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To: [email protected]
Subject: Re: LF: SAQ
From: DK7FC <[email protected]>
Date: Fri, 26 Oct 2018 10:01:18 +0200
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Hi Domenico,

Thanks for the feedback.
Is the auto-notch filter necessary at 17.2 kHz? Hopefully it doesn't trigger on the SAQ signal and removes it ;-)
Maybe not a suggestion, just my point of view (you asked about it):
The noise blanker may improve the S/N but listening to a CW signal doesn't sound so good when a noise blanker is used. Also i would choose a smooth filter, low pass + high pass, maybe combined with the brick wall filter. There is/was that Russian transmitter arround 18.2 kHz, very strong here. It will lower the performance of the noise blanker.
Here i used vtread | vtcat -p | vtmix | vtmult -f16500 | vtmix -c1,j | vtfilter -h hp,f=850,poles=10 -h lp,f=550,poles=10 -g1000 | vtresample -r4000 | vtraw -ow > audio.wav
..., i.e. i convert the SAQ signal to 700 Hz, apply a rough band pass filter, resample to 4 kS/s and then i read the wav file in SpecLab and apply a rough noise blanker (high threshold, just for the few very strong sferics), then a better CW filter and then a slow AGC.

Are you regularly archiving the VLF stream from your soundcard to a database (using vtwrite) ?

73, Stefan

Am 26.10.2018 00:20, schrieb Domenico IZ7SLZ:
Hi Stefan,

Thanks for informations on your temporary setup for SAQ.

For this last SAQ reception, I have used vlfrx-tools to acquire (vtcard) and record (vtwrite) the VLF signal coming from E-probe antenna.Then filters, NB and vtvorbis have been used for stream the 'RF' to the other pc where Spectrum Lab is running.

vtfilter -v -h bp,f=17200,w=3000 @input:1  |          # Pre-filter before blanking
vtfilter -v -a th=5 |                                                # hum filter
vtblank -v -a15 -d0 -t300 |                                    # sferic blanker settings
vtresample -v -r 48000 |                                       # Reduce to 48000 sample pairs/sec
vtvorbis -v -ep -t -q1 -kn 1236                               #port 1236

So, in a similar way as your setup Stefan,  Spectrum Lab here uses this stream, a  bp filter with a frequency shift (to obtain a central frequency of  700 Hz) and an AGC (slow) before the D/A converter (see attached screenshot).

Maybe there is a better way to stream the raw audio to a Windows PC but this i'm using is working ok.
I also use this method to stream LF audio (filtered and timestamped by vlfrx-tools) into another pc  where SL and OPDS are running.

Any suggestions and improvements are welcome.

73, Domenico IZ7SLZ

On Thu, 25 Oct 2018, 09:30 DK7FC, <[email protected]> wrote:
Hi Domenico,

Which filter settings did you choose to obtain the CW filter?

I also used vlfrx-tools to extract the SAQ message. The SNR was much
stronger on the N-S loop than from the E field. Mixing E and H
(including a small portion of the E-W loop) resulted in an SNR increase
of > 6 dB.
But in the end i used SpecLab to build the CW filter, a rough noise
blanker for the very loud sferics (12 dB threshold) and finally a slow
AGC which made it even more comfortable to listen to...

73, Stefan

Am 24.10.2018 23:35, schrieb Domenico IZ7SLZ:
> Good reception of SAQ here in JN80nu thanks to very low QRN and to
> Paul's vlfrx-tools used for acquiring, filtering and stream the audio
> to Windows PC.  Then i have used Spectrum Lab to convert to analog and
> send the audio to the speaker. Path is about 2300 km.
> This is the audio recordered:
> In perfect 'vintage' marine radio officer style, i have prepared the
> text sent by SAQ :
> a nice text about the Peace. Thanks to the SAQ staff for their activities.
> 73, Domenico IZ7SLZ

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