[Laser] minimum frequency PSK31

[TWOSIG at aol.com]

Paolo
 
Thank you for your response.  Let me try to reply to you in pieces,  rather 
than all in one message.
 
"Well, then you revert back to plain BPSK, so why undergo the hassle of  
generating a clean PSK31 signal if it gets "destroyed" by a non-linear  
the modulation process?"

 
Indeed, why bother to use the sound card to generate a clean audio tone  
intended for linear conversion and amplification, when the intent is to drive a  
non-linear device using a channel that, for now at least, will generate  
interference to no-one?
 
I think the answer is inertia.  If you are going to receive PSK31, I  know of 
no better way than to use a computer sound card.  If you have to  have the 
computer and sound card, why not use the programming and hardware that  already 
exists for the transmitter.
 
To my knowledge, no one has modified a PSK31 program so that it has an  
output of the transmit bit stream or modulated square waves.  It would  simplify 
the interface to drive a laser or most other light sources used by  light 
communication experimenters.  I do not have the skills to modify such  a program, 
but if I found someone that both could and was interested enough to  do it, I 
could certainly suggest what I would like to see in a version modified  for 
light communication experiments.  ( While I am wishing, I would wish  for light 
comm versions of CW, MCW, RTTY, MFSK, MT63, BPSK31,  QPSK31..........  Oh, just 
about anything that might prove interesting to  test. )
 
If on the other hand, you only wish to transmit PSK31, and only on an  
optical channel, you could use very different methods.  If you wanted an  optical 
beacon for system testing, or perhaps to use as one of the payloads on a  high 
altitude balloon.  The bit stream for a PSK31 signal would be  relatively easy 
to generate in a simple microprocessor or even from a ROM.   The bit stream is 
fed to one input of an Exclusive OR gate.  The other  input is fed from a 
clock source. ( The audio frequency, or any frequency up to  the limit of the 
gate if you wanted to test something that high. )  The  output is BPSK.  It is 
only a matter of interfacing the logic level output  to drive the light source.  
 
If you do not need an external clock source, and the  microprocessor is fast 
enough for the output you want, the function of the gate  can be done in 
software as part of the microprocessor.  Similarly, I am  sure that QPSK can also 
be done with hardware, but unless there is a strong  reason to have an external 
clock, it probably should be done with a  microprocessor.
 
 
 
James
N5GUI