Could anybody tell me why should zi made FFT for ecg signals? For me it seems more simple to use time-domain, but I saw some ecgs in frequency domain and I don.t know how to interpret the results.
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It depend on a way you see ECG signal, you could use time domain or frequency domain or ..
In general, i think physicians and physiologists work with time domain representation of ECG, but as a lot of ECG feature would not be appear in time domain, signal processing specialist and biomedical engineers uses frequency (such as fft) or frequency-time domain (such as wavelet) beside time domain representation of signal.
In frequency domain you can observe what type of sine waves (amplitude and frequencies) are combined to obtain the signal. This can be quite useful for example if you want to see if there power lines on the walls (50/60Hz) are causing any disturbance. You can also observe high frequency components related with acquistion noise and many other stuff.
On the other hand, physicians are used to observe the time domain signal on a 12 (or more) ECG. These are just voltages that are related with the heart anatomy. When you know the anatomy and the location of the electrodes then you can easily tell what is working well and what is not.
In the end, the use of time or frequency domain are only dependent on what you are trying to find in the signal.