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... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Thank you, Bjoern, for an excellent example. Here is what is visible from my location -- your location should be able to see most of these signals. This capture was made at 11pm EST, about one month ago.
MSK signals are visible at 18.1kHz, (Russia), 19.6kHz (Anthorn, UK), 20.9kHz (Rosnay, France), 21.4kHz (Laualualei, Oahu, Hawaii), 22.1kHz (Skelton, UK), 22.7kHz (North West Cape, Australia -- a rare catch), 23.4kHz (Rhauderfehn, Germany -- almost obscured by the signal at 24kHz), 24kHz (Cutler, Maine -- appears so broad because it is the most powerful VLF transmitter in the world, and is less than 150 miles away), 25.2kHz (North Dakota, US), and 37.5kHz (Keflavik, Iceland). In order to have a fair shot at SAQ, you should be able to see some of these in the evening. During the day, nothing will be visible except perhaps the signal at 24kHz from Cutler -- that is strong enough to be received during daylight throughout the northeastern US.
I'm not sure what the pulsing carrier you see there is. It is probably local noise. At some point each night, the 21.4kHz signal from Hawaii switches to a long dash format, something like 15 seconds long with 5 seconds between dashes, but that is the only dashed signal I have ever seen on VLF.
The wandering carrier is fairly typical of an electric motor -- they radiate a lot of noise. The most common source for that sort of noise around here is a vacuum cleaner, though I have picked up similar noise from electric saws, as well as cars driving by. Some cars produce a surprising amount of alternator noise.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The pulsing carrier seems to be drifting a lot so it is not a proper transmitter. Even a RC oscillator would be a lot more stable. Looks like electric noise to me. Maybe a regulator of some sort.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Here are the spectrograms of more than two dozen VLF receivers, updated every ten minutes: . None of them are showing any similar signals. Closest thing is the Russian Alpha stations, but they are much lower in frequency and the pulses are not constant. If it is a VLF transmitter, it is an extremely short range one, with a curiously unstable signal. It's certainly possible, but I can't imagine what purpose it would serve.
At any rate, mysteries aside, if you are seeing the MSK signals, you're in a good position to be able to hear SAQ, so that's good news.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
To decode SAQ by ear, yes, a very narrow filter will be needed, as the band in this region is extremely noisy. 50Hz would be good, 30Hz would be better. Good results have been reported with 15Hz filters, but keep in mind that SAQ is powered by an RF alternator, so the frequency can drift a bit. Also, at 15Hz, you are pushing things close to the Nyquist limit... dots and dashes may start to run together a bit. Probably the best approach is to just capture the full 48kHz (or 44.1kHz) sampled audio, and try different filters at your leisure.
For visual decoding, a wide filter will suffice, since the FFT and your eyes do the work of a filter -- so long as you have selected an appropriate bin width. Your eyes will easily be able to pick SAQ out from the powerline harmonic that is 20Hz below, even if it is significantly stronger. Your ears probably won't be able to do that without the aid of a narrow filter.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Spectrum Lab's own FFT filter worked great for me. I just set it to a few hundred Hz bandwidth and had it output the down-converted signal from my PC speakers.
My code is pretty rusty, but I made out the callsign and the word "Alexanderson." I can't remember if I posted a MP3 last time.
I've also demodulated AM radio using a similar setup with a VFO and Tayloe downconverter before the soundcard inputs. Speclab is a complete software-defined radio transceiver, though the user interface tries pretty hard to hide it.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I did not get anything either that I can say for sure was not noise, thousands of people getting out of bed and turning on their appliances made it very messy.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
I definitely received it, but I do not believe any of the code is copyable. I would not have received it at all, but by an amazing coincidence, there was a blackout from about 0750UTC until 0850UTC. I saved a raw dump of the soundcard output. I will try to clean it up and post it. For now, I've got to get to bed!
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SAQ Transmission - December 24th, 2007 - 0800 UTC
