[time-nuts] Phase modulation detection/NIST plan

[Bob Camp]

Hi

In this case the data format and it's contents are highly "computable". If you have a good local clock *and* an initial lock, the rest of what follows is predictable. That of course assumes we know the real format ….

Bob

On Jul 8, 2012, at 6:58 PM, J. Forster wrote:

> Hi Peter,
> 
> That's be the hard way, but yes, if the message BPSK coded is computable
> and of a known format. If the message contained more than time, like solar
> flux, it gets more complicated very rapidly.
> 
> A similar thing was done with the Equatorial system 30+ years ago. In that
> case, each data bit was broken into something like 32 or 64 chips (I don't
> remember). There were two maximally distant, orthogonal chip patterns,
> representing 1 and 0. The incoming BPSK message went through a 0 or 180
> degree switch, then the IF stages. The switch was driven from a local
> (known pattern) chip generator, so that if everything was synced up the
> narrow band IF would put out the 0 or 1 that had been encoded. BTW, this
> trick vastly improved the system S/N becaust it narrowed the receiver IF
> bandwidth many times.
> 
> If the chip pattern is not known (fixed) or computable (like a correct
> TOD) things go to pot quickly.
> 
> Rather than building such a kludge, it would be easier to use the locked
> clock in a newly designed receiver and phase compare that to your local
> standard directly.
> 
> -John
> 
> ==================
> 
> 
> 
> 
> 
> 
>> Any possibility of using the decoded signal to un-do the modulation and
>> feed the reconstituted signal to the older receiver?
>> 
>> 
>> 
>> On 7/8/2012 12:56 PM, paul wrote:
>>> Ei
>>> Sorry if I have your name reversed. By taking this approach it
>>> eliminates the ability to use wwvb as a frequency reference because it
>>> destroys that traceability.
>>> Thats what we are trying to preserve. Or at least re-establish for the
>>> older phase measuring receivers.
>>> Regards
>>> Paul
>>> 
>>> On 7/8/2012 12:10 PM, Tofurk Ei wrote:
>>>> If the changeover you are talking about is this one:
>>>> http://www.nist.gov/pml/newsletter/radio.cfm as a proof of concept a
>>>> DVB-T
>>>> dongle/upconverter combo could almost certainly handle PM easily to
>>>> output
>>>> whatever it encodes, when paired with gnuradio..
>>>> 
>>>> The RTL2832U chip might also be able to handle some low band signals
>>>> directly, using direct sampling. No upconverter.
>>>> 
>>>> Regardless, then the data would be fed into gnuradio - the gnuradio
>>>> developers GUI is called "gnuradio companion" It has a nifty way of
>>>> doing
>>>> this kind of thing, one builds a "flow graph" where the actual
>>>> demodulation
>>>> is simply laid out graphically and tested.
>>>> 
>>>> When everything works to one's satisfaction the file is saved and it
>>>> gets
>>>> compiled - then it can run - its basically a python script.
>>>> 
>>>> If the modulation scheme is public, I think you can be almost certain
>>>> that
>>>> gnuradio might be quite useful to rapidly design a tool to demodulate
>>>> it.
>>>> Perhaps very quickly.
>>>> 
>>>> For the money, one really couldn't hope to beat the flexibility of this
>>>> combination in any other manner. If I were interested in trying this I
>>>> would join the gnuradio mailing list and ask there. Perhaps the
>>>> answer is
>>>> surprisingly simple.
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>>> 
>>> 
>>> 
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> 
> 
> 
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