# Translations:Fourier transform - book/4/en

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where *A*(*f*) is the amplitude spectrum, where is the phase-lead spectrum and where is the phase-lag spectrum. The phase-lag spectrum is the negative of the phase-lead spectrum. Either one can be called the phase spectrum. However, because the exponential in the Fourier transform carries a negative sign, the choice of , which also has a negative sign in the exponential, seems to be favored. For that reason, we choose to let the word *phase* refer to the phase lag and not to the phase lead . For example, the Fourier transform of the unit delay (which is 1 for *n* = 1 and 0 otherwise) is simply