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Project Number: P13323
OPTO-ELECTRIC GUITAR PICKUP (REV. 2)
Chris Steele
Mechanical Engineering
Chris Perry
Electrical Engineering
Robert Paul Hoops
Electrical Engineering
Rachel Arquette
Electrical Engineering
Joe Mauger
Electrical Engineering
Ye Kuang
Electrical Engineering
Caroline Lichtenberger
Electrical Engineering
ABSTRACT
The vibrational movements of stringed instruments have been detected primarily through the
use of fluctuations in a magnetic field. The problem is that magnetic pickups produce a weak
signal and have a limited frequency response. The purpose of this project was to design and
create a guitar pickup that accounts for the disadvantages of magnetics, through the use of optical
sensors. The vibrational signal of a stringed instrument will be detected by the intersection of a
string with a light beam between an infrared LED and a matched photo-transistor. The incoming
signal will then be selectively filtered by either digital or analog signal processing. The sensing
unit of the pickup is portable, and removable from the sound-board of a guitar, without any
required modifications or permanent fixtures.
INTRODUCTION
Current market options for guitar string pickups are limited to magnetic and piezo-electric
types with the former being most common. The magnetic type, using wire wound permanent
magnets under each string, are fastened to the guitar permanently or integrated into its design.
The use of wound magnets will cause damping of the true string vibration and may result in the
unintended detection of any uncontrolled RF in the vicinity (fluorescent light ballasts, power
supply transformers, etc). Piezo-electric systems are not prone to electrical noise, but are limited
to detecting the vibrations of the guitar body itself, since they cannot discriminate between
strings, and therefore have mechanical noise concerns as well.
The goal of this project is to create a non-contact, optically driven guitar pick-up using
infrared sensors that are temporarily affixed to a guitar and that reproduce string tones through
an analog or digital filter set. This is a completely optical system for each string so that each note
is filtered and reproduced individually, thereby minimizing the influences of mechanical and
electrical noise interference, while being easily removable. The pick-up mounts completely
under the strings by the rear bridge (with the exception of a small upper photo-transistor mount)
and does not impede playing the guitar for most users. To be used effectively, it is mechanically
adjusted to the guitar’s string spacing and automatically adjusts to any difference in signal offset
coming from the sensors which results from string type and diameter differences. This prototype
system will demonstrate the effectiveness of optical pickups and hopefully create interest in an
unexplored area of opto-electric technology.
DESIGN
Copyright © 2013 Rochester Institute of Technology
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Specifications
This unit is foremost designed to be temporary, with no modifications to the guitar required.
It has to satisfy the desire to switch digital and analog signal filtering quickly, with minimal lag
time between them so that the user can compare the effects of each (filtering should encompass
20 Hz-20 kHz signals). The infrared optical pick-up system must be versatile enough to detect
both fundamental and harmonic string tones out to approximately the 20th harmonic, but it cannot
interfere with the original acoustic sound. Noise from the ambient environment will need to be
dealt with as well. Sound reproduction must be clear and output should be from a standardized
connector and at a standardized level (¼'' TRS, 894mV peak-to-peak).
Additionally, the unit must be ergonomic and have a small learning curve so that it can be
picked up and used by anyone. The enclosure and sensor will need to be aesthetically pleasing,
with no exposed, loose components and the entire unit must be powered from one source.
Battery life should be at least a couple songs. All of the aforementioned specifications must be
developed and produced for under $500.
A few designs for the sensors were formulated, having beam positions that are parallel,
perpendicular, or diagonal to the soundboard. Circuitry enclosure locations were considered both
on and off the guitar, and many filtering methods were theorized. The final product uses a space
saving perpendicular sensor arrangement, with one set for each string. The enclosure is located
on the hip to relieve ergonomic constraints on size that would be present if it was mounted to the
guitar. Filtering is performed by active high-pass and low-pass Butterworth filters of the second
order in the analog path and high-pass and low-pass Butterworth filters of the sixth order in
digital path. Power is provided from a sole 9 V battery and output is from the common ¼'' TRS
jack with signals at 894mV peak-to-peak.
