Hi The other answer is that DSP was not really available when the original waveforms were developed. A modern system would not have a "must be able to work with manual delay lines and an oscilloscope" requirement on it.
Bob On Oct 5, 2010, at 6:56 PM, Poul-Henning Kamp wrote: > In message <4caba343.8c581...@cox.net>, WB6BNQ writes: > >> Please explain to me how spread spectrum would enhance any process >> of frequency or time recovery ? > > Ok, it is late and I'm probably going to botch this, but I'll try: > > The really short explanation is that your carrier transitions have > random-ish looking signs, which, if properly designed, allows you to > balance out pretty much any kind of CW or random noise. > > This is, in essence, why you can separate the different GPS > sattelites, even though they all send on the same frequency. > > Technically speaking, Loran-C is spread spectrum, but they botched > this aspect slightly, by not properly balancing the signs of (all) > the codes. > > The Austron 2000 has a switch that allows you to disregard certain > bits in the codes to balance them, this increases the imunity to > CW interference. > > So given that you can trivially get a good OCXO today, I would design > our "low-power-time-transmitter" to send one fix per hour. > > For instance 127 bits of PRNG at 28 seconds per bit with a four > second gap before the next timestamp (send ID ?) > > On the receiver side, you know what time it is +/- one 28sec bit, > so you digitize the signal and correlate the PRNG in a window around > your local clock. > > After an hour, you pick the correlation bucket that correlated best > and have an instant estimate of the difference between your local > clock and the average of that hours transmissions. > > If xmitted as NFSK at around 100kHz and digitized at 1MSPS, you > would get 1µsec resolution without resorting to interpolation. > > By choosing all your magic numbers to be nonprime to normal > CW signals inside their respective periods, you supress those > by averaging. > > This is why the NELS LORAN-C chains got new 4-digit GRI's: they > are imune to pretty much traditional CW interference because > they do not divide seconds or kHz on relevant timescales. > > Poul-Henning > > PS: DCF77 already does SS, but on a second to second basis. > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > p...@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. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.