What the Hell??
This is how the signal looks on Spectrogram using settings suitable for quite fast "QRS"....
Doesn't make much sense does it?
But see below where the resolution is increased and the scrolling slowed down, this time using Spectran on the "0.12" bandwidth setting.
Yes, the secret is revealed. It's a slow version of Hellschreiber which Andy has developed using a DDS to generate the different frequencies. This is his 10Hz bandwidth version, received through considerable noise on Saturday morning. The 5Hz version below shows better contrast under the same conditions.
Download Andy's article (Word 2 format).