[Laser] RTTY over light, anyone tried it?

[Paolo Cravero]

Garnier Yves wrote:

> The simplest is to use PC to PC optical links with sound cards and 
> squared output signals to drive your LED.
> You can use many TX RX protocols.
> With rather strong signals you can use all the PSK31 programs.

Let's go for it! Squared soundcard output to drive the LED. If the bandwidth 
rolloff of my OPT201 sensor circuit is at few kHz I should be able to send 
even SSTV.

I am a bit skeptical about PSK31, since the squarer would not take into 
account the amplitude envelope of this mode, resulting in a poor bandwidth 
efficiency (who cares about adjacent channels anyway!). So, better stick to 
FSK-derived modes, even if it would probably work.

Moreover sending the test/tune sequence, or a one-way link test, can be done 
without a computer: just record the audio output and play it in an endless 
loop on a MP3 key. Any length. I've done this for an automated SSB CQ'er.

Hopefully tonight I will draw a modulator circuit and share it with the group 
for feedback. Then also complete my second barebone (no lenses) receiver 
circuit for indoor testing.

Paolo IK1ZYW