AN EXAMPLE OF THE CEPSTRUM FOR A VOWEL
Below is an example (from Noll) that demonstrates a typical cepstrum sequence for a vowel. The cepstrum is computed every 10 msec.


From this example, we can see two important things:

• At the onset of the vowel, where the signal is not quite periodic, the peak in the cepstrum at the fundamental frequency is not well-formed. The amplitude of this peak grows as the signal becomes more regular (periodic). The same phenomena is true for the autocorrelation function.
  
  
• It is clear that the low order coefficients of the cepstrum contain information about the vocal tract, while the higher order coefficients contain primarily information about the excitation. (Actually, the higher order coefficients contain both types of information, but the frequency of periodicity dominates.)

Hence, for speech signals, it seems the vocal tract response and the excitation signal can be separated using simple windowing in the quefrency domain.