DIGITAL GUITAR EFFECTS UNIT AND AMPLIFIER by Kevin Salvador Senior Project ELECTRICAL ENGINEERING DEPARTMENT California Polytechnic State University San Luis Obispo i Table of Contents List of Tables and Figures............................................................................................................... ii Acknowledgments.......................................................................................................................... iii Abstract .......................................................................................................................................... iv I. Introduction ..................................................................................................................................1 II. Background .................................................................................................................................1 III. Requirements .............................................................................................................................3 IV. Design Approach Alternatives ...................................................................................................3 V. Project Design .............................................................................................................................4 Blackbox Diagram ...............................................................................................................4 Definition of User Interface .................................................................................................5 Overall System Functional Diagram ....................................................................................5 Analog to Digital and Digital to Analog Converter .............................................................6 Digital Signal Processor .......................................................................................................7 Digital Equalizer ......................................................................................................8 Echo .......................................................................................................................11 Chorus ....................................................................................................................11 Flange .....................................................................................................................12 Reverb ....................................................................................................................12 Auto-Wah ...............................................................................................................13 Distortion ...............................................................................................................13 Power Amplifier.................................................................................................................14 Speaker ...............................................................................................................................16 Power Supply .....................................................................................................................16 VI. Physical Construction and Implementation .............................................................................18 VII. Integrated System Test Results ..............................................................................................20 VIII. Conclusion ............................................................................................................................28 IX. Bibliography ............................................................................................................................29 Appendix ........................................................................................................................................30 ii A. Specifications ................................................................................................................30 B. Parts Lists and Costs .....................................................................................................30 C. Schedule – Time Estimates and Actual .........................................................................30 D. Program Listing.............................................................................................................30 E. Source Code...................................................................................................................30 MatLab Code .........................................................................................................30 C-Programming code .............................................................................................35 iii List of Tables and Figures Figure 1: Analog vs Digital signal ................................................................................................ 1 Figure 2: Blackbox Diagram .......................................................................................................... 4 Figure 3: Functional Block Diagram ............................................................................................. 5 Figure 4: FFT of guitar signal at highest frequency (E string 24 th fret) .................................... 7 Figure 5: Pole-Zero diagram for band pass filter with Fc of 512Hz ........................................... 9 Figure 6: dB magnitude and phase response vs of bandpass filter with Fc = 512Hz ................. 9 Figure 7: Signal Flow graph for the Direct Form II Transposed Difference Equation .......... 10 Figure 8: LTspice simulated circuit of voltage gain stage and power output stage .................. 15 Figure 9: LTspice simulated waveforms of figure 7 ................................................................... 15 Figure 10: LTspice simulated circuit of dual power supply ....................................................... 17 Figure 11: LTspice simulated dual rectified and smoothed waveforms .................................... 17 Figure 12: Setup used during the Senior Project Exhibition. Not included is board’s power supply ............................................................................................................................................ 18 Figure 13: Close up of DSK6713board and setu of user buttons .............................................. 19 Figure 14: Power Amplifier and Speaker ................................................................................... 20 Figure 15: Input and Output waveforms displaying the delay time used for Echo effect ........ 21 Figure 16: Input and Output waveforms displaying the delay time used for Echo effect ........ 22 Figure 17: Input and Output waveforms displaying the delay time used for Echo effect ........ 22 Figure 18: Input and Output waveforms displaying the delay time used for Echo effect ........ 23 Figure 19: Frequency Response Plot of each individual filter .................................................. 24 Figure 20: Secondary Winding voltage from transformer ......................................................... 25 Figure 21: Positive and Negative rails of power supply ............................................................. 26 Figure 22: input voltage vs output voltage of op amp volume stage .......................................... 27 Figure 23: input voltage vs output current of power amplification stage ................................. 27 iv Acknowledgements I would like to thank my Senior Project advisor Dr. Pilkington for his help and enthusiasm throughout the process. His knowledge of music specifically guitars and the effects used with them helped tremendously. Without the knowledge acquired from his Digital Signal Processing course, which was taking concurrently with the first quarter of senior project, none of this would be possible. I’d also like to thank my lab partners from the DSP laboratory, David Smith and Eric Spinks whom help code the design functions used in this project. Also thank you to Dr. DePeiro for providing the template C code for implementing the digital filters of TI’s DSK boards. v Abstract This report outlines the design and implementation of a digital guitar effects unit and amplifier. The main portion of this project consisted of the digital equalizer and effects. Several commercial equalizers were researched in order to decide the typical frequency bands and average amount of bands total. Eventually 8 bands were selected. A range of approximately 20Hz-3kHz was chosen based on test data of guitar signals. Popular effects that were incorporated in this project include Distortion, Echo, Reverb, Chorus and Flanger. The digital processor chosen was a Texas Instruments c6713 floating point processor. Designs for the various filters were done in MatLab and implementation on the processor was done through TI’s Code Composer Studio. 1 I. Introduction This DSP and amplifierunit is targeted towards guitarists. Electric guitars require an amplifier in order for the sounds to be audible. This project also provides them with signal processing “effects” to change the tones of their instrument. An alternative for these customers are analog effects filters such as simple passive filters. Since this project will make use of digital filters and a programmable processor, there is more flexibility and user control for desired output. The use of digital filters also reduces the size and number of components required for more complex processes. The