Guitar Effects Pitch Scaling
|
Wah Wah The wah effect is produced by moving the centre frequency of a bandpass filter slowly up and down in frequency. This sounds like a baby crying therefore giving it the name wah wah. Figure 9 shows the block structure for this effect.
Figure 9: Block structure of the wah wah effect The speed of the wah, the centre frequency of the filter, and how the centre frequency is moved up and down in frequency are the effect variables. A 2nd order tunable bandpass filter has been implemented based on that described in Mitra. This is shown in block form in Figure 10. This implementation allows two variables control the effect. a controls the bandwidth, and b the centre frequency of the filter.
Figure 10: Block structure of the tunable bandpass filter
Figure 11: Bandpass filtered chirp waveform (DC to 12000Hz) for two different sets of a and b Figure 11 above shows how varying a and b effects the centre frequency and bandwidth of the filter. With a = 0.9 and b = 0.9, the centre frequency is approximately 2kHz with a narrow bandwidth. When these values are reduced to a = 0.6 and b = 0.6, the centre frequency moves to approximately 3kHz with a much wider bandwidth. To get a good guitar effect, the centre frequency is positioned around 1kHz, with a variation from that limited to 500Hz either side. The DSP code implements the filter structure in Figure 10. With very iteration of the filter code (every time a new sample arrives from the AIC), the b variable is incremented or decremented, depending on if the centre frequency is increasing or decreasing. The frequency of this variation can be altered and it is implemented as a simple triangular waveform. When b reaches a maximum or minimum (0.9 or 0.1), it immediately starts going in the other direction.
|