Optical Sensors
The flow of information throughout the system originates at the string sensors. Each sensor
consists of an 870 nm infrared LED and photo-transistor pair positioned vertically in a breakbeam setup mounted near the bridge. At rest, each string should lie directly between the LED and
photo-transistor pair, creating the maximum physical impedance of the infrared light beam. As
the strings are plucked/strummed, they oscillate between their respective sensor pair. The
oscillation is detected by the photo-transistor in the form of the correlated oscillating light
intensity caused by the string moving through the light beam. The photo-transistors translate the
oscillating light intensity to an oscillating voltage. The signal obtained then goes through a
processing stage to be outputted. The schematic for the sensor circuit is shown below in Figure 1,
with the analog path circuit.
Processing Paths
As requested by the customer, the information gathered by the photo-transistors is filtered
using either analog or digital implementation. First, the signal from the photo-transistor is passed
through a gain stage to optimize the voltage range upon inputting to the microprocessor for the
digital implementation. Depending on the position of the analog/digital switch, the signal is then
passed through both a high-pass and low-pass filter using either an analog or digital
implementation. The digital implementation is designed to mimic its analog counterpart. The
cutoff frequencies for the aforementioned filters allow for the specific frequencies of each string
and up to two harmonics to pass through un-attenuated. Each of the high-pass filters are designed
to attenuate low frequency noise typically present around the DC level as well as the 60 Hz noise
which may be present due to interference from the electrical outlets.
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The information flow finally terminates in this system at the ¼” jack output. Both the analog
and digital implementations contain a summing stage, which sums the output of all six strings as
well as reducing the peak-to-peak voltage to a level compatible with commercially available
headphones and amplifiers.
Analog Filtering Path
Low Pass Filter Design:
A second-order Butterworth Filter was chosen for both the high- and low-pass filters. The
poles are always the same in a second-order low-pass filter. The expression for normalized H(s)
is shown below in Equation (1).
(1)
Element values in the normalized filter network designed so far – H(s) above – must be
frequency-scaled in order to make the 3-dB frequency of the network fit the attenuation
specifications given. In this case, the attenuation is 1. Equation (2) below is used to determine
the ω3dB.
(2)
In the filter network designed above, leave the resistance values as they are. Replace each
capacitance C by
. In general, if the frequency-scaling factor is kf, then
replaces each
C. Table 1 shows the calculated results for all 6 strings as well as real-world values for
capacitors. This refers to commercially available elements, each with a tolerance generated by
the manufacturing process. When the calculated values are replaced with real-world values, the
LPF passes frequencies slightly higher than the planned cutoff.
Table 1: Calculations for Low Pass Filter Passive Values
High Pass Filter Design:
To design a second-order Butterworth high-pass filter, first design a low-pass filter with
high-pass cutoff frequency and apply the RC-CR transformation, which consists of replacing
each R in the LPF by a capacitance of value (1/R) Farad and replace each C in the LPF by a
resistance of value (1/C) Ohms. The resulting network is a HPF with the 3-dB frequency at 1
rad/s. When the calculated values are replaced with real-world values, the HPF passes
frequencies slightly lower than the planned cutoff. Table 2 shows the calculated results for all 6
strings as well as real-world values for capacitors.
Table 2: Calculations for High pass filter
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Full Analog Path:
The full analog path, containing gain stage, low pass filter, high pass filter, and summing
amplifier, is shown as a PSpice model in Figure 1 for the low E string. For completeness, the
sensor circuit is also shown on this model.
Figure 1: Low E String Analog Path
Digital Filtering Path
Microcontroller and Hardware:
The basic functionality of the digital processing path is to take the analog signals from each
of the six guitar strings, convert them to their respective digital formats, filter using the same
filtering format as the analog path, sum the signals together, convert the processed signal back to
an analog signal, and send the analog signal to the output jack. The analog to digital conversion
(ADC) and all relevant signal processing algorithms are handled by the microcontroller. The
digital to analog conversion is handled by an external digital to analog converter (DAC). Since
six ADC channels are required in order to read each string, a feature known as the direct memory
access controller (DMA) is used. The DMA allows for the transfer of data between peripheral
modules and memory. The DMA reduces overhead by avoiding processor intervention (i.e.
interrupts) for data transfers. As a result, the performance of the processor is improved and
software design is simpler. Much like in the analog path, each string is filtered with a series of
high pass and low pass filters before summing the signals together. The implementations of the
high and low pass filters are their finite impulse response (FIR) implementations. FIR filter
implementations run faster in software and are simpler to implement with given DSP library
functions.
