Guitar Effects Pitch Scaling
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Introduction Traditionally, guitar effects have been performed using analogue foot pedals, where a guitarist can plug the output of the guitar into the pedal, and the output of the pedal into an amplifier. This amplifier contains signal amplification circuitry coupled with a loudspeaker. Each individual effect usually requires its own pedal, and the foot controls if the effect is on or off. If multiple effects are to be used at the same time, the individual pedals need to be chained together. This results in a few problems. Firstly, each pedal introduces its own level of noise to the signal. This noise can add up to a significant amount, therefore placing a limit on the amount of effects used at once. Secondly, having to move ten pedals around and then chaining them together adds technicality that a guitarist would rather avoid. A solution to both these problems is to have all the effects available in one small unit. The first major part of this project has been to take popular effects and implement them digitally on a DSP device. They have not however, been made to chain together, which is left to future work in the project. By having multiple effects in one unit, the guitarist gets value for money. By digitally implementing these effects, a wide range of variables can be altered to change the result of the effect. These variations will be discussed in this report, along with how the effects are generated. The second major part of this project has been to refine the SOLA algorithm so that a guitar signal can be scaled in pitch. Pitch scaling (more commonly known as pitch shifting, or key shifting) is the process of moving up or down the frequency of the guitar signal. A capo is a device that clamps down the strings of a guitar at a specified fret, effectively making the strings shorter thereby increasing the pitch from their original tuning. Capos can only increase the key, and effectively reduce the number of frets that a guitarist can use. Adding to these limitations is the fact that a guitarist needs to stop playing the instrument to change to a new key. By implementing pitch scaling electronically, these disadvantages are removed, and the change can be made quickly. It is also possible to achieve very low and very high frequencies that conventional strings cannot produce. Strings require a minimum tension level to produce a good note. The SOLA algorithm can take a note played at an adequate tension level, then scale the pitch of that note down to below where a clean note can be generated by conventional detuning. This also applies where the tension is too great to enable any further pitch increases. Therefore, this electronic scaling can dramatically increase the frequency range of the guitar. This SOLA approach is described in this report, and the effect of the major parameters on signal quality are investigated. The maximum amount of pitch deviation from the original signal is examined, as well as how the quality and accuracy change with ever increasing deviations. Both effects and pitch scaling are implemented in real time on a DSP. The methodology used to get from design to DSP implementation is discussed; with the software packages MATLAB and Code Explorer (DSP development board software interface) featuring heavily in this section. The project report can be viewed in PDF Format here
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