[pskmail] Re: Improved FEC for PSK modes

  • From: Pär Crusefalk <per@xxxxxxxxxxxx>
  • To: pskmail@xxxxxxxxxxxxx
  • Date: Mon, 23 Nov 2009 17:21:07 +0100

Hi John,

This is excellent, just what I was saying a while ago I should do.. :-)
I would guess that lightning and static crashes would be better handled
by an interleaver coupled with fec. I'll test this asap!

73 de Per, sm0rwo


mån 2009-11-23 klockan 17:33 +1100 skrev John Douyere:
> Hi all,
> 
> I have done more work on the FEC option for BPSK modes:
> 
> I added a second Viterbi decoder shifted by one bit and compare the
> output of both, then I select the output of the one with the less
> errors.
> 
> Cpu load has not increased significantly, but re-synchronisation is
> now fast (typically one to 3 characters).
> 
> I have also changed the way we send the preamble and postamble for
> PSK: I pass both through the encoder/decoder as this gives the chance
> for the encoder and decoder to be fully synchronized before the frame
> data and to flush out the remaining characters at the end of the frame
> too. This gives a different sound to the preamble and postamble of
> this mode compared to the traditional BPSK modes.
> 
> I am still doing hard decoding.
> 
> The result seems good based on my tests today. Still not the
> robustness of the THOR or MFSK for lighting strikes, but much better
> at handling white noise and regular qrm it seems.
> 
> In my tests, most of the time I could get better results than the BPSK
> mode one speed below, which is a fair comparison in terms of
> throughput. And certainly much better for the same BPSK mode without
> FEC.
> 
> PSK500 with FEC gives good results. I even did some tests mobile and
> got good copies when generally the quick fades are not very kind to
> the PSK modes.
> 
> I feel that version is really worth a try. It could give us a nice
> speed downgrade/reliability increase for the automatic mode changes.
> 
> Attached are two files: psk.h in the src/include directory and psk.cxx
> in the src/psk directory. These should work with any source of Fldigi
> from at least 3.13AY onwards.
> 
> To access these modes, use the QPSK modes on the Fldigi gui, with the
> exception of QPSK31 which is PSK500+FEC. This is of course a temporary
> and quick solution since we do not use the QPSK modes in Pskmail.
> 
> I will also do some comparisons with the Multipsk implementation of
> these FEC modes to see if I am not missing something on the coding
> gain.
> 
> Please let me know of your results if you can test.
> 
> Best regards,
> 
> John (VK2ETA)


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