Editing Analog signal processing (section)

===Fourier transform===
The [[Fourier transform]] is a function that transforms a signal or system in the time domain into the frequency domain, but it only works for certain functions.  The constraint on which systems or signals can be transformed by the Fourier Transform is that:  
:<math>\int^\infty_{-\infty} |x(t)|\, dt < \infty</math>
This is the Fourier transform integral:
:<math>X(j\omega) = \int^\infty_{-\infty} x(t)e^{-j\omega t}\, dt</math>
Usually the Fourier transform integral isn't used to determine the transform; instead, a table of transform pairs is used to find the Fourier transform of a signal or system. The inverse Fourier transform is used to go from frequency domain to time domain:  
:<math>x(t)=\frac{1}{2\pi}\int^\infty_{-\infty} X(j\omega )e^{j\omega t}\, d\omega </math>
Each signal or system that can be transformed has a unique Fourier transform. There is only one time signal for any frequency signal, and vice versa.