1. P13323: Opto-Electric Guitar Pickup
Sponsored by the EME Department of RIT

2. P13323 Team Members Rachel Arquette (EE) Ye Kuang (EE) Joe Mauger (EE)
Analog design
Ye Kuang (EE)
Joe Mauger (EE)
Caroline Lichtenberger (EE)
Digital Design
Chris Perry (EE)
Power regulation design
Paul Hoops (EE)
Data analysis
Chris Steele (ME)
Mechanical design

3. Agenda Project Description Customer Needs Engineering Specifications
Concept Summary
System Architecture
Design Summary
Test Results
Project Evaluation
Opportunities/Suggestions for Future Work

4. Project Description
Design and build a guitar pickup using optical methods, as opposed to traditional magnetic or piezo-electric pickups.
Allows for easy attaching and detaching for any guitar, without causing damage to the guitar or interfere with ability to operate the guitar.
Can be selected for analog filtering or digital filtering.

5. Customer Needs A system that will not modify the guitar!
Has to be temporarily attached and easily removable.
Ergonomically unobtrusive and intuitive for beginners.
The signal filters must be both analog and digital!
Switching must be able to happen quickly for sound comparison.
Recognition of fundamental tones and some harmonic tones, too.
The sound must be clear!
Sensing and reproduction cannot change the original sound.
Battery life must be long enough for a few whole songs.

6. Engineering Specifications

7. Concept Summary Mechanical Overview Electronics Overview
Two height adjustable mounting surfaces per sensor pair, plus a sensor guide to allow for string separation variation.
Allows single design to fit many sizes and shapes of strings.
Straps hold the custom fit sensor guide to the guitar.
Processing unit is located “on the hip” via a belt clip.
Electronics Overview
The signals obtained by each of the six sensor channels are sent through a gain amplifier to allow for proper voltage ranges required by the microprocessor.
After the gain stage, the signals are processed using either analog or digital means, depending on the status of the Analog/Digital switch.

8. System Overview

9. Design Summary - Mechanical
Individual sensor mounts
Provide adjustment for string to sensor offsets.
Repositionable sensor posts
Slide along a guide bed for zeroing the sensor output.
Low profile sensor system
Allows the unit to fit in the area under the wrist while playing, without interference.
Soft Nylon strap, Velcro fasteners
Allows quick, firm tensioning of the straps and does not mar the finish.
Remote electronics enclosure
Mounts on the hip to reduce clutter on the guitar body.

10. Design Summary - Sensors
Each string has an emitter-detector pair consisting of an infrared LED and a phototransistor.
Signals are generated by the string oscillations creating variations in the amount of light detected by the phototransistor.

11. Design Summary - Analog Processing
After the gain stage, the signal from each channel will go through a second-order Butterworth high-pass and low-pass filter set to minimize frequency components due to noise sources and allow the first two harmonics of each note to pass through un-attenuated. Then, the channels are summed into one signal using a summing amplifier and sent to the ¼” jack as the output.

12. Design Summary - Analog Processing
This processing path uses two matching PCBs, as shown.Each PCB contains the hardware for three string channels.

13. Design Summary - Digital Processing
Each string has its own dedicated ADC. Each string has its data passed through a second order Butterworth filter (just like the analog path) before being summed together. The digital board then communicates to an external
DAC mounted on the motherboard through an I2C interface.

14. Test Results - Mechanical
Set screw stripping torque spec.
Book suggested 18.2 N-cm
Testing resulted in 12.0 N-cm
Failure mode was torsional shearing of the driver bit, threads did not fail before deformation!
Guide Bed positional freedom
Minimum movement required
Maximum movement of 1mm at the extremes of the guide, rotating around center
Had little effect on frequency output, but a 25% movement of zero position.

15. Test Results - Analog Processing
Each component of the system was tested individually with a high rate of success
The analog path has been successful in recreating the original signal and outputting the result to an amplifier for auditory confirmation

16. Test Results - Digital Processing
There had been several setbacks concerning the DAC throughout the design process.
The image shown depicts a test signal sent through the ADC, passed through the microprocessor with no filtering, and then sent through the DAC.

17. Project Evaluation Successful: Unsuccessful:
Battery life exceeds minimum requirements
Frequencies from the guitar can be reproduced out to ~5kHz, well above the highest note.
Both analog and digital paths reproduce input frequency for at least two harmonics in addition to the fundamental tone
Developed a harness that is detachable and does not permanently modify the guitar
Unsuccessful:
Mounting accuracy can be improved using 1 DoF sensor adjustments and more permanent fixturing, but may require guitar modification.
Human hearing range (20Hz thru 20kHz) was not met.

18. Opportunities/Suggestions for Future Work
Combine the pickup with an electric guitar.
Make the design fit a more compact enclosure.
Design BPF to save power, money and space.
Use surface mounted discrete PCB components
Have more than one set of arrays of IR LED’s/detectors at different points on the neck of the guitar.
Reduce the power consumption of the pickup.
Constrain the sensor mounts to limit unintended motion during adjustment.
Produce a sensor unit that installs faster.
Order more accurate capacitors and resistors for better sound quality.
In the digital path, use higher order filters than can be reasonably constructed with discrete components.

19. Questions?