Formant Filter

Speech Synthesis

In speech synthesis, formant filters are often used to simulate formant filtering by the vocal tract. Passing a "buzz source" through only two or three formant filters mimics different vowel speech sounds. As a result, speech is fully intelligible through the telephone bandwidth (nominally only 200-3200 Hz). A formant is a resonance in the voice spectrum. Thus, A single formant may be modeled using one biquad (second-order filter section). For example, in the vowel (a) as in "father," the first 3 formant center-frequencies have been measured near 700,1220, and 2600 Hz, with half-power bandwidths 130, 60, and 170 Hz. In principle, the formant filter section is in series. As a result, there is no need to specify the gains for the formant resonators; only the center frequency and bandwidth are necessary to identify each formant, leaving only the overall scale factor unspecified in a cascade (series) formant filter bank.

Emulating the Vocal Tract

The formant filter emulates the vocal tract in the musical world to add vocal quality to synthesizers and other instruments. When your mouth produces vowels such as "eee eye oh," the resonances of different areas inside your mouth create simultaneous high-Q acoustic band-pass filters. This is one way we recognize people by their voices. The fundamental pitch of speech depends on the tension of the vocal cords, but just as important is the complex filtering that occurs acoustically inside your mouth. Where a wah-wah filter comprises a single narrow band-pass filter, a formant filter applies three or more high-Q that you can tune independently. The relationship between center frequencies and how each frequency band's amplitude changes over time makes each person's voice unique.  

Pitch Correction

Vocal tuning, or pitch correction, is standard in pop music. Sophisticated formant filtering has improved the effect considerably compared to early software plugins. Small changes in the fundamental pitch of the voice can pass unnoticed. Nevertheless, a difference large enough to create harmony benefits from applying formant filtering similar to that of the original voice. This reduces the "chipmunk effect" substantially because of a less processed sound.  

Formant Synthesis

In formant synthesis, the goal is to simulate the solid resonant structure of many real instruments. Here, one or more formants dominate the spectrum of the output sound. Some analog synthesizers have one simple high-pass filter after the low-pass filter. This gives them additional control over the bandwidth of sounds; thus, it is a simple formant type.