[time-nuts] DCF77 question

Dennis Ferguson dennis.c.ferguson at gmail.com
Wed Oct 12 21:11:40 EDT 2011


On 12 Oct, 2011, at 16:03 , asmagal at fc.up.pt wrote:
> QUOTE:
> 
> "DCF77 marks seconds by reducing carrier power
> for an interval beginning on the second".
> 
> UNQUOTE
> 
> This is not a good (Heaviside step) time marker.

For what it is worth, the amplitude modulated on-time
second pulse isn't the only time marker DCF77 transmits,
it also transmits a phase modulated 512 bit pseudo-random
sequence in the remainder of the second.  See

    http://http://en.wikipedia.org/wiki/DCF77

or, for the best technical description I've seen, google

    "Zeit- und Normalfrequenzverbreitung mit DCF77"

The phase modulation is much, much better than the
amplitude modulation for a pile of reasons.

> Is it possible to decode that signal by any very
> simple "on-my-shelf gear", as for instance a PC
> sound card,to recover a good seconds time mark?
> 
> Do you know some software to perform the above task?

I don't yet.  I've been playing with DCF77 with a high-zoot
RFspace DSP receiver located in Toronto, however, and have
found that even though the waterfall display shows only noise
at 77.5 kHz, so there's no chance of measuring the amplitude
modulated time marker, I can usually detect the phase modulation
sequence in the noise for quite a few hours per day using the
brute force approach of convolution with an  800 ms long
filter matched to the sequence.  When I find the time to
actually finish this I want to begin recording propagation
delays when the signal is detectable.

If you end up wanting to build your own detector I'd highly
recommend chasing the phase modulated sequence rather than
the amplitude modulated second marker.  BPC on 68.5 kHz transmits
a similar phase modulated code (according to a PTTI paper they
presented) which there may be some hope of detecting on the
North America west coast.  While they don't publish the details,
I've been planning to take the receiver and an antenna with me
when I next go to Hong Kong and see if I can record enough
good quality signal to eventually reverse engineer the
encoding out of it.

Dennis Ferguson




More information about the time-nuts mailing list