The microcontroller selected for the digital signal processing (DSP) path of the guitar is the
Atmel AT32UC3L064. Major factors in the selection of the processor for this project included
ADC resolution and speed, the number of ADC channels, processor speed, processor cost, and
processor power consumption. The AT32UC3L064 is a 32 bit AVR architecture with 64 Kbytes
of Flash memory, clock speeds up to 50 MHz (64 DMIPS), 8 12-bit (28 ksps) ADC channels, 12
DMA channels, and DSP instructions. The AT32UC3L064 typically operates between 1.62 and
3.3 V. Assuming the processor is running on 3.3 V, the ADC uses 14.9 µA/MHz; the TWIM
uses 5.1 µA/MHz; and the worst case for the digital filtering algorithm is 260 µA/MHz.
As it is uncommon to find a DAC on most microcontrollers, an external DAC had to be
selected. The DAC chosen under terms of cost, resolution, and simplicity of interfacing with the
microcontroller is the Microchip MCP4728. The MCP4728 is a 12-bit quad DAC with on-board
EEPROM. The communication interface for the DAC is the two wire interface, I2C. The I2C
protocol is compatible with the processor’s TWIM (two wire interface master mode) interface.
The schematic for the digital path is shown in Figure 2.
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Figure 2: Digital Circuit
Software Layout:
The software file structure is given by Table 3. The idea behind this software structure is to
allow for ease of software expandability, readability, and re-usability. Therefore, it makes sense
to break down the embedded code into source files based on the microcontroller peripheral
feature being used.
File Name
Description
OptoGuitar.c
Main source file (i.e. main(void) function call)
OptoADC.h
ADC configuration header file
OptoADC.c
ADC configuration source file
OptoDMA.h
DMA configuration header file
OptoDMA.c
DMA configuration source file
OptoTWIM.h
TWIM configuration header file
OptoTWIM.c
TWIM configuration source file
OptoGPIOConfig.h
GPIO configuration header file
OptoGPIOConfig.c
GPIO configuration source file
Table 3: Software File Descriptions
In OptoGuitar.c, all calls to the peripheral functions defined in their respective source files
are made. All other source files listed in Table 3.1 relate to their respective peripheral functions.
Figure 3 shows the software flowchart. The flowchart describes how the software for the
system goes together. After initializing each peripheral, the software remains in an infinite loop,
triggering an interrupt every 48 kHz (0.021 ms) in order to maintain a fixed sampling rate
equivalent to that of high quality audio recording devices. Once the software enters the interrupt
service routine (ISR), all ADC channels are read, the signals are processed, converted back to an
analog signal, and sent to the output.
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Figure 3: Software Flowchart
Power Management
The power regulation consists of three main sections, the battery, the battery indicator, and
the voltage regulators. The battery was chosen based on the selected components, the required
energy to run entire circuit efficiently, and the need to power the circuit for a set amount of time.
The battery chosen is a Tenergy 9 volt lithium ion battery, with a 500 mA output. The battery
indicator section consists of two zener diodes at 4.3 volt forward voltage, two 10 kΩ resistors,
and an LED. The indicator circuit will remain lit up, indicating a good battery life, and will turn
off when the battery goes below approximately 7.25 volts. The voltage regulators will convert
the incoming 9 volts into steady ±5 volt rails, giving the required two voltage rails.
Figure 4: Schematic for Power Regulation and Battery Indicator
PCB Layout
The Printed Circuit Board (PCB) was designed using ExpressPCB software. The PCB
contains four layers: components are soldered to the top layer with traces routed to the ground
plane and positive plane, both located on the inner layers, and signal trace on the bottom layer.
