Brad Geltz
EGRE631 – Embedded Systems
April 15th, 2009

Introduction
• Project motivations
• Commercial products

Sound
• Transmission/Reception
• Signals/Harmonics

Guitars
• Music theory
• Signal generation (pickups)

Tuner
•
•
•
•
PIC18F4550 Microcontroller
Features used
Program outline
Results
 Commercial product comparison

Conclusion
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Project
motivations
• Instrument tuning not a trivial problem
• Personal interest
 Commercial
products
• Inexpensive
• Accurate
• More functionality
• E.g. Korg GA-30
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Transmission
• Signal converted to wave
• Transmit
 Electrical signal applied to
coil
 Creates a electromagnetic
field
 Magnet and coil interact
 Begin oscillations
Basic Loudspeaker
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Reception
• Wave converted to signal
• Wave causes oscillations in
material of receiver (e.g.
human ear, microphone)
• Microphones (condenser)
 Vibrations cause charging and
discharging of capacitors
 Generate signal
Microphone
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Signals
(sound wave)
• Properties of waves
apply
• Amplitude
 Effects how loud/soft the
sound is
• Frequency
 Effects how high/low the
pitch is
Example sound waves
440Hz to 880Hz
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Music
theory
• Standard guitar
 6 strings
 Each string tuned to a different note (frequency)
 Tuning – How close the frequency of the signal generated
by the instrument is to its defined value
 All notes have a defined value in terms of Hertz
 Equal temperament – Every adjacent note has an identical
frequency ration between them
 Based on Concert A at 440 Hz (key #49)
 Calculations based on 12 tone equal temperament (12-TET)
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Frequencies
• Standard 6 string
guitar
 1 - E4 – 329.6 Hz
 2 – B3 – 246.9 Hz
 3 – G3 – 195.9 Hz
 4 – D3 – 146.8 Hz
 5 – A2 – 110.0 Hz
 6 – E2 – 82.4 Hz
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Harmonics
• For a given sound wave, the harmonic is an
integer multiple of that wave’s frequency
 440 Hz A4 (Concert A)
 880 Hz A5 (Harmonic)
• Problem
 Harmonics present when base note played
 Typically not recognized as separate notes
 “Noise” introduced to signal
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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
Acoustic
• Guitarist plucks note
• String vibrates
• Transferred to the body
(insides)
 Bridge (where string
connects to body)
• Projected through sound
hole
• Specifically engineered to
maximize resonance
 Maintaining maximum
amplitude at a particular
frequency
• No way to pick up sound
wave for amplification…
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Electric
• Pickups (transducers)
 Captures or “picks up” the
vibration of the string and
converts it to electrical
signal
 Wire coiled around
magnet
 Vibrations modulate
magnetic flux and thus AC
 Signal weird to output jack
on body of guitar
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Signal
(voltage)
• Varies
 Type of pickup
 Guitar model
 Etc.
• Typically between
100mV and 1V
• Multiple sources of
noise
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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Brad Geltz | VCU | EGRE 631 - Embedded Systems
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 Problem
• How to determine frequency of analog signal of
varying amplitude with a microcontroller?
 Frequency counter
 Analog comparator
 Timing algorithms
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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
Microcontroller
• PIC18F4550
 8MHz +/- 2% internal oscillator
(up to 48MHz externally)
 Numerous other oscillator modes
(31 KHz, 2MHz, 4MHz, etc.)
2 analog comparators
4 timers (1-8 bit, 3-16 bit)
35 I/O pins
USB Programmer (PICkit2)
40-pin DIP
32K flash (program) memory
2K bytes SRAM, 256 bytes of
EEPROM (data memory)
 10 bit, 13 channel A/D converter
 Wide Operating Voltage Range
(2.0V to 5.5V)
 Numerous books/guides for C
programming







Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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
Microcontroller
• PIC18F4550
 RISC architecture (8051 is
CISC)
 Large number of registers
 Small stack
 95% of instructions
executed in 1 clock cycle
 Load/store versus memory
manipulation of data
 One instruction cycle
consists of 4 oscillator
periods
 Instructions execute at
CLKSPD/4
 2MHz based on 8MHz clock
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 IDE
• MPLAB IDE
 Direct from Microchip
 Full debugging utilities
• MPLAB C18
 Free for academic use
 In-line assembly
 Included and user
created libraries
 I2C
 ADC
 Delays
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 PICkit
2
• Wire to PIC
• Program direct from
MPLAB
• Provides debugging
• Power for PIC
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Features
used
• I/O ports
 LCD screen
 Newhaven NHD-0216BZRN-YBW
 Push button
• Voltage reference for
comparator
• Analog comparator
• Priority interrupts
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Analog
comparator
• Set threshold voltage
(VIN-)
 Between VDD and VSS
• Set analog source (VIN+)
• Output is low until signal
crosses threshold
voltage
 Raised to VDD
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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
Threshold Voltage
• Initially with voltage
divider
 2 resistors of equal size
yields VDD / 2
• Voltage reference
generated by PIC
 Setup to use source or
external voltages
 (X/24) * VDD
 X between 0 and 24
 .208V
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4/15/2009
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4/15/2009
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Brad Geltz | VCU | EGRE 631 - Embedded Systems
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 Frequency
counter
• Solutions
 Comparator drives external interrupt
 Too complex
 Utilize interrupt triggered by output change on
comparator
 Have to differentiate between rising and falling edge
 Flag set in comparator configuration register
 Increment variable on interrupt
 Report counted Hz after a specified amount of time
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Determine
tuning
• Compare counted frequency versus stored value
for particular note
• If the counted frequency is less than the store
frequency the note currently being played is
know as “flat”
 String needs to be tightened to increase frequency
• Frequency greater than the stored value is
known as “sharp”
 String needs to be loosened to decrease frequency
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Algorithm
• Main()
 Wait for change in frequency variable
 Output to LCD if change
 Slider




