Message-ID: <56EADC81.2000107@abelian.org>
Date: Thu, 17 Mar 2016 16:34:09 +0000
From: Paul Nicholson <vlf0403@abelian.org>
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Subject: Re: LF: 8270Hz
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Status: O


Markus wrote:

 > Running the decoder with 61 rather than 60 s offset happened to
 > reduce the number of symbol errors from 388 (0.4 dB) to 369
 > (1.3 dB Eb/N0).

Thanks Markus, perhaps there's a sample offset error
in the pipeline somewhere.  I reproduced the same result with
the Linux decoder too: needs T offset +1 to get 1.2 dB when
using the 1s/s signal.

 > when using the SpecLab FFT-export method with very slow
 > samplerates, I noticed that I had to add about four samples
 > worth to the time offset for best decodes

An out-by-one error is an easy mistake for a novice programmer
to make.   An out-by-four error requires much more skill and
generally needs an experienced programmer.

I'll investigate.

If anyone's interested, following are the commands I use to
receive Uwe's signal.

The first pipeline extracts the signal from a raw store which
has been filled by vtwrite, and mixes it down to baseband I/Q
centered at the signal frequency,

  vtread -T2016-03-14_03:00:00,+11900 /raw | # Extract 11900 seconds
   vtmix -c 0.866,0.500,0.45/-105.71 | # Steer loops and add E-field
   vtfilter -h bp,f=8270,w=3000 |            # Filter 3kHz bandwidth
   vtblank -a1.2 -d0 -t1 |                          # Remove sferics
   vtmult -f 8270 |                   # Mix with LO at 8270.00000 Hz
   vtresample -r240 > dj8wx.160314a.vt      # Resample to a low rate

The magic numbers given to vtmix are the loop coefficients for
steering, followed by the coefficient/phase_shift of the E-field.

Then another pipeline does the decode

  vtraw -oa dj8wx.160314a.vt |               # Convert to ASCII data
   ebnaut -dvp8K19A -N19 -S10 -r240 -PS -c4 -L20000         # Decode

--
Paul Nicholson
--