Cepstral Liftering

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Introduction

A cepstrogram is similar in many ways to a spectrogram. The only difference is that each vertical 'strip' is a cepstral slice and not a spectral slice as in a spectrogram. The method of obtaining a cepstral slice can be investigated using the Pipeline Processing MAD demonstration (pipeline).

A cepstral slice exhibits a very interesting property: there is a prominant peak in the slice at the point corresponding to the signal's pitch. In the cepstrogram, the series of these peaks appears as a smooth contour. It is known that if the cepstral slice is liftered (the cepstral-domain equivalent of filtering) in a specific place to separate it into two portions, two of the signals characteristics can be examined independently: its spectral envelope and its pitch and harmonic information.

This demonstration allows the user to compare the original spectrogram with the two resulting spectrograms by interactively liftering the cepstrogram in different places.

The demonstration

 

Type 'ceplift' to launch the demo. When the window appears, use the load menu (1) to load a sound file. After a short wait the spectrogram, cepstrogram together with the smooth and pitch spectrograms will appear. There is a cursor on the cepstrogram which defines the liftering position (2). Move this cursor to see how the differing positions affect the smooth and pitch spectrograms. The parameters of the windowing function can also be altered (3).

Things to investigate

  1. At what point does the smooth spectrogram begin to contain pitch and harmonic information?
  2. What happens when the liftering point is moved close to the bottom of the cepstrogram? Why is this?
    Conversely, what happens when the liftering point is move close to the top of the cepstrogram? Why is this?
  3. Identify the pitch contour. Above this, it may be possible to see a fainter contour with a similar shape. What is this contour?
  4. Does altering the window type alter the plots? If so, why?

References

Brown, G.J. and Cooke, M.P., "COM325 Speech and Hearing" Course Notes. Department of Computer Science, University of Sheffield.

Further reading

See also the demonstration for pipeline processing (pipeline).

Credits

Produced by: Stuart N Wrigley

Release date: January 20 1999

Permissions: This demonstration may be used and modified freely by anyone. It may be distributed in unmodified form.