[Laser] minimum frequency PSK31

[TWOSIG at aol.com]

PSK31 is a system developed for radio to provide  digital communications  at 
a conversational speed ( approximately what  a person can type ).   Typically 
it is generated by the sound card of  a personal computer and fed into the 
microphone input of a transceiver.
 
There has been some discussion of adapting it for use with light  
communication systems.  PSK31 when used on RF has narrow bandwidth and  requires linear 
conversion from audio to RF, but there seems to be no strong  reason to 
continue that for light systems.  Non-linear conversion and  amplification cause 
distortion and wider bandwidth ,harmonic rich  transmission, but since there other 
users on other frequencies are unlikely in  the optical channel, they cause 
no harm.  The "wasted energy" in the  transmission may be more than compensated 
by the simplicity of the system  used.  Non-linear systems may even convert 
less of the system input energy  into heat than a linear converter and 
amplifier would for similar signal  transmission.
 
It may also be possible to use the harmonic energy to improve the detection  
of signals.  However, I do not know if anyone has tried to do so.
 
 
In previous discussions, the issue of light receiver bandwidth has come up  
in regard to receive noise.  I am curious if it really makes any difference  
what tone frequency is used for a PSK31 signal as far as the noise it has to  
"fight".
 
The software that generates the PSK31 signal generally allows the output  
freqency to be set between 300 Hz and 3000 Hz.  That pretty well matches  the 
audio input limits of the radio transceiver it is expected to be sent  to.  On 
the receive side, it is not possible to identify what audio  frequency was used 
to generate the signal.  If the transmitter is tuned to  14.080 MHz USB and 
the tone sent into it is 1 KHz, the result is a signal at  14.081 MHz.  It would 
also have been generated by a 500 Hz tone sent to a  transmitter tuned to 
14.085 MHz.
 
The noise at the receiver depends on the bandwidth used.  If the  receiver is 
set for 14.08 with its "filter" set for 300 to 2,800 Hz audio, it is  seeing 
2.5 KHz of noise along with the signal.  It could also be tuned to  14.080200 
with its filter set for 650 to 900 Hz audio so that it only sees  250 Hz of 
noise.
 
With light communication systems, we cannot tune the receiver to compensate  
for transmitter tone.  If the sender uses a 1.5 KHz tone, the receiver is  
going to process that tone.  The light sensor is not a narrow band  device.  It 
is going to pick up a lot of noise in addition to the  signal.  ( I do not know 
that it makes any difference to our applications,  but I understand that even 
a very good laser will have more than 250 MHz of  phase noise.  Maybe why we 
don't notice is because we have systems that  only respond to a few tens of 
KHz. )  Once the optical detector has done  its job, the rest of the signal 
chain can have filters to narrow range of  frequencies delivered to the signal 
detector ( typically a computer sound card  ).
 
Does it really help to put a narrow band audio filter in line between the  
optical detector and the sound card?  Can't the sound card just choose the  
frequencies that it is decoding?  If so the detection process would seem to  only 
deal with the sound in that freqency range, which would have passed through  
the outboard filters anyway.
 
 
Another question I have is, if there really is a practical reason to narrow  
the bandwidth of the signal from the optical detector, does that mean that a 
500  Hz tone is better for PSK31 than a 1KHz tone?  Carrying on that logic, 
then  250 Hz is better, and successively 125.  What is the limit?  If the  signal 
is 31.25 is the limit arround 65 Hz?   From a truely practical  stand point, 
can the typical sound card even operate at that low of a  frequency?  Should 
we be trying to work on a system that uses 65 Hz BPSK  for the improved signal 
to noise ratio that is inherent in the narrow bandwidth,  then "lifting" the 
signal up to 400 Hz so the sound card can process it?
 
 
For that matter, if there is real benefit to sending low frequency, why not  
send the digital bit stream with on-off keying of the light source?   Instead 
of modulating an OOK or sine wave audio tone that has to be more than  twice 
the frequency of the data stream, send just the data.  Doesn't that  result in 
an automatic two to one decrease in the noise bandwidth?
 
 
James 
N5GUI