Recording the Data

Joachim Köppen Kiel 2018

The best way to record the data is by using a portable magnetic tape recorder. Whether it uses a Compact Cassette, a Mini- or Microcassette (for dictation recorders) is of little importance. The audio signals from the VLF receiver are faithfully recorded; the only disadvantage of the magnetic tape can be that the sensitivity may fall off at high frequencies. And one has the inconvenience that after an observing session one has to play back the tape and convert the audio signal into a computer file for further analysis.

The tape recorders are nowadays replaced by digital voice recorders. They are even smaller, have a lot of memory space, store also the time of reording, and their operation is very easy. However, they have one fault: They have a liquid crystal display, which is refreshed by electric pulses, which are easily picked up by the receiver's antenna. The sharp pulses produce a wide band interference which consists of harmonics from about 75 Hz which extend up to 3 kHz:

Click to hear the sound file
Fortunately, there is a very simple cure for this: It suffices to cover the display screen with a metal shield which must have electrical connection to the ground terminal of the recorder. This ground connection is made by letting the metal shield have good contact with the outer part of the microphone (or earphone) plug. Below a shield is shown, which was fabricated from the metal case of a 9 V battery.
The small notches on its top match the positions of the sockets for microphone and earphone, so that they make contact with the 'earthy' part of an inserted plug. During recording I hold the shield tightly to the display and also in good contact with the receiver ground. This completely eliminates all interference from the display!
A digital voice recorder can have another disadvantage: Usually they record the audio in a compressed format, such as *.WMA or *.MP3 files. While these are very efficient and space-saving, but the record is no longer an accurate reproduction of the data. The compression algorithm may work well with human speach, but it tends to produce artifacts when confronted with the sounds of spherics and tweeks.
Only an umcompressed format like *.WAV records the data faithfully. So if the recorder has this option, use it!

Here is a direct comparison between a tape dictation recorder and a digital voice recorder: two recordings were taken with the same receiver on the beach on 20 apr 2018 shortly after another. Many weak whistlers could be clearly heard. This is what the recorders made out of that:
As the magnetic tape recording sounds a bit too bassy, it was put through a gentle high pass filtering with the Audacity program. Because strong spherics pulses triggered Audacity's automatic volume control, each pulse is followed by a short silence, giving this 'breathing' sound:

Click to hear the sound file

The whistlers are equally well heard with the digital voice recorder, but the steady noise by the spherics appear as somewhat musical sound, as if the data compression algorithm tries to make something 'sensible' from the noise. But once one gets accustomed to this artefact, the recordings are quite useful for most purposes.

Click to hear the sound file

With a bit of filtering (with Audacity) one can reduce the noise at frequencies between 6 and 10 kHz, which makes the whistler stand out better:

Click to hear the sound file

In any case, these whistlers are one-hop greetings from southern Africa, because it takes about 0.6s for the frequency to fall from 5 to 2 kHz.

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last update: Apr 2018 J.Köppen