[time-nuts] Receiving the MSF time signal on cheap radio modules

Poul-Henning Kamp phk at phk.freebsd.dk
Tue Feb 6 16:19:42 EST 2018

In message <CAGJ4F+54Sxqc34fpNefNW3q3emHmq+uC-V-Lfc5Bnfk7h7qWUw at mail.gmail.com>, "Deirdre O'Byrne" writes:

>With a blame algorithm in place it should be possible to recover these signals.

Yes, easily.

At distance MSF is significantly harder to receive than DCF77.

One of the reasons is that USA also operates two 60kHz transmitters
also very precisely on frequency, so there are areas of the world
where the three signals cancel and areas where they reinforce
each other.

I tried to model this many years ago, but I don't trust the result,
somebody with better HF-propagation chops than me should look at it.

In addition to that problem, switch-mode designers seems to just
*love* 60 kHz, and at least here in Denmark there is a lot more
"hash" around 60 kHz than 77.5 kHz.

Finally, the modulation scheme of MSF is a bit on the overengineered
side, which makes pulse discrimination needlessly hard - as you have
also found out.

The big advantage of the blame algorithm is that since it is so
tolerant of missing pulses, you can be throw everything away which
isn't 100% clearcut.

If you look at the top of the dcf77.c file, you can see how I did
that for DCF77, but the complex modulation of MSF needs a much
more complex state engine there.

Finally, many of the small "clock-receivers", like the one you use,
are optimised for battery-life and therefore they use very resonant
filters, often crystal-filters, and heavy low-pass after demodulation,
and that trows away a LOT of information which would be useful to
have to discriminate the pulses.

If you go for the SDR approach, you will have much more information
available, and can use much more well-behaved filters to detect the
pulses, and one added advantage of carrier-tracking is that the
power-modulation is carrier-synchronous, which makes them much
easier to spot.

So really:  Get yourself an 1MSPS ADC chip and go that route instead.

(In theory, certain modern sound-cards should be usable for this if
you can rip out their low-pass filters.  Havn't tried.)

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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