[Numpy-FFT] data output format

[Peter Hansen]

Giovanni Ciampaglia wrote:
> 
> Hello everyone,
> Could someone explain me what is the "steady-state" value contained into the
> array returned by FFT.fft(data,n=None,axis=-1) at its first position?
> Is it one of the values calculated by the FFT algorithm or some sort of control
> value different from the other elements of the array?

That would likely be what I'd have called the DC (from Direct Current) value,
representing a constant offset from zero in the signal being transformed.

Basically thing of it as 0 Hz (the unchanging portion of the signal), therefore 
no different from a constant offset, or "steady-state" as the man said.

Note that depending on how the FFT is calculated, this steady-state value will
likely include some amount of the near-zero frequencies, since each bin really
represents the contributions of a range of frequencies, rather than just 
single, precise frequency values.

To answer the latter question then, it's just one of the values calculated
by the FFT, "no different" from the others (except for its uniqueness in being
from the unchanging portion of the input).

-Peter