Guitar Effects Pitch Scaling
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Chorusing Chorusing is the effect of multiple instruments being played at once, even though there is only one present. This simulates the variances in pitch and timing that occur naturally when two or more people play the same piece of music at the same time. This effect is achieved by breaking up the signal and passing these parts into flange units. These flange units are slightly different from each other and distort the signal in amplitude and frequency by adding a delayed signal to itself causing phase distortion. These processed signals are then added together to form the output. The delay in each flange unit varies with time. The rate in which this varies is different in each unit. The gain values between units are also selected to be different, and this result in each altered signal having different amplitudes. Figure 12 shows the block diagram for this effect.
Figure 12: Block structure of the chorusing effect Each gain factor and modulation frequency of the delays can be modified, allowing a wide range of resultant sounds. The number of flange units is also selectable, with a large number (4+) giving a fuller sound. For a chorus effect, the frequency that the delays are modulated by is around 1Hz. With this structure, it is possible to implement a vibrato effect. Vibrato is the periodic pitch modulation of a signal. A guitarist can generate such an effect manually by rapidly moving the fretting finger side to side, effectively varying the length of the string, therefore the pitch by a small amount. By having the delays modulated at around 7Hz, instead of 1Hz, the distortion periodicity becomes audible to the ear, and the effect is of a chorus of guitars performing vibrato. By only allowing one modified signal to the output, the chorusing effect is removed. DSP implementation involved setting up a single flange unit, with a delay line of length 2400 samples (100 ms at 24kHz sample rate). This length is the maximum delay available in this effect. Every time a sample arrives, the signal is broken up into n parts, where n is the number of instruments (flange units) selected. Each of these parts is then passed through the flange code with its own unique variables, such as length of delay, and gain factor. Once all of the parts have been processed, they are added together and sent to the output. At this point, the delays for each flange unit are updated. The next delay point is found from the frequency of delay modulation that the guitarist gives.
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