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

[Adrian Musceac]

Hi Thomas,

I use it for 4FSK, it does work, not optimally it seems, but I attribute that to my lack of understandimg of the algorithm.
Regarding the square wave issue, I use a low pass filter in front of it with bandwidth = samp-rate / samples-per-symbol.

Regards,
Adrian

On April 2, 2018 11:54:21 AM UTC, Thomas Habets <address@hidden> wrote:

I just seem to recall that being said.

I've used M&M successfully in the past, but it does require some massaging of the data to not be "square wave" before it'll work. Also does it work (well) for M-FSK?

On 2 April 2018 at 11:16, Adrian Musceac <address@hidden> wrote:

Hi,

Why is the M&M algorithm discouraged?
I use successfully both in FSK and PSK demodulators. It seems to perform fine.

Regards,
Adrian

On April 2, 2018 8:48:00 AM UTC, Thomas Habets <address@hidden> 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

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