Guitar Effects Pitch Scaling
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Reverb Reverb is an effect to simulate various acoustic spaces. It consists of early reflections, which are the first reflections that arrive back to your ear, and the later reflections, which do not arrive at a constant rate. Figure 15 shows the impulse response of a room and how these early and late reflections contribute.
Figure 15: Impulse response of a room To achieve this in a digital system, the early reflections are generated from an IIR comb filter structure as shown in Figure 15.
Figure 16: IIR comb filter structure An allpass reverberator is used to generate non-constant reflections, therefore simulating the late reflections of true reverberation. Figure 17 shows this structure.
Figure 17: Allpass reverberator structure By combining these two structures, a natural sounding reverberation can be achieved. Figure 18 shows this configuration.
Figure 18: Block diagram of natural sounding reverberation Each IIR comb and allpass filter has gains and delays that can be altered to give a simulation of an acoustic space. Because of the complexity of all this, presets for a few suitable sounding reverberations are included in the code. With further investigation, many hundreds of presets could be developed for a guitarist to utilise. Because of time restrictions, this effect was not implemented on DSP. However, all that is required would be to set up one allpass and one comb filter structure. The comb filters would be processed in a similar way to that of the chorusing effect, with the delay and gain variables being replaced and updated with every iteration of the IIR comb code. The addition of these filters in parallel would then be passed to the allpass filter, where the output of one would be fed back and processed again with a new set of variables.
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