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Re: LF: Coherent BPSK on LF using EbNaut

To: [email protected]
Subject: Re: LF: Coherent BPSK on LF using EbNaut
From: Andy Talbot <[email protected]>
Date: Fri, 25 Sep 2015 17:20:50 +0100
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That gives me an idea ...
For some time I've been toying with the idea of writing  a utility that reads audio in blocks based on UTC timings and saves to a .WAV file . It woudl form part of a long term plan for playing with writing data mode software, using a similar timing protocol to WSJT, ie blocks of data timed on the minute, or 30s or whatever.   Using blocks like this makes the software an awful lot simpler to write than trying use real time, continuous streaming audio.

I was originally just going to save the audio to a mono .wav,  but it would be easy enough to filter, decimate and downconvert to baseband.  That way would allow a standalone prog for receiving  EbNaut instead of a custom setting in Spectrum Lab
The idea of using a .WAV file for storing baseband IQ data has a certain appeal, and one I hadn't thought of, even though stereo I/Q is used in real time for all the Softrock-type SDRs out there..

Andy  G4JNT

On 25 September 2015 at 15:37, Domenico IZ7SLZ <[email protected]> wrote:
Hello Andy,  all
thanks for interesting at this test.

Yes, Spectrum Lab is the right program to manage the audio from receiver also in this case. The SL configuration used in LF_EbNaut experiment basically is the same of the DF6NM OPDS setup  downloadable at Markus' site:

Markus, as usual, includes all informations necessary to do the proper changes according to the hardware at disposal.

In my setup, i used SL to do many things: calibrate the sample-rate,  manage the ptt at scheduled time,  generate a precise tone to drive my SSB exciter,  create some screenshots of the waterfall and - most important -  export, every 10 minutes,  the FFT raw data of the reception periods.

Markus will explain better than me this matter, but exporting FFT instead of .WAV file seems a good solution to avoid some issues encountered with the  .wav  recording function of SL.
He wrote a program (named ebnaut_ifft2.exe) able to convert FFT raw data  to wav file,  inserting the appropriate timestamp.

After this conversion,  Ebnaut-rx can be used, but it needs of some - very sensitive -  offset entry. For that,  you need to become skilled with the use of some other Markus' tools that plot the symbol phase before the full EbNaut-rx search.
But, if the clock  and the  frequency have very small drift in both TX and RX sides, than you can use the same measured parameters to catch the next transmitted messages.

73, Domenico (iz7slz)

On 25 September 2015 at 13:23, Andy Talbot <[email protected]> wrote:
EbAnut is a new one here - I hadn't heard of it.   After Googling I found the page and read up / downloaded the software.   Unfortunately, I see it is not standalone, and requires Spectrum lab to act as the audio "front end" to take in the Received audio, downconvert to a baseband I/Q and store that as a .WAV file.   So downloaded Spectrum lab and tying to wade through how to use that.

Could you perhaps give a short cut simple explanation of how to set up Spectrum lab to do that - it looks to be a very complex bit of software to drive.
the EbNaut Tx and Rx programmes, in contrast, look very simple to use and it would be goo dto get more people using sensitive coherent signalling such as this.

Some years ago I proposed GPS / UTC seconds locked coherent signalling on LF, but didn't have the expertise to write the software - and certainly not to implement strong FEC !

Andy  G4JNT

On 23 September 2015 at 01:04, IZ7SLZ <[email protected]> wrote:
Hello LF,

I am pleased to report a successful "Coherent BPSK" QSO between DF6NM and IZ7SLZ on LF using EbNaut program of Paul Nicholson.
QSO took place in the night between Friday 2015-09-18 and Saturday 2015-09-19 .

I think it was the first time that anyone tried this on 137 kHz !

Frequencies used were 137510.00 Hz (DF6NM) and 137502.12 (IZ7SLZ).

We exchange messages of 16 characters in coherent BPSK with UT synchronous symbols, using a symbol period of 1 second and 8K19A scheme coding.

Software used were the Windows' versions of EbNaut-TX and EbNaut-RX (tnx to P. Nicholson: your software is working without problems!).
Also Spectrum Lab (tnx Wolf !) was used to export FFT data, in a similar way of the Markus' OPDS setup.
Other programs,  written by Markus,  were used: one  to convert FFT exported data for the EbNaut entry and another to search for the best frequency and time offsets.

There was not much phase drift during the qso and in other test transmissions, since we reach a good frequency stability at both ends thanks to Rubidium Reference Oscillator (Markus side) and a good quality OCXO from my side.

You can see reported symbol / carrier EbN0 and also the messages exchanged,  in the attached screen-shots taken at both ends.

We hope now to joint other stations that want to challenge the timing and frequency accuracy, to have a QSO in LF close to the Shannon limit.

Markus DF6NM, will report soon all particulars and his comments of the experiment. Thanks in advance Markus!

73, Domenico (iz7slz)

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