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Audio beacon is generating different frequency between 18 khz to 19 khz in DIGITAL(square wave) formate.I'm trying to record all frequency using AudioRecord Api and trying to convert time domain to frequency domain. I refered this link How to get frequency from fft result?. I think this link will work only for sine wave and how to apply fft and hanning window function for digital signal.

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The ideal square wave contains only components of odd-integer harmonic frequencies (of the form 2π(2k − 1)f).

reference : https://en.wikipedia.org/wiki/Square_wave

Using these harmonics in frequency response one can find the sinosoidal frequency components presents

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how to apply fft and hanning window function for digital signal.

Windowing technique works similar as it works on any other time domain input signal you have to just multiply the hanning window with the input digital signal.