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From: Paul Nicholson <vlf0403@abelian.org>
Message-ID: <603f2e72-38b9-0d93-31c1-416bf05f4ed6@abelian.org>
Date: Sun, 31 Dec 2017 16:13:21 +0000
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Subject: Re: LF: VLF NA Ebnaut
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Domenico wrote:

 > can you suggest us some commands for stacking  vlfrx-tools
 > raw audio

The script I use is attached, save it to your working directory
and make executable with

  chmod +x stack

First I'll describe how to phase shift.

Assume you've extracted each repeat and mixed down to base
band at a low sample rate, eg with some command like

  for day in 27 28 29 30
  do

     vtread -T 2017-12-${day}_22:30,+27900 /raw | # Retrieve signal
       vtcat -p |                      # Pad over any timing breaks
       vtfilter -h bp,f=8270,w=3000 |      # Filter before blanking
       vtblank -a12 -d0 -t100 -v |      # Typical nighttime options
       vtmult -f 8269.9 |                    # Mix down to baseband
       vtresample -r 240 | vtresample -r1 > w4dex-${day}.vt

  done

You end up with a set of files:

   w4dex-27.vt w4dex-28.vt, w4dex-29.vt, w4dex-30.vt

each containing two channels, I and Q with the signal at
zero frequency.

You don't usually have to adjust phase but in this case the
transmitted phase changed from night to night so we need to
change the phase of three of the files so they all share
the same phase.  Let's leave the 27th with its original
phase and adjust 28, 29, and 30 to match.  The required
adjustments are:

  28: +118 deg
  29: +236 deg
  30: +260 deg

Run each of the three files through vtmix, with the coefficients
calculated by awk.  Eg for the 28th,

  angle=118

  mix=`awk -v "A=$angle" 'BEGIN{
    sinA = sin( A * 3.14159/180)
    cosA = cos( A * 3.14159/180)
    printf( "-c %.3e,%.3e -c %.3e,%.3e\n", cosA, sinA, -sinA, cosA);
  }'`

  vtmix $mix < w4dex-28.vt > w4dex-28.pt

where I'm using .pt to stand for the phase shifted version.

Now stack the four files using the script:

  ./stack w4dex-27.vt w4dex-28.pt, w4dex-29.pt, w4dex-30.pt > stack.vt

Then you can just decode stack.vt in the normal way:

  vtraw -oa stack.vt |
   ebnaut -dp16K21A -r1 -S30 -k20 -N3 -PU -L500000 -v

Of course, there isn't as much typing as it looks.  Anything you run
more than once or twice you'll type into a text file and make it
executable with chmod +x so that you can run that file as a command.
You soon end up with a nice suite of signal processing scripts to
make everything quick and easy and reliable.

Oh, if the decode fails, try weighting with power instead of
amplitude:

  ./stack -p w4dex-27.vt ... > stack.vt

and/or try a -a option to ebnaut:

  ebnaut -dp16K21A -r1 -S30 -k20 -N3 -PU -L500000 -v -a 50

--
Paul Nicholson
--

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