Guitar Effects Pitch Scaling
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Distortion Distortion is the process of distorting the waveform of a guitar signal to add high order harmonics. There are many distortion effects that operate directly on the waveform, and these are known as wave shaping algorithms. However, the effect implemented performs a threshold on the input signal to achieve the high order harmonic components. If the input signal is above 0.1 or below –0.1 then the output is forced to either 1 or –1. In between these two values the output is set to zero. This signal is then passed through a filter to reduce the very harsh effect of a threshold. The result is then added back to the original undistorted signal to produce the final output. Figure 7 shows the block structure of this effect, with Figure 8 demonstrating the distorted waveforms. The type of filter used and the filter characteristics determine the sound of the distortion. Best results are found when using either band pass or low pass filters.
Figure 7: Block structure of the distortion effect
Figure 8. Waveforms of distortion effect Variables identified in this effect are the type of filter and filter characteristics, as well as the gain values for wet mix (how much of the processed signal is allowed through to the output) and dry mix (how much of the original signal is allowed through to the output). The level of distortion is selected by changing the cut frequencies when using a lowpass filter, and by varying the selected bandwidth when using a bandpass filter. A cut frequency of 3-6kHz for the lowpass filter, and a passband of 2-3kHz gives the best results. The DSP implementation is focused on using a tunable bandpass / low pass filter configuration which would allow the guitarist to change the filter characteristics at the press of a button. While currently the lowpass filter implementation has a fixed cut-off frequency, a tunable bandpass filter demonstrated in Mitra has been implemented and is shown in Figure 10.
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