[Laser] Re: RTTY over light, anyone tried it?

[C. Turner]

Having done a few different modes over light, as well as messed with 
various RTTY-like modes, there should be no problem in doing what you 
propose.

As for PSK31, there's no penalty whatsoever in doing brute-force 
XOR-type modulation as there aren't adjacent-channel users to contend 
with:  The demodulator really doesn't care a whit what the signal beyond 
the 31Hz bandwidth looks like:  Peter Martinez (one of the inventors of 
PSK31) and I exchanged a few emails on this topic back in '99 when we 
had a discussion of minimalistic methods of generating PSK31 streams.

Also used are the WSJT modes with great success - and there are enough 
of those to suite most needs, although the messaging capabilities may be 
limited.  Related modes also include the WOLF modes used for LF 
communications.  Another clever scheme would be multi-frequency Hell in 
which plain text would magically appear on one's waterfall display - and 
of course, just plain Hell would work too - even at very low S/N!  (I'm 
speaking of Hellschreiber, of course!)

As pointed out in other correspondence, the best range is usually 
accomplished by using as low a modulation frequency as possible to take 
advantage of one of the intrinsic properties of a typical silicon 
photodetector in which the ultimate sensitivity of higher frequencies is 
limited by the detector's capacitance.  For relatively short-range 
communications, using a Transimpedance amplifier (like the OPT202) works 
reasonably well, but these devices - while very convenient - are rather 
poorer performers when compared to relatively simple (and cheaper!) 
circuits constructed of discrete components - on the order of 20dB or 
more worse for speech-range operations.

As it turns out, many PSK31 programs can be set to ridiculously low 
frequencies (<100 Hz) so it is practical to set the center frequency 
down that low.  One would want to stay as far away as possible from AC 
line frequencies (primarily 120/100 Hz and its harmonics - but the 60/50 
Hz component can occasionally be potent as well) so PSK31 operation in 
the 80 Hz area would work - provided that one's sound cards worked 
properly at such frequencies.

FWIW, simple (and more complicated) linear modulator schemes may be seen 
here:  
http://modulatedlight.org/optical_comms/LED_linear_modulator.html  (I 
would recommend figure 2 or 3 for a quick 'n dirty modulator circuit.)

73,

Clint
KA7OEI