km from me. Down in the mud, there's also the NML station that is in North Dakota, about 1600km from me. The clicks 'n pops are 'spherics mostly caused by distant lightning. I pre-process the entire input bandwidth (DC-48KHz) to remove *all* harmonics of 60Hz, prior to doing anything else with it. That cleans up the spectrum considerably! The RMS signal power for up to 4 narrow-band VLF stations is also recorded in real-time, to measure ionospheric absorption. Doing this audio-shifting trick was more for amusement than any scientific value. The whole digital portion of the signal processing chain is done in Gnu Radio. I have a magnetic loop antenna outside the house, about 300' of wire wound onto a 1.8M square frame, and I'm using a cheap Behringer microphone pre-amp as a low-noise amplifier in front of the sound card. If the sound card was a little quieter, I wouldn't need the pre-amp, but there's just a little too much noise on the card to allow the signals to be really clean, so an audio pre-amp for VLF radio signals! Cheers -- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
