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Re: Re: Observed AO-40 FEC coded telemetry



Jonathan Naylor wrote:

> The problem is that dpsk_demod is synchronous and the data coming down from
> AO-40 is asynchronous.

Just to fill in the details, I know exactly what the problem is and what 
has to be done to fix it, but it's a little more work than I thought it 
would be. It also doesn't help that I hadn't looked at this code in over 
a year -- you forget what you used to have all in your head!

In the existing dpsk_demod program, I assume that frames arrive 
synchronously every 13 seconds. Since the 65-bit sync vector is 
interleaved with the data, that means I could take any arbitrary 
"window" of 13 seconds of received data and find some complete cyclic 
(circular) shift of the sync vector regardless of where that 13 second 
window happens to lie with regard to the boundaries of an actual frame.

This assumption is violated by the present asynchronous format, but if 
you happen to pick a 13 second window that includes part of a FEC frame 
you'll still capture that fraction of its sync vector, and if that's 
enough to detect sync the demodulator will still work. This is what 
Viktor has been doing.

I'm currently restructuring dpsk_demod to eliminate this assumption.

Phil

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