Implementation of digital audio effects on an FPGA

Abstract

Rapid improvements in digital signal processing (DSP) are taking place and various applications are heavily dependent on them. In particular, this applies to digital audio effects (DAFX), which play an important role in the music and filming industry. Implementing effects, such as Echo, to a stream of signals for substantially a long time may be computationally expensive if implemented in software running on a generic processor. Thus a hardware implementation is often sought such as in the design of an audio mixer although this may be relatively expensive. A Field Programmable Gate Array (FPGA) implementation of a DAFX system provides a cheaper solution to the existing solutions and has the added advantage of being highly reconfigurable. The aim of this project was to capture sound via a microphone or an alternative input source such as a music player; convert the signal in the digital domain, apply effects, such as Echo, Flanger, Phaser and Chorus; and finally output the processed signal to a speaker after some amplification and filtering is applied in the analogue domain. The digital effects were implemented using z-transform transfer functions, using Very High Speed Integrated Circuit (VHSIC) Hardware Definition Language (VHDL) code, after being simulated and tested using MATLAB. Besides the FPGA, the hardware system implementation consisted of an analogue-todigital converter (ADC) and a printed circuit board (PCB) with some required signal conditioning circuitry. The ADC is responsible of converting the input that is either coming from the microphone or from the music player to a digital format, to which effects can be applied. The PCB consists of three independent signal conditioning circuits; the microphone circuit, the level shifter, and a Sallen-and-Key filter that works together with a Pulse Width Modulator (PWM) digital-to-analogue (DAC) converter block implemented in VHDL.