Re: [Discuss-gnuradio] Best in-tree blocks for clock recovery of 8FSK

The gr-op25 code handles 4FSK. The demod and slicer code might be
expanded to 8FSK.

On 04/02/2018 06:22 AM, Jeff Long wrote:
> This looks like an amateur M-FSK signal. You might have luck with fldigi
> and save yourself some work. It uses FFTs and then analyzes frequency
> shifts. You could do some of this in GR, but syncing to the signal would
> be hard because you would miss a few transitions trying to get synced.
>
> On 04/02/2018 04:48 AM, Thomas Habets 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
>> <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
>> <https://blog.habets.se/tmp/ft8-burst-10k.raw> (10k samp_rate)
>>
>> GRC:
>> https://blog.habets.se/tmp/ft8_decode.grc
>> <https://blog.habets.se/tmp/ft8_decode.grc>
>>
>> Screenshot:
>> https://blog.habets.se/tmp/ft8_decode_grc.png
>> <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
>> <https://blog.habets.se/tmp/ft8_packet.png>
>>
>>
>> [1] Like https://www.youtube.com/watch?v=rQkBDMeODHc
>> <https://www.youtube.com/watch?v=rQkBDMeODHc> which I turned into
>> https://github.com/ThomasHabets/radiostuff/blob/master/gr-habets/python/magic_decoder.py
>> <https://github.com/ThomasHabets/radiostuff/blob/master/gr-habets/python/magic_decoder.py>
>>
>>
>> --
>> typedef struct me_s {
>>   char name[]      = { "Thomas Habets" };
>>   char email[]     = { "thomas@habets.se <mailto:thomas@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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>>

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