Hellschreiber is a method of sending text over radio. It basically is a kind of primitive fax machine: it sends each character as a 7×14 matrix of dots. If the receiver hears a signal, it plots a dot, otherwise it plots a space. Each character is scanned left to right, bottom to top, and are sent at 2.5 chars per second.
But all this playing around with QRSS makes me want to try for something different. I wanted to send a character every 20 seconds or so, so I represent each of the 14 rows of the character by a oscillator. Each oscillator is spaced 1Hz apart. All rows for a column are thus sent simultaneously, and we send each column for three seconds.
If you don’t have any noise, it’s really easy to decode:
Same message, with lots of noise…
Here is what the audio sounds like with the noise in place. I can barely hear the signal most of the time, but it does sort of warble in every once in a while.
I’ll have to try an on-the-air test sometime soon.
Addendum: You can see little spurs at the beginning and end of each column. That’s because each of the oscillators snaps on to full volume. That generates a little click, which contains lots of energy. Slowing on that pop would help keep the signal narrow. But the reality is that in practice, it probably doesn’t matter at all.