倒频谱（cepstrum），顾名思义，就是将频谱（spectrum）的英文前四个字母反过来写。倒频谱是为了某些时候，为了计算方便，将原来信号的频谱先转成类似分贝的单位，再作逆傅里叶变换，把它视为一种新的信号做处理。倒频谱有复数倒频谱，及实数倒频谱。

倒频谱被定义在1963的论文（Bogert等）。定义如下：

复数倒频谱拥有频谱大小跟相位的信息，实数倒频谱只有频谱大小的信息，各有各的不同应用。

A cepstrum (/ˈkɛpstrəmˈˌˈsɛpstrəmˈ/) is the result of taking the Inverse Fourier transform (IFT) of the logarithm of the estimated spectrum of a signal. It may be pronounced in the two ways given, the second having the advantage of avoiding confusion with 'kepstrum' which also exists (see below). There is a complex cepstrum, a real cepstrum, a power cepstrum, and a phase cepstrum. The power cepstrum in particular finds applications in the analysis of human speech.