Why does FFT produce complex numbers instead of real numbers?

Solution 1:

The FFT is fundamentally a change of basis. The basis into which the FFT changes your original signal is a set of sine waves instead. In order for that basis to describe all the possible inputs it needs to be able to represent phase as well as amplitude; the phase is represented using complex numbers.

For example, suppose you FFT a signal containing only a single sine wave. Depending on phase you might well get an entirely real FFT result. But if you shift the phase of your input a few degrees, how else can the FFT output represent that input?

edit: This is a somewhat loose explanation, but I'm just trying to motivate the intuition.

Solution 2:

The FFT provides you with amplitude and phase. The amplitude is encoded as the magnitude of the complex number (sqrt(x^2+y^2)) while the phase is encoded as the angle (atan2(y,x)). To have a strictly real result from the FFT, the incoming signal must have even symmetry (i.e. x[n]=conj(x[N-n])).

If all you care about is intensity, the magnitude of the complex number is sufficient for analysis.

Solution 3:

Yes, it is possible to represent the FFT frequency domain results of strictly real input using only real numbers.

Those complex numbers in the FFT result are simply just 2 real numbers, which are both required to give you the 2D coordinates of a result vector that has both a length and a direction angle (or magnitude and a phase). And every frequency component in the FFT result can have a unique amplitude and a unique phase (relative to some point in the FFT aperture).

One real number alone can't represent both magnitude and phase. If you throw away the phase information, that could easily massively distort the signal if you try to recreate it using an iFFT (and the signal isn't symmetric). So a complete FFT result requires 2 real numbers per FFT bin. These 2 real numbers are bundled together in some FFTs in a complex data type by common convention, but the FFT result could easily (and some FFTs do) just produce 2 real vectors (one for cosine coordinates and one for sine coordinates).

There are also FFT routines that produce magnitude and phase directly, but they run more slowly than FFTs that produces a complex (or two real) vector result. There also exist FFT routines that compute only the magnitude and just throw away the phase information, but they usually run no faster than letting you do that yourself after a more general FFT. Maybe they save a coder a few lines of code at the cost of not being invertible. But a lot of libraries don't bother to include these slower and less general forms of FFT, and just let the coder convert or ignore what they need or don't need.

Plus, many consider the math involved to be a lot more elegant using complex arithmetic (where, for strictly real input, the cosine correlation or even component of an FFT result is put in the real component, and the sine correlation or odd component of the FFT result is put in the imaginary component of a complex number.)

(Added:) And, as yet another option, you can consider the two components of each FFT result bin, instead of as real and imaginary components, as even and odd components, both real.