[time-nuts] PN sequence generation using GPS

[Bob Camp]

Hi

If I can reset the sequence at any time I feel like while transmitting, I am modifying the sequence. The PN used to run my radio will not be the "official" PN they have legislated. 

On a practical basis - I suspect they use autocorrelation to monitor you. As long as it's the right sequence, started where ever, their gear will work. The second issue would be that making gear that inter operated would be really tough if the sequence did strange things every so often. Manufacturer A would have a tough time talking to manufacturer B.

Bob


On Feb 17, 2011, at 5:46 PM, Hal Murray wrote:

> 
> lists at rtty.us said:
>> The key item here is that the system is going to work via amateur radio here
>> in the US. The FCC only lets you use three very specific PN sequences. The
>> three are called out explicitly in the rules. The requirement that they not
>> be reset while transmitting except by feedback is also called out explicitly
>> in the rules. So no fancy stuff with multiple spreaders and the like.  
> 
> Thanks, but I'm having trouble understanding that.
> 
> The problem is "not be reset while transmitting except by feedback".  I 
> usually use reset to mean set the system to a specific state no matter what 
> the current state.
> 
> I think they are trying to say you can't use something like a PPS signal to 
> reset the LFSR.  If so, I would have said something like "can't be reset.  
> The next state can only be influnced by feedback."  (Which also rules out the 
> self-synchronizer type approaches which use the data bit rather than the LFSR 
> output.)
> 
> 
> Are there any constraints on picking the initial state?  (There has to be 
> some time when the transmitter starts.)  Can I send packets?  Can I start 
> each packet on a second boundary (and not reset if it goes over 1 second)?
> 
> What do you call a clump of voice when something like a PTT is used?
> 
> 
>> There's a much easier way to do it and keep everybody in sync. GPS gives you
>> time of day along with the 1 pps signal. You agree that at this or that
>> time, everybody starts in sync. It's just time math to figure out where you
>> should be at any 1 pps point. If you aren't where you are supposed to be,
>> you take action.  
> 
> That makes sense to me.
> 
> I think you have to figure out what to do if the transmission crosses 
> midnight.  You can shut down and start over, or just keep going.  In the 
> latter case, you have to agree (out of band) which day you started in, but 
> that only matters if you start near midnight.
> 
> 
> 
> 
> 
> 
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
> 
> 
> 
> 
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