how we can get the fundamental frequency of speech waveform in the Z transform?
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I think you're better off not just relying on the DFT (Z-transform on the unit circle) to estimate fundamental frequency. For a voiced speech segment, you'll see multiple peaks spaced at the fundamental frequency. For that reason, you're better off using the cepstrum which detects the periodicity in the Fourier transform, or a zero-crossing detector on a lowpass filtered and rectified form of the speech signal, or an autocorrelation-based technique to find the fundamental frequency.