Guitar Effects Processor Using DSP Alex Czubak Gorav

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Guitar Effects Processor Using DSP Alex Czubak Gorav Raheja Advisor: Dr. Thomas L. Stewart

Problems Need for real-time effects generation for live performances and recording Effects pedals exist, but better used for presets

Overview Overall Project GUI Interface Filter Design Results and Conclusions

Overall Project • Guitar signal to DSP • Passes through filters • User controls filters through GUI • Signal outputs to speaker

Project Specs 8 Effects filters in both parallel and series Sampling Rate = 48, 000 samples/sec GUI controls effects filters Real-time processing at a speed at most 1/sampling rate

Cancelled Original Spec Noise Filter – Eliminates 60 -Hz hum from single-coil pickups – Sampling rate made design difficult Pole and zero practically overlap Z-Transform Function if sampling rate = 48000 Multiband approach considered

Cancelled: Noise Filter FFT of signal shows pointlessness of filter FFT of Strat: A-110 Hz Noise Component of Strat Signal

Guitars Used Squier Stratocaster – 3 Single-coil pickups – 5 -way selector switch Squier Telecaster Custom – 2 Humbucker pickups – 3 -Way Selector Switch

Amplifiers Used Fender Frontman 15 R Guitar Amplifier – External Reverberation Potentiometer set to 0 so designed Reverberation effects can be tested – Distortion channel, ignored to test designed filter – Output: 15 watts into 8 ohms

DSP Board Used Spectrum Digital TMS 320 C 6713 DSK – DSP Chip: Texas Instruments C 6713 @ 225 MHz – 16 MB SDRAM – 512 KB Flash Memory – AIC 23 Stereo Codec allowing 8 -k. Hz to 96 -k. Hz sampling rate – A/D and D/A Converters only handle 2 Volt amplitude max

Programs Used MATLAB and Simulink – – Quick conversion of audio for testing Graphical modeling of filters GUI design links with models Auto C-code generation for DSP board Code Composer 3. 1 – Programs code to board – Only way to interface with board

Overview Overall Project GUI Interface Filter Design Results and Conclusions

GUI Allows user to select and control effects Interface simple and easy to use

GUI Software Design

GUI Testing Initial examples to understand GUI in MATLAB Uploading/playing wave files Tying GUI through Simulink models Connecting GUI through Simulink to DSP board for “real time” implementation

Tying GUI and simulink

Sample testing (no effects)

Sample test (with effects used)

Overview Recap of Overall Project GUI Interface Filter Design Results and Conclusions

Filter Design: Overview 8 user-modifiable filters • Distortion • Volume Envelope • Octaver • Flanger • Phase Shifter • Chorus • Delay/Echo • Reverb Control Signal: A @ 110 Hz

Filter Design: Distortion Boosts and clips signal Gain values from 1 to 50 Saturation at -0. 5 and +0. 5 Telecaster – A 5 th Chord Simulink Distortion Model

Filter Design: Volume Envelope Signal attack is eliminated Violin-type sound – Signal gradually approachs full value – Does so for each plucked note

Filter Design: Octaver Octave Down – Halves frequency of signal – Output at same speed as input A @ 55 Hz from MATLAB Code

Filter Design: Flanger Signal is split Passed through variable-time delay – Controlled by sine wave at <= 1 Hz – Maximum delay = 10 ms Flanged “NBC” Simulink Model of Flanger Effect Filter

Filter Design: Phase Shifter Similar to Flanger, but: – All-pass filter instead of just delay – Creates non-linearly spaced notches – 2 filters = 1 notch 8 filters for 4 notches

Filter Design: Chorus Similar to Flanger, but: – 4 splits instead of 1 Creates multi-guitar sound User selects how many are on – Sine waves run at 3 Hz and 6 Hz

Filter Design: Chorus Simulink Model of Chorus

Filter Design: Delay/Echo Signal is split Fixed delay determined by user Audio Test of Delay/Echo Model

Filter Design: Reverb Delay-based filter – Attenuated feedback decays sound – Delay determines length of decay Reverb Test Filter

Overview Recap of Overall Project GUI Interface Filter Design Results and Conclusions

Results MATLAB – – 5 of 8 effects fully functional Octaver slows signal Volume Envelope only for first note Phase Shifter not adding any effect

Results GUI – Connection to Simulink easier versus direct hardcoding – No real-time application, but Simulink connection is the step before that

Results Real-Time – 3 of 8 effects work properly – 3 have variable-time delay issue “Popping” occurs Happens if effect is present in system – 2 haven’t been finalized in Simulink

Results Real-time demo

Conclusions Effects – Simulink and Code Composer allow quick real -time development – Real-time variable-time delay a complicated matter – Embedded M-file block not a simple solution for Volume Envelope

Conclusions GUI – Significant learning curve at first – Need to define all variables from workspace if used – No info on connecting GUI to DSP board – Simulation environment demonstrated good functionality

Conclusions Future Ideas – Effects Improved variable-time delay algorithm for DSP Board Add more effects to system – Wah-wah – Talk-Box Add pedal functionality to select effects on the fly USB or PC Card connection to laptop for portability – GUI Allow real-time implementation More variable control for more dynamic effects

And so… We were able to bring the house down!!!

Questions?

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