# Translations:Fourier transform/23/en

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In this book, we use the hybrid convention. Working with negative powers of *Z* is cumbersome. The hybrid convention avoids this contingency but keeps the form of Fourier transform that electrical engineers use. Under the hybrid convention, the *transfer function* of a filter is obtained formally by the substitution of in the filter’s *Z*-transform. In other words, *the transfer function is the (electrical engineering) Fourier transform of the impulse-response function*. We notice that except for the case of the constant filter , the transfer function always depends on the frequency *f*.