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I am working on a simple MFSK modem for use on the HF band. I'm having trouble recovering the symbol timing information from the stream.

With 2-FSK it's easy - just subtract one frequency from the other, and use the 0-crossing point to determine how far off you are (ideally, the 0-crossing point should be right in between 2 symbols)

I'm unsure how to scale this up to more than 2 frequencies. Is there a standard way that symbol synchronization is performed on MFSK signals that I could look into?

Here is a graph of one of my 4-FSK signals, with some noised added in. Each colour represents one of the different frequency bins of interest. The pink lines are the center of each symbol.

enter image description here

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  • $\begingroup$ what's the vertical axis in this? If it is decibel, then it would look like you have an SNR of 200 dB – which would be quite surprisingly good. But it would make things a lot easier if the SNR is this good! $\endgroup$ Commented Jan 15, 2023 at 16:26
  • $\begingroup$ @MarcusMüller This is the raw output from the FFT. The unit is effectively meaningless as it depends on the analog signal magnitude itself. $\endgroup$ Commented Jan 15, 2023 at 16:43
  • $\begingroup$ a negative magnitude? $\endgroup$ Commented Jan 15, 2023 at 16:43
  • $\begingroup$ @MarcusMüller My apologies, I uploaded the wrong image. I've corrected it now $\endgroup$ Commented Jan 15, 2023 at 17:02
  • $\begingroup$ nice, this looks a lot more challenging. I assume the red, yellow and green traces are wide because you were observing multple frequencies being active? $\endgroup$ Commented Jan 15, 2023 at 17:55

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