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From: Paul Nicholson <vlf0403@abelian.org>
Message-ID: <64a5498c-8ac8-09c2-6b90-0826830a8f8e@abelian.org>
Date: Thu, 9 Nov 2017 17:36:18 +0000
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Subject: Re: R: Re: VLF: EbNaut message over 2877 km, DK7FC to RC4HAA
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 > Can you suggest and give some indications how to stack raw audio
 > of two or more different days using vlfrx-tools?

I have a little shell script to do the stacking, which I've
attached.

First, extract each transmission in the usual way, eg something
like

  vtread -T2017-11-07_15:58,+45360 /raw |
    vtcat -p |
    vtfilter -h bp,f=8270,w=3000 |
    vtblank -a12 -d0 -t100 -v |
    vtcat -S120 |
    vtmult -f 8270.1 |
    vtresample -r240 | vtresample -r1 > dk7fc-t1.vt

I start 2 mins early to give the blanker some time to settle
its auto threshold, then remove the leading 2 mins with
vtcat -S120.  I also add a minute or two at then end.

  Message duration = 45120 (use the signal calculator web page)

  45120 + 120 + 120 = 45360 seconds.

The vtcat -p pads over any timing breaks that might be in
the raw recording.

vtblank -a12 is good for night reception, I use about -a22 for
daytime.  vtblank should report a 'dropfactor' of somewhere
around 5 to 30%, eg I got

  vtblank: dropfactor 0 2.745e-01

reported on this one, which means 27.45% of the signal was blanked.

After mixing to I/Q, I drop the sample rate to one sample pair
per second, which needs 2 stages of vtresample.  If the symbol
period is shorter you may want to use a higher sample rate.

(I've left out the antenna E and H mixing in this example)

Run all the transmission as above, eg

  vtread -T2017-11-08_15:58,+45360 /raw ... > dk7fc-t2.vt

so you end up with a collection of files, one for each
transmission.

Then run the stacking script: list all the files on its
command line and direct the output to another file, eg

  ./stack dk7fc-t1.vt dk7fc-t2.vt > dk7fc-stacked.vt

The script shifts each file to a common start time using
vtcat -a, then sums them all into a single I/Q stream
using vtjoin and vtmix, with vtmix normalising either by
amplitude or power.   Amplitude is default, use a -p option
to normalise by power instead:

  ./stack -p dk7fc-t1.vt dk7fc-t2.vt > dk7fc-stacked.vt

You can list as many files as you like.   The script uses
/tmp/stack as scratch space, you might want to edit the
script to put these somewhere else.

Then run the stack file into ebnaut in the usual way:

  vtraw -oa dk7fc-stacked.vt |
    ebnaut -dp16K21A -r1 -S60 -k9 -N3 -PU -L1000 -c8 -v

--
Paul Nicholson
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