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Re: LF: ebnaut & speclab question

To: [email protected]
Subject: Re: LF: ebnaut & speclab question
From: Markus Vester <[email protected]>
Date: Sun, 26 Feb 2017 04:34:21 -0500
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Hi Jacek,
short answer:

Yes (if you have a soundcard without dropouts on the input). It will work on many modern PC's. For example, RN3AUS has achieved successful decodesd from DK7FC using samplerate sync to GBZ.

Longer answer:

The samplerate detector will track the frequancy and the phase of the reference signal. However if a buffer overrun occurs, an unknown number of samples may be missing in the data stream. Then the phase of the reference frequency will be reconstructed, but the phase on other frequencies will still be wrong. For example, if you are using a 10 kHz reference, a 1 ms dropout will swallow exactly one cycle, so the SR detector will simply not see it. On the other hand, the same 1 ms step will introduce a 97.2° phase step on 8270 Hz, which would ruin an EbNaut decode. With a non-integer kHz reference (e.g. 22.1 kHz), a phase change would be detectable on the reference, but the correction would still not scale correctly to the receive frequency. So  to be able to apply a correction to any frequency, we need to know the absolute duration of the outage.

One theoretical solution would be to track multiple different off-air references simultaneously, which will experience different phase steps, which could then be used to solve for the length of the data gap. However such a function is not quite trivial and not available in SpecLab.

The ambiguity is conveniently resolved using 1pps pulses from a GPS, allowing correct identification and elimination of glitches which are shorter than +-0.5 s. In addition with a PC clock, the 1 pps pulse can be allocated to it's time of day, so absolute phase can be retrieved on all frequencies after arbitrarily long outages or after a restart. If PC time cannot be kept within +-0.5 s of UT (eg. due to non-availability of NTP time, or multiple-second latency variation on an Icecast stream), serial time from the GPS (NMEA) can be fed to SpecLab to identify the pps second.

However while 1pps can be superimposed on antenna signals and eliminated a posteriori by the noise blanker, feeding NMEA to the soundcard requires dedicating a second soundcard channel solely for this purpose. This is the main reason why only one of two loops is available on Stefan's tree-grabber. To solve the issue, several mnonths ago I have found and developed a reliable way to identify the pps second using signals available from the antenna, i.e. either the 3.6 s Alpha sequence, or the DCF77 minute marker (which is conveniently downsampled by the soundcard to 2*48-77.5 = 18.5 kHz). I still haven't given up the hope that Stefan may some day decide to make use of these scripts...

Best 73,

-----Ursprüngliche Mitteilung-----
Von: Jacek Lipkowski <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: So, 26. Feb 2017 8:53
Betreff: LF: ebnaut & speclab question

Is it possible to decode an ebnaut transmission using only the sample rate
correction using external VLF transmitters in Spectrum Lab?

VY 73

Jacek / SQ5BPF

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