Implementation of an electric guitar effects pedal on an FPGA

Abstract

Implementing real time digital signal processing can be computationally expensive when it is developed as a software to be run on a generic computer platform. A solution to compensate for the high computational demand, is to implement it in as a dedicated or application specific hardware. This can be done in various ways such as on a field programmable gate array (FPGA), which permits an inherent parallelised architecture that facilitates real time processing. The aim of this project was to develop and implement different digital audio effects using a hardware definition language. In particular, this work presents the implementation of a guitar effects pedal board based on a FPGA. The Atlys Spartan-6 FPGA trainer board was used since it features an AC-97 audio codec with line-in, lineout, microphone and headphone interfaces, which facilitated the implementation. A number of effects were implemented including echo, reverb, chorus, distortion, tremolo, phaser and flanger. The implementation of these effects was aided by considering the ztransform transfer function of each respective effect. The implementation was designed around the AC-97 audio codec hardware driver developed by Mr Tony Storey and obtained from Digi-key. This driver runs at a 48 kHz sampling rate and features an 18- bit signal representation. An analogue signal conditioning amplifier, based on an OPA 350 operational amplifier was also designed and implemented on a printed circuit board, to adequately amplify the signal coming from an electrical guitar prior to feeding it to the ADC of FPGA trainer board. The selected effect is applied to the raw digitised signal, which is eventually converted to the analogue domain via the on-board DAC and outputted to an amplified speaker. The performance of the implemented system was tested and verified while comparing the measured results with those obtained from the ideal implementations carried out in MATLAB. Depending on the nature of the effect, the frequency response and/or the transient response of each effect was verified by means of a high-speed digital oscilloscope and a signal generator. The AC-97 audio codec has its own frequency dependent phase shift contribution which had to be taken into consideration while carrying out the various tests.