I’ve been pondering the amateur mode known as FeldHell or Hellschrieber. It’s a very old mode, dating back to a patent in 1929. I’ll let Randy, K7AGE explain it with a demo on youtube:

For fun, I sat down and wrote and encoder for it. It tries to be really good about bandwidth, I haven’t checked it out yet for that, but it sounds relatively unclicky at the moment. Here’s an example mp3 file of a FeldHell transmission. Perhaps later I’ll get the time to write up why I think FeldHell is interesting.

Addendum: It seems to mostly work, although I might have a timing issue, and I’m uncertain as to how good the filtering is. I played it back into gMFSK which has a feld decoder, and the images come out significantly slanted. There are variants of FeldHell which run a bit slower, maybe that’s the issue. I’ll have to ponder it more. I adjusted the timing in the recording, added some gaussian noise, and got the following image:

Addendum²: I discovered an error in the code that I put in to add Gaussian noise (embarrassing). It tended to add in considerable low frequency power. Not sure what overall effect it will have on the signal, but it sure made the spectrum look funny. I think the pulse shaping is reasonably correct.

Addendum³: As some evidence that my program works… here’s the power spectrum of the message above, with the improvement in the gaussian noise generator.