Guitar EffectsI. IntroductionII. System OverviewIII. Hardware RequirementsI. Audio CodecII. Effects Processor1. Distortion Module2. Vibrato Module3. Chorus ModuleIV. Software RequirementsV. Appendix AGuitar Effects March 22, 2007 Navarun Jagatpal Fred Rassam Young Jin Yoon Elton Chung 1TABLE OF CONTENTS Guitar Effects...................................................................................................................... 1 I. Introduction................................................................................................................. 3 II. System Overview........................................................................................................ 3 III. Hardware Requirements.......................................................................................... 3 I. Audio Codec ........................................................................................................... 3 II. Effects Processor..................................................................................................... 4 1. Distortion Module............................................................................................... 4 2. Vibrato Module................................................................................................... 5 3. Chorus Module.................................................................................................... 6 IV. Software Requirements........................................................................................... 6 V. Appendix A................................................................................................................. 8 2I. Introduction It is common for electric guitar players to make use of ‘pedals’ in order to achieve certain effects such as ‘distortion/overdrive’, ‘delay/echo’, ‘chorus’, ‘flanger’ and ‘pitch/phase shifter’. Typically these pedals make use of analog electronics to yield the desired effects. Our goal will be to use the Altera DE2 FPGA board as a sampler and DSP (Digital Signal Processor) to produce a few of these effects. The sampling will be done in real-time with mono audio input from a guitar connected to a vacuum tube preamp into the FPGA. The output will be played on a pair of speakers. II. System Overview The diagram fig 1 below is a top level block diagram of our guitar effects system. 16 CPU Effects Processor 16 DACBuffer1bit1bit ADCBufferDAC (Not built by the group) ADC (Not built by the group) fig. 1 Top level diagram The ADC(Audio Digital Converter) and DAC (Digital Audio Converter) will be done by the Wolfson WM8731 audio CODEC provided by the FPGA via the sound card. We will be designing a ADC buffer which will take the serial digital input received from the ADC and buffering it so our Effects Processor can modify the data to the desired effect before re-serializing the data out to the DAC through the DAC buffer. The CPU will be used to switch the effects, increase/decrease volume and increase/decrease the effect. III. Hardware Requirements I. Audio Codec We will be using the Wolfson WM8731 audio CODEC for doing our analog digital conversions (ADC) and digital audio conversions (DAC). The CODEC 3provides us with stereo and mono microphone level audio inputs as well as an array of programmable functions ranging form mute, volume control, bias voltage output, I2S, DSP, 16/20/24/32 bit Word Lengths and Master or Slave clocking mode. The CODEC samples the input with a range of frequency from 8kHz-96kHz II. Effects Processor 16-bit input to DAC bufferDistortion Module Vibrato Module Chorus Module DemuxE 16 16 16 161616Mux Eeffect_selectparameter1parameter21616 16-bit output from ADC buffer Memory-mapped I/Ofrom CPU 2 Fig. 2 Effects processor block diagram The effects processor consists of a mux, demux and three effect modules. A 16-bit sample input feeds into the demux which supply inputs for the three separate effects, Distortion, Vibrato and Chorus, modules. The CPU will dictate which effect module to use and supply a gain and phase input parameter. The output will then be fed into a mux for aggregating the signals into a 16-bit buffer for the DAC to process. 1. Distortion Module 416-bit output from Demux E Fig. 3 Block Diagram of Distortion Module The 16-bit input will be fed into two separate comparators. The first comparator will output a ‘1’ if the sample is greater than ‘N’. The second comparator outputs a ‘1’ if the sample is less than -N. (‘parameter1’ = ‘N’), ‘N’ being the clipping level 2. Vibrato Module Fig 4. Block Diagram of Vibrato Module D Q Compare MuxCompare Mux×-16 1616(sample of music) 16×g 16-bitinput to Mux Eparameter1 16 bits parameter2 (parameter2 = g)D Q 16 16 bits (sample of music) z -d[n]D Q 16 (parameter2 = g)D Q 16 (buffer for varying delay) d[n] = 221cos[2πfn]+882 (lookup table) (parameter1 = f) 16-bit 16-bitoutput from Demux E input to Mux E×g parameter1 16 bits parameter2 16 bits 5The 16-bit signal will be re-evaluated with a complex cosine look-up table. A frequency parameter will be fed in from the CPU to help with the final calculations. The re-imaged signal will then be amplified by ‘parameter2’ which is also passed in through the CPU. 3. Chorus Module Fig. 5 Block Diagram of Chorus module This module is similar to the Vibrato module except the resulting sample after re-imaging will be added with the original signal before sending it out to the mux. IV. Software Requirements The selection of which guitar effect to use will be done in software. This process will be mapped to different keys on a keyboard and the user will have the ability to choose which effects should take place. D Q 16 (parameter2 = g)D Q 16 z -d[n](buffer for varying delay) d[n] = 221cos[2πfn]+882(lookup table)(parameter1 = f)×g + Adde(sample of music) 16 1616-bit output from Demux E 16-bitinput to Mux E16parameter1 16 bits parameter2 16 bits 6APPENDIX 7V. Appendix A Verifying algorithm on C code Because we need to verify the algorithm we used, we need to build a simple program in C to test our algorithm to implement functionality that we have. In our C program, basically it reads the sample from the file, applies the algorithm that
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