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I just started a while ago with DSP and slowly staring to understand the concept of I/Q and what it means. I successfully could decode some bfsk, bpsk and wanted to move to msk which as not easy to understand. Lost of blog posts explain very well how to deal with as OSQSK or CPFSK. In those posts we always work with I/Q data, but what if my signal is already FM demodulated an I am left with in my case a 16bit signed integer representation of the signal. Can I just reconstruct the Q part by shifting by 90° to recreat my Q? Or how would demodulation work then. Or do I just compare my signal to a sin starting from the same time to track phase changes?

Thanks

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If you already have frequency then IQ is not needed. You could use the IQ waveform if you had it to create a frequency discriminator, but this appears to already have been done for you. This existing post provides further insight onto the phase (as determined by I and Q) and frequency versus time for the MSK waveform:

Difference between MSK and GMSK?

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  • $\begingroup$ Thanks, clear now. Do you know any sources for working with none IQ data with the digital modulations. Basically tones. I guess in my case I need to recover the orig freqs. (which I know) and sync them to see if it they in which direction the phase shift was performed. I am trying to understand this on a rather low level before $\endgroup$
    – Tim
    Commented Feb 22, 2023 at 8:10
  • $\begingroup$ Glad it helped Tim. Please accept this if it sufficiently answered your question as posted so we can close this one. The topics your asking about in your comment above are all covered extensively in an online course I teach coming up next month! More info here: dsprelated.com/course/DSP_For_Software_Radio_2023 $\endgroup$ Commented Feb 22, 2023 at 13:01

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