(Hearing an envelope follower controlling a BPF. (figure : 3))
Links : LPF Envelope Follower, LFO Modulation, Humanizer.
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Figure 2 : The auto wah effect is applied to the highlighted section of the wave file.
Auto wahs change frequency controls on parametric equalizers. They can control the bass (low pass filter (LPF)), midrange (band pass filter (BPF) or band reject filter (BRF)) or treble (high pass filter (HPF)). The wah pedals that are controlled manually, by foot, are sweepable BPFs. They are also the most common type of filter controlled by wahs.
Figure 3 : Digilogue BlueFilter control settings for a BPF controlled by an envelope follower.
Auto wahs are VCFs (voltage controlled filters ; effecting frequency response) that are controlled in two different ways :
Envelope Followers
The volume envelope of the instrument changes a frequency control on a parametric equalizer. This means the louder you play the instrument, the more the sound changes.
Modulation
Triangle wave LFO modulation is the most common type of modulation used in the auto wah effect, because it creates a constant, periodic, sweep of the filter spectrum.