On 24 May 2012 12:23, Graham Bryce <gra...@visible-support.com> wrote:
> Good afternoon, everyone.****
>
> ** **
>
> Can I suggest that, rather than a rely on single packet “header” to
> achieve synchronisation like D-Star, that consideration is given to a
> distributed sync system like the NATO STANAG 4285-based scheme that Murray
> Greenman and Con Wassilief used for their “CMSK” HF-optimised GMSK modem?
> ****
>
> This involves burying a repeating patern of 32 isolated sync bits into
> the datastream to establish synchronisation, and because the sync detection
> is a correlation process over a long-ish period, it makes it more robust in
> the presence of burst noise or high bit error rates.****
>
>
I had a quick look at that system. As far as I can see it works in a much
similar way to David's current HF modem. In that it relies on the
characteristics of PSK to get either 16 or 32 symbols in a segment. So
while you know your 16 or 32 bits are aligned correctly without a physical
sync, anything beyond that is not known without a secondary method.
>From looking at the FDMDV modem a similar method is used to send data in
blocks of 28 symbols, 2 of which made up a codec2 frame. Which is the first
half and second half is decided by a single "sync bit". Sorry David if I
understood the code wrong. But that's how I read it.
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.
> A basic “back of an envelope” calculation indicates that, trying to
> carry a 1400 baud Codec 2 data “payload” over a 2400 baud GMSK modem, you
> could apply 2/3 rate convolutional error correction to increase the payload
> rate to 2100 baud, and then packetize the resulting stream into pairs of
> 11-bit frames (11 payload bits in one frame, 10 payload bits plus a single
> bit from a slow 100 baud “metadata” stream in the other) followed by a
> single sync bit, with these 12-bit sub-frames repeating every 5ms.****
>
> The 32-bit PRBS sync sequence would repeat every 160ms, and provide
> sync for both the slow data stream, the convolutional coding puncturing
> sequence, and any interleaving that has been applied to make the payload
> less susceptible to burst errors.
>
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.
>
> Just a suggestion…
>
It's probably a good one, and I'm just missing something. Like I say it's
fine if someone else wants to do this, I'm not trying to be awkward. When I
started on the documentation, no-one was working on anything for VHF/UHF to
my knowledge. So I was just trying to get the cogs moving, so to speak. :)
Best regards,
Peter - M6DGI.
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