Difference of observed value from stored value (Z)
Left if flat (Z < 0)
Flash “PERFECT PITCH” if in tune (Z = 0)
Right if sharp (Z > 0)
 If button press, advance to next note
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Algorithm
• Interrupts
 Comparator Output
 Checks to see if positive transition
 If so, increment frequency counter
 Timer 0 (2 Hz)
 Multiplies frequency count by 2
 Updates LCD frequency display variable
 Resets frequency count variable
 Configured in priority fashion
 Two interrupt vector locations
 Each interrupt can be configured as high or low priority
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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/*
Guitar Tuner
Author: Brad Geltz
*/
#include <p18f4550.h>
#include <string.h>
#pragma config WDT = OFF, PBADEN = OFF, BOR = OFF
#pragma config DEBUG = OFF, MCLRE = OFF, LVP = OFF, FOSC = INTOSCIO_EC
#define button PORTCbits.RC6;
void chk_isr(void);
void chk_low_isr(void);
void T0_ISR(void);
void CM0_ISR(void);
Brad Geltz | VCU | EGRE 631 - Embedded Systems
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//Set code for high and low priority interrupt
#pragma code My_HiPrio_Int=0x0008
void My_HiPrio_Int (void){
chk_isr();
}
#pragma code My_LoPrio_Int=0x0018
void My_LoPrio_Int (void){
chk_low_isr();
}
//Set ISR for high priority interrupts
#pragma interrupt chk_isr
void chk_isr (void){
if (PIR2bits.CMIF == 1){CM0_ISR();}
}
//Set ISR for low priority interrupts
#pragma interruptlow chk_low_isr
void chk_low_isr (void){
if (INTCONbits.TMR0IF == 1){T0_ISR();}
}
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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//Timer 0 ISR - Interrupts at 2Hz
//Multiplies freq_count by 2 and stores it in frequency
//Resets frequency count
//If the button is not pressed, write the pitch indicator
void T0_ISR(void){
TMR0H=-0xF2;
TMR0L=-0x24;
INTCONbits.TMR0IF=0;
frequency = 2 * freq_count;
freq_count = 0;
if (!PORTCbits.RC6){
set_cursor_loc(0x40);
write_string(dashes);
set_cursor_loc(0x40);
write_pitch_indicator(frequency - freqs[note_index]);
}
//DEBUG - Uncomment to see frequency
//set_cursor_loc(0x40);
//Set cursor to beginning of next line
//write_int(frequency);
}
//Comparator 0 ISR - Interrupts on output transition
//If it was a positive transition, increment the frequency count
void CM0_ISR(){
if (CMCONbits.C1OUT){freq_count++;}
PIR2bits.CMIF = 0;
}
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Error
• Priority interrupts increase accuracy
• Off by 2 Hz in the worst case
 Signal running at 31 Hz
 15 Hz / 16 Hz
 Bounce between 30 and 31 Hz
• 8 MHz only accurate to +/- 2%
• Signal noise
 False comparator interrupt
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Results
• Successfully determines pitch
 Very briefly
 Intense vibrations
• Not perfectly accurate
• Cost ($17)
 PIC - $5
 ¼” Jack - $3
 LCD - $9
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Future iterations
• Timer 0 counts between comparator interrupts
 No need to wait for frequency count for valid output
 Requires accurate system clock
• Band pass filters
 MAX263
 Pin programmable
 1Hz to 57 KHz
 Eliminate harmonic noise
• Decrease VDD to decrease threshold voltage
 Increase valid tuning time
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Commercial
products
• Korg GA-30
 Auto ranging
 Supports multiple instruments
 Multiple string configurations
 Built-in speaker
 Reference pitch
 Microphone/Line input
 Inexpensive (~$15)
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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 Conclusion
• Microcontroller successfully tunes guitar
• Future iterations can greatly improve
functionality
 Algorithm tweaks
 Filters
• Add features similar to Korg tuner
Brad Geltz | VCU | EGRE 631 - Embedded Systems
4/15/2009
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