04 November 2008

I received Hell

While MCW allows optical communications with simple equipment and doesn't require a computer, it does require two Morse-enabled operators.

In order to be able to involve more people in my (our) optical experiments, I wanted to try to send computer generated data over light. With all those soundcard programs and modes, there must have been a way to do it!

The people on the laser reflector were very helpful and provided a lot of info and encouragement. So I went on, my way.

Rather than implement a linear modulator that would also allow for AM voice (as suggested), I kept my original idea of simply squaring the audio drive and feed it to the LED. Sidebands are not an issue on a optical channel, where you're very likely to be alone.

For testing I recorded three 60" messages, two in RTTY (standard HAM baudot) and one in FeldHell. These were copied to an mp3 pen and played individually in an endless loop.

I built a squarer transmitter chain as follows:
  • AFSK (or whatever) source, mp3 pen or computer soundcard
  • LM386 amplifier at max gain
  • NPN switch to drive LED
The receiver is general purpose. RTTY got through easily once TX and RX were aimed at eachother, in a dark room (except for laptop TFT backlight) at 1m distance. For FeldHell I needed to lower the software squelch level, but then...

... words started flowing on the screen.
At the end of the second row ("the binary install" words) I tried aiming the receiver somewhere else, simulating path attenuation: the text is still visible and in extreme situations can be reconstructed by the human brain processor.
I had very strong AC hum on the receiver side, that visualizes as slanted upgoing lines, but do not interfere too much with FeldHell.

Thinking of on-the-field experiments, RTTY would provide a constantly lit LED easier for aiming, while FeldHell blinks it causing more interest in occasional onlookers.