Two PCBs operating in parallel fully accommodate the six strings of an acoustic guitar. The
boards are identical, with the first board populated with three strings of the analog path, all of the
digital path, and the power regulation and the second board populated with the remaining three
strings and the sensor circuit resistors. The layout currently in use is shown in Figure 5.
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Figure 5: PCB Schematic Layout
Mechanical Components
Mounting and operation of the system in a simple, intuitive
mannerr is required. The whole system consists of two major
components: the optical sensor system and the processing enclosure.
Each sensor unit consists of an LED / photo-transistor pair that mount
on pods (Fig 6) in a break beam arrangement with the string, varying
the amount of light permissible to the photo-transistor. Adjustments to
sensor position upon installation to a guitar can be made in two ways
(two degrees of freedom) that will not cause movement in any other
direction than the one intended.
The first degree of freedom, offset height from the string to each component, is adjustable via
set screws acting on a guide. This gives the correct distances between the string, photo-transistor,
and the emitter to provide the photo-transistor with a signal that will not cause saturation at any
point in the strings movement, as saturation gives no data as to string position. A second degree
of freedom provides the horizontal placement of the pair to account for variations in string
spacing, which is also positionable with set screws and a guide on the guide bed, so that as the
string swings back and forth, the lowest light level detected by the photo-transistor is at the
strings resting (or zero offset) position. String swath was accounted for in the dimensions of each
sensor pod and under heavy strumming the string will never contact another surface. The guitar
system uses 6 sets of identical mounts, each tuned individually to their string, but all identical in
specifications when manufactured. All components are modular and can be replaced individually
if required, without dismantling the entire setup.
The enclosure for housing both analog boards, the digital board, and the battery is remote
from the guitar and is attached to the player with a belt-clip. This relieves the problem of placing
the enclosure on the guitar in an ergonomic location (of which there are very few). Inside the
enclosure, the digital board is mounted to the back (the removable cover being the front) by four
standoffs (one in each corner). Each analog board is then mounted on two standoff adapters that
run through the digital board and connected in between to limit flex and remove any breakage
concern during rough use. The battery is mounted to the bottom in a clip, next to a 1/4” TRS
jack. The jack is on the bottom to aid in strain relief of the cable. Input from the guitar comes in
the left side, as it is expected that the unit is worn on the left of a right handed guitar player. Six
switches are mounted on the top surface (one for each string) along with the power switch for the
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unit and the jack switch. The unit is designed to be ergonomic to use while playing in both sitting
and standing positions and has a short learning curve when first introduced.
RESULTS
The system meets most of the customer needs and specifications. There were several changes
made to the specifications with approval of the customer, due to unforeseen design constraints.
Each subsystem functions properly.
The sensors were tested under normal conditions and output expected results to be sent to the
processing paths. The analog path was observed using an oscilloscope at every stage to ensure
functionality. The result of a single strum of the guitar strings after being processed and summed
is shown in Figure 7. The results of the digital path testing gave less than expected results, due to
the current DAC’s clock samples not being quick enough for an accurate 48 kHz sampling
frequency. This is due to clock speed limitations of the I2C bus. The output of the DAC with the
poor sampling rate is shown in Figure 7. The voltage regulators in the power system were tested
to maintain an output of ±5 volts from a range of 9 V to approximately 7.25 V.
Figure 7: Oscilloscope Captures of Outputs, Analog Path (left) and Digital Path (right)
CONCLUSION AND RECOMMENDATIONS
Overall, this project was a success because the analog path and digital path both proved their
functionality. The team learned a lot about processing, both using analog means and digital
means. Though the system functions as expected, there is a vast amount of room for
improvement. Future work for this project could be explored with regards to sound quality, ease
of use, efficiency of resources and/or power, and even processes for commercial availability.
ACKNOWLEDGEMENTS
Optical Pickup Team would like to thank our guide Les Moore and our customer and
technical advisor Dr. Patru. We would also like to acknowledge other faculty support from
Professors Slack, Indovina, and Barrios, Dr. Puchades, and Ken Snyder. In addition, the Team
would like to thank fellow student Alex Coleman for valuable input throughout the MSDI
process.
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