9.3.1 Preemphasis

The first stage in MFCC feature extraction is to boost the amount of energy in the high frequencies. It turns out that if we look at the spectrum for voiced segments like

vowels, there is more energy at the lower frequencies than the higher frequencies. This drop in energy across frequencies (which is called \textit{spectral tilt}) is caused by the nature of the glottal pulse. Boosting the high frequency energy makes information from these higher formants more available to the acoustic model and improves phone detection accuracy.

This preemphasis is done by using a filter $ ^{1} $ Fig. 9.9 shows an example of a spectral slice from the first author's pronunciation of the single vowel [aa] before and after preemphasis.