On 24 May 2012 16:32, Graham Bryce <[email protected]> wrote:
Hi Graham,
There are a few point here that I think I should clarify…****
>
> * *
>
> *The PSK systems are great for HF. But these designs are specifically for
> VHF/UHF use over an FM radio. I don't know how well this would perform over
> that medium. Which is the reason the current suggestion centre around the
> use of systems known to work on FM, GMSK/4FSK with sync patterns to mark
> the start of useful data.
> *
> I think you’ve been looking at another modulation mode to the one I was
> talking about (see http://www.qsl.net/zl1bpu/CMSK/cmsk.htm) – GMSK is the
> obvious choice for VHF and above, if only because of it’s constant
> amplitude, and tolerance of non-linear (and so power efficient)
> amplification. My suggestion was concentrating on the synchronisation
> mechanism within the transmitted datastream…**
>
> **
>
Yeah, I was looking at this
http://www.wavecom.ch/onlinehelp/WCODE/WordDocuments/stanag42851.htm which
is describing various versions of PSK. I see you're just using the
distributed sync element of the mode.
> **
>
> ** **
>
> **
> *I'm still trying to work out how PRBS alone will help. As far as I can
> work out PRBS is effectively the same as the scramble stage in D-star, and
> used to minimize the occurrence of too many of the same state being sent
> over the link, which could cause sync issues with the actual bit stream.
> I'm happy to hand over the design of a VHF/UHF system to someone that can
> better do so using these mechanisms. In my head I can't quite see how sync
> can be attained using a PBRS sequence alone. Of course if you know the
> original data beforehand you can easily do so (meshing the sequence until
> you get the result you expect). But with unknown data, I just cannot see
> how it can establish sync. If I can't understand it, I can't specify for it.
> *
> * * You seem to have misunderstood the reference to a PRBS – the
> sequence of 32 sync bits that are interleaved with the payload data (one
> per 11 “real” bits) are generated by a PRBS to make the sequence look like
> noise, but be predictable. The PRBS isn’t used to modify the transmitted
> data as it would be in energy dispersal application like the one you
> describe. The sync detection process looks at every 12th bit received,
> and attempts to correlate this sequence to the reference sequence generated
> by the PRBS at the receiver. It’s only when the sequences match (and stay
> matched) for a while that a lock is established. The idea is that the
> chances of that pattern and distribution of bits is very unlikely to occur
> and be maintained in the actual payload data, and so, although a false lock
> might occur, it won’t be sustained.****
>
> **
>
It's an idea. Not sure it's something to put in the first versions. I agree
with Kristoff on keeping the initial versions simple. Surely it takes more
memory/processing time to detect this sync though? Because we don't know
where we are at all. It'd need the full most recent 100+ bits, and check
all permutations of 12 bits apart. But being able to make a "best guess"
early on after a sync loss is a nice thing I'll agree.
Best regards,
Peter - M6DGI.
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