Guitar Effects Pitch Scaling
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Delay The delay effect is simply the addition of multiple delayed signals to the original signal. This gives an echo effect. Figure 13 gives a block description for the effect.
Figure 13: Block structure of the delay effect The length of the delay k is selectable, with a range of 200 ms down to a few milliseconds. The number of delayed signals (n) added to the output is also selectable, up to a maximum of five. The gain multipliers g1 to gn can be altered to preset values, rather than the guitarist having to manually change each one. Figure 14 demonstrates two presets of gain multipliers. The one on the left gives a standard decay rate, similar to that of echoes. The right one gives a peak in the middle, resulting in an interesting guitar signal.
Figure 14: Two gain preset graphs for the delay effect In the DSP implementation, a delay line of 4800 samples (200 ms at 24kHz sample rate) has been set up. This is simply accessed at the required point to get the delayed samples, which are then multiplied by a gain factor. All of these delays, along with the original signal are added together to form the output. This design is not memory efficient, and if this effect were to be part of a chained system, then only the delayed samples required would have to be stored, rather than all of them. An alternative to this is to use an IIR comb filter structure (Figure 16, section 3.6) that relies on feedback to achieve the delays.
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