Hi @fubar3 - I made a recording of the VLF signal during the 2023-12-24 SAQ transmission. You can find it here: https://privat.bahnhof.se/wb748077/radio/2023-12-24-grimeton/ - the file to download is telegram-17200.wav.
Besides the signal from SAQ there is noise, and signals from other VLF transmitters, so some kind of bandpass filtering may be needed.
Besides the signal from SAQ there is noise, and signals from other VLF transmitters, so some kind of bandpass filtering may be needed.
Hi @fowlay. thank you for the recordings. I noticed an optical illusion when viewing the unprocessed file. Sometimes when scrolling through the file with Audacity, some blocks would appear that looked like CW Morse. They would disappear in about 2 seconds. Since they were the same size, they were not Morse. Probably just a software artifact.
The raw signal level is too low for my circuit but I see you have a successful decode using Gerke even though the noise spikes were getting through. Noise spikes trouble my design they because they look like the start of a dit or dah.
My attached schematic removes some of the noise from a recording. It interrupts an ESP8266 which sends time-stamped signal events to the PC for decimation and decoding. So far, it works only with ideal Morse Code, not VLF.
The raw signal level is too low for my circuit but I see you have a successful decode using Gerke even though the noise spikes were getting through. Noise spikes trouble my design they because they look like the start of a dit or dah.
My attached schematic removes some of the noise from a recording. It interrupts an ESP8266 which sends time-stamped signal events to the PC for decimation and decoding. So far, it works only with ideal Morse Code, not VLF.
Attachments
There is great software for decoding CW radio signals but I wanted the DIY experience using less abstract and simple hardware and software. I have an analog PCB and a digital PCB. The analog converts the audio input from the recorded transmission into pulses for the digital. Interrupt routines on the digital produces a 100 microsecond clock for timestamping the analog events which occur at about 750hz. These are output to the PC USB serial port at a high rate. A PC program interprets the timestamps to reveal the Morse Code timing. Ok, it's an elaborate simplicity. 
Morse Code was designed for wires with the signal either on or off. Unfortunately, there is no off state with wireless since the CW transmitters cannot turn completely off. And if they did, noise and interference would take over. There are sharp spikes even under good conditions. They may change the timing when the transmitter is on but they cause fake dit-dahs when the transmitter is off. It is possible to double check a possible spike, throw it away if it is less than a time unit. But that is too fancy for me.
I decoded an SAQ July recording from weaksignals.com. There were some spikes but most of the file worked.
? cq cq cq?desaqsaqsaq?? thisisgrimetotradio etation?saq in a transmission using the?alexanderson 200tw awternator from 192? on?17??2?khzr? in the summ?i oe 19fh? the machine perts for the two alexanderson a?ternators arrtved to grimeton frym the?usa? a special team from gene?al electric arriied a?d assem?ly could beetn? at the same tite the power line to g?imettn wis?x?mpleted and conneceed to the transeormer buildi?g? stauf housint begins tone buirt and the site is expandine atarapted aace?? signed? aoeldhere?ege grimeton radio station and th? alexander grimeton friendshii association?? for q?l info?please readour website? waa?alexanderaen?se r eesa
lines processed = 132699
Conclusion: My thing is good for machine generated files but will rarely work for wireless ones.
Morse Code was designed for wires with the signal either on or off. Unfortunately, there is no off state with wireless since the CW transmitters cannot turn completely off. And if they did, noise and interference would take over. There are sharp spikes even under good conditions. They may change the timing when the transmitter is on but they cause fake dit-dahs when the transmitter is off. It is possible to double check a possible spike, throw it away if it is less than a time unit. But that is too fancy for me.
I decoded an SAQ July recording from weaksignals.com. There were some spikes but most of the file worked.
? cq cq cq?desaqsaqsaq?? thisisgrimetotradio etation?saq in a transmission using the?alexanderson 200tw awternator from 192? on?17??2?khzr? in the summ?i oe 19fh? the machine perts for the two alexanderson a?ternators arrtved to grimeton frym the?usa? a special team from gene?al electric arriied a?d assem?ly could beetn? at the same tite the power line to g?imettn wis?x?mpleted and conneceed to the transeormer buildi?g? stauf housint begins tone buirt and the site is expandine atarapted aace?? signed? aoeldhere?ege grimeton radio station and th? alexander grimeton friendshii association?? for q?l info?please readour website? waa?alexanderaen?se r eesa
lines processed = 132699
Conclusion: My thing is good for machine generated files but will rarely work for wireless ones.
Attachments
@w5jag said: mentally tune out noise and interfering signals.........
Well humans and songbirds have had millions of years to detect sounds of interest in a babble.
Ideal Morse Code found in ARRL practice files decode perfectly because the timings are consistent and there is no noise. However, recordings of CW transmissions are GIGO - garbage in, garbage out. I can interpret them in the Audacity view but my hardware and software cannot handle the noise and erratic timing.
The WWVB Atomic clock system also has noise problems. I am on the fringe at 2200km. The receiver is flickering all the time unless I cover the antenna with metal. Perhaps I can learn something if I get it working. Here is a nifty project by Bruce Hall W8BH https://github.com/bhall66/WWVB-clock
Well humans and songbirds have had millions of years to detect sounds of interest in a babble.
Ideal Morse Code found in ARRL practice files decode perfectly because the timings are consistent and there is no noise. However, recordings of CW transmissions are GIGO - garbage in, garbage out. I can interpret them in the Audacity view but my hardware and software cannot handle the noise and erratic timing.
The WWVB Atomic clock system also has noise problems. I am on the fringe at 2200km. The receiver is flickering all the time unless I cover the antenna with metal. Perhaps I can learn something if I get it working. Here is a nifty project by Bruce Hall W8BH https://github.com/bhall66/WWVB-clock
Below is my suggestion for implementing Q multiplication and a brief description. Please excuse my crude drafting.
I assume an antenna connected to a parallel L-C resonator and a low-noise opamp with voltage gain of +10. As a guess, I’m assuming the resonant tank circuit has unloaded Q of about 30 and have modeled the losses as a parallel resistor (Rp = 33.9k) in shunt with the inductor. Resistor Rf applies positive feedback to the the resonator to raise the Q. Threshold of oscillation is reached when loop gain is 1.0. That occurs when Rf = 9 * Rp --- that is, when the attenuation of the Rf and Rp voltage division is 1/10, and the forward gain of the opamp restores the gain to unity.
I’ve arranged Rf as a combination of a fixed R and a pot so that the multiplier Q can be driven from low Q into full oscillation, but of course these are designer options. I’m remembering a Heatkit Q multiplier from my teenage years.
View attachment 1262563
Q multiplication is an interesting idea and is worth looking at further.