Hi Steve,
Thanks for the additional info. I will look forward to your further
progress with the sfrsd decoder.
> By the way, one thing that I’ve already noticed is that some
> files produce lots of identical un-decodable received symbol
> vectors. Since I have to try a whole bunch of virtual received
> signal vectors before I can decide that these are un-decodable,
> my decoder bogs down on these whereas KV seems to be able to
> blow right past them. I’m guessing that these events result
> from interference, or birdies - I can’t think of anything
> else that would produce a slew of identical vectors. It may
> eventually be necessary to figure out a way to identify these
> events before they are sent to the decoder.
Have you looked at what's going on in extract.F90 ? Birdie suppression
takes place there, before kvasd is invoked. If sfrsd (temporarily
renamed to kvasd) is used instead, it will be given symbol vectors from
which birdies have supposedly already been removed.
> Re JTMSK - I am very interested in it, but I haven’t had time to
> look at what you are doing there. If you have carefully read the
> relevant sections of Proakis, then you certainly know more about
> CPFSK techniques than I do. I am aware of the fact that it should
> be possible, in principle, to do some form of block demodulation,
> perhaps using the Viterbi algorithm, as opposed to symbol-by-symbol
> demodulation. While this could produce significant gains - it would
> seem that this approach will require the channel to be reasonably
> well behaved (i.e. no significant frequency offset or random phase
> walks), no?
When I find time I will try implementing a simplified block demodulation
scheme perhaps using only 2 or 3 consecutive symbols. That should be
fairly simple, and Proakis shows that you can achieve most of the
potential advantage that way.
As for the channel behavior: I have been surprised to discover how well
phase coherence is preserved over meteor-scatter pings. I designed the
JTMSK message structure to include a synchronization pattern with three
copies of the "Barker-11" code spaced at non-commensurate intervals.
Also three "even-parity" bits that ensure that an even number of
lower-tone intervals occur between each of the B11 codes. This makes it
possible to look for a known pattern of 33 bits that extends (with gaps
containing the 198 information bits and 3 even-parity bits) across the
full message length. With this information the frequency offset can be
detected and measured quite easily. After correction for that offset, a
plot of measured phase for all 234 symbols shows surprisingly little
scatter.
-- Joe, K1JT
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