DAFX: Digital Audio Effects, Second Edition

2.4 Time-varying Filters

The parametric filters discussed in the previous sections allow the time-varying control of the filter parameters gain, cut-off frequency, and bandwidth or Q factor. Special applications of time-varying audio filters will be shown in the following.

2.4.1 Wah-wah Filter

The wah-wah effect is produced mostly by foot-controlled signal processors containing a bandpass filter with variable center frequency and a small bandwidth. Moving the pedal back and forth changes the bandpass center frequency. The “wah-wah” effect is then mixed with the direct signal, as shown in Figure 2.20. This effect leads to a spectrum shaping similar to speech and produces a speech-like “wah-wah” sound. Instead of manually changing the center frequency, it is also possible to let a low-frequency oscillator control the center frequency, which in turn is controlled based on parameters derived from the input signal, e.g., the signal envelope (see Section 3.3). Such an effect is called an auto-wah filter. If the effect is combined with a low-frequency amplitude variation, which produces a tremolo, the effect is denoted a tremolo-wah filter. Replacing the unit delay in the bandpass filter by an M tap delay leads to the M-fold wah-wah filter [Dis99], which is shown in Figure 2.21. M bandpass filters are spread over the entire spectrum and simultaneously change their center frequency. When a white noise input signal is applied to an M-fold wah-wah filter, a spectrogram of the output signal ...