Time‐domain speech processors, that analyze the short‐time autocorrelation function in terms of a set of orthogonal functions, are shown to yield parameters that are the coefficients of a generalized Fourier cosine series representation of the speech power spectrum. Two examples are described, one using a Laguerre filter and one using a tapped delay line as in the autocorrelation Vocoder. In the case of the autocorrelation Vocoder, the derived power spectrum representation reduces to a simple Fourier cosine series in the frequency domain.