Hi Thomas,
This is a very nice demodulation! Congratulations!
Regarding frequency sync: I count roughly 80 bits in your packet. Is there some
synchronization preamble perhaps in the beginning, where the quad demod jumps
around?
Best regards,
Marcus
On 2 April 2018 10:48:00 GMT+02:00, Thomas Habets <tho...@habets.se> wrote:
>Hi.
>
>As an experiment I'm trying to decode an FT8 signal I've captured. I've
>gotten far enough that I can clearly see the packet (
>https://blog.habets.se/tmp/ft8_packet.png), but now I want to actually
>turn
>that into bits.
>
>I know it's 8FSK (the above screenshot is quad demod of that), but I
>have a
>few questions on clock recovery:
>
>* Is the GFSK block only for BFSK? If so then that's out.
>* Is there a "best" clock recovery block nowadays? I seem to recall
>"Clock
>Recovery MM" being discouraged in favor of "Polyphase Clock Sync".
>* I'm trying to read up on the parameters Polyphase Clock Sync wants,
>but
>any pointers would be helpful.
>* Would it be a good idea to throw in a costas loop for frequency
>tuning?
>* Does Polyphase Clock Sync have the same dislike of "staircase"
>inputs?
>That is, I should try to make the center of the bits more "pointy"?
>(e.g.
>lowpass filter them)
>
>I've done some custom OOT decoders before, so I'm not shy about that.
>Maybe
>the best thing is some whole-packet clock recovery[1]. But if I just
>write
>a block that takes the quad demod (see above screenshot) and finds the
>"platforms", outputting a message that is a list of floats, and then
>another block that takes a list of floats and the number 8 and decodes
>it
>as 8FSK, well it seems like I may be reimplementing things where I'm
>guessing someone might say "oh just use this block".
>
>Also for experimentation and my own understanding I'd like to turn it
>into
>bits using in-tree blocks, if possible.
>
>Maybe a correct Polyphase Clock Sync of the quad demod followed by a
>Constellation decoder? I could use the float as the phase, with
>magnitude
>1. Does that make sense? Is it a good idea?
>
>What I have so far is:
>Data:
> https://blog.habets.se/tmp/ft8-burst-10k.raw (10k samp_rate)
>
>GRC:
> https://blog.habets.se/tmp/ft8_decode.grc
>
>Screenshot:
> https://blog.habets.se/tmp/ft8_decode_grc.png
>(everything off-screen to the right is failed clock recovery
>experiments)
>
>Quad demod
> https://blog.habets.se/tmp/ft8_packet.png
>
>
>[1] Like https://www.youtube.com/watch?v=rQkBDMeODHc which I turned
>into
>https://github.com/ThomasHabets/radiostuff/blob/master/
>gr-habets/python/magic_decoder.py
>
>--
>typedef struct me_s {
> char name[] = { "Thomas Habets" };
> char email[] = { "tho...@habets.se <tho...@habets.pp.se>" };
> char kernel[] = { "Linux" };
> char *pgpKey[] = { "http://www.habets.pp.se/pubkey.txt"; };
> char pgp[] = { "9907 8698 8A24 F52F 1C2E 87F6 39A4 9EEA 460A 0169" };
> char coolcmd[] = { "echo '. ./_&. ./_'>_;. ./_" };
>} me_t;
--
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