Academic journal article Journal of Singing

What Is Inverse Filtering?

Academic journal article Journal of Singing

What Is Inverse Filtering?

Article excerpt

MOST VOCOLOGISTS UNDERSTAND THE CONCEPT of filtering. In the voice technology world, it means the enhancement or attenuation of selected frequencies from a given spectrum of frequencies. The vocal tract acts like a series of band-pass filters, one for each resonance. Thus, a spectrum of frequencies from the source (the glottal airflow) is filtered by the vocal tract to produce the output at the mouth. The perception of vowels and consonants is largely determined by this filter rather than the source. Nevertheless, the source determines the strength and the tone quality (timbre) of the sound produced.

Less understood is the concept of inverse filtering. The spectrum of frequencies produced by the source is not easy to measure directly because microphone or airflow detector placement at glottal exit is invasive. Investigators generally resort to a technique known as inverse filtering, basically undoing what the vocal tract does as a filter.1 Either the acoustic pressure radiated from the mouth or the acoustic flow at the mouth can be inverse filtered to obtain an estimate of the glottal airflow wave shape.2

Figure lb shows an example of an original glottal wave shape and an inverse filtered wave shape obtained from the radiated pressure in Figure la. The inverse filtered flow wave shape was obtained without any apriori knowledge of the vocal tract (i.e., no knowledge of the vowel or any vocal tract dimension). …

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