[Laser] minimum frequency PSK31

[Paolo Cravero]

TWOSIG at aol.com wrote:

Hi James

> PSK31 is a system developed for radio to provide  digital communications  at 
> a conversational speed ( approximately what  a person can type ).   Typically 

I'd add that PSK31 has lower a S/N requirement for proper reception if 
compared to RTTY or other digimodes. So, for the same output power, a 
weaker signal can still be decoded (in a hypothetical PSK31 vs RTTY race).

> 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 

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?

> 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".

I haven't read past discussion. According to my knowledge, put the tone 
where the receiving system has less noise, and build a narrow filter 
around that frequency.

> 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.

14.0805 MHz. Or 14.0815 MHz LSB too (@500Hz).

> 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

As long as the light detector responds to few kHz, you don't need to 
care of a broad TX phase noise. It will "see" only its few kHz of noise. 
Am I wrong?

> 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.

It does help. An example.
A typical soundcard has 24 kHz BW, which it samples at 48 kHz. If it is 
not too-cheap, it will have a lowpass filter at 24k or so, in order to 
avoid the unwanted "aliasing" effect. A soundcard will always sample 
that bandwidth (unless you reduce the sampling rate, but the input 
filter BW IMHO remains unchanged).
If the optical receiver has a broader BW, without any audio filter, 
everything it sees will go to the soundcard. Say the optical RX has 50 
kHz BW. Your signal is at 1 kHz but you've go a very strong 
"interference" at 40 kHz: it will saturate the soundcard input (which 
might have an automatic AGC as well) and mask the wanted signal.

I prefer to do narroband filtering outside the PC, and then let the 
software deal with a cleaner, narrow-BW, signal.

> 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, 

I think you've mixed up two concepts: tone center frequency and signal 
bandwidth.

Without interfering sources, so with a white background noise, if the 
receiver has a definite and constant BW (say 250 Hz) centered on your 
tone, there are no improvements operating at 300 Hz or 2576 Hz: 
bandwidth at the receiving end will always be the same.

> 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?

I see no improvements here, and how would you lift the signal up to 400 
Hz? A mixer can be highly non-linear!

To answer one of your questions, although soundcards are specified 
20-20kHz, you better stay away from those limits not to incur in 
distortions and certain bandpass attenuation.

> 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 

Do you mean sending "baseband". That requires theoretical infinite 
bandwidth and noiseless medium (ie optical fiber).

Those above are my views according to what I've studied, read and 
understood. Given that I'm not using these concepts in my everyday job, 
I might have written some nonsense. Please forgive and correct.

Paolo IK1ZYW