[Laser] minimum frequency PSK31

[TWOSIG at aol.com]

In a message dated 11/13/2006 9:50:33 AM Central Standard Time,  
KY1K at verizon.net writes:

System 1  is a dc to 400 Hz bandwidth receiver listening to a PSK31 
signal at 300  Hz.

System 2 is a dc to 800 Hz receiver listening to a PSK31 signal at  600 Hz.






Art
 
It seems straight forward to say that for two systems which are identical  
except as you describe above, that is the receive bandwidth from dc to X, system 
 2 will receive twice the noise as system 1.
 
It does not follow ( at least not to me ) that the 300 Hz signal will be  any 
stronger than the 600 Hz one.  Nor does it seem that the noise density  of 
one system is higher than the other.
 
If we presume that the sound card can process the input signals and noise  
from 100 Hz ( not from DC ) up to at least 10 KHz, what is to prevent a talented 
 programmer from designing two digital filters each with unity gain and 100  
Hz bandwidth, one centered at 300 Hz and the other at 600 Hz.  Run the  
digitized audio input through the filters ( or build external ones ) and then  
decode the PSK31 signals.  Don't we then have the same noise figure for  both 
signals?  And presumably the same S/(S+N).
 
>From a practical standpoint, I think what you are saying is that  if someone 
wanted to experiment with digital signals, then the system  should be built 
with lower rather than higher frequencies in mind, and that the  receive chain 
should have low pass characteristics with the "knee" above the  expected 
signals.  This sounds like a practical suggestion.
 
To me, it still "feels" like using bandpass filtering should work as  well.  
At least if you stay well within the operating range of the sound  card you 
are using to decode.  Perhaps there is a way to run experiements  with a system 
that has a 400 Hz cutoff ( DC to 400 ) compared to a system with a  1600 Hz 
cutoff ( DC to 1600 ) followed by a 1200 Hz cutoff high pass  filter.
 
 
Thank you for trying explain this.