Eguitar tuner Mark Stocker Eastern Kentucky University Department of Technology
OUTLINE • Automatic Guitar tuning • Amplification of signal • Frequency Measurement • Harmonic Interference
MOTIVATION • Beginners often have trouble tuning their guitar properly. • Self tuning guitars such as the Gibson Robot are extremely expensive ($4000!) • http://www.youtube.com/watch?v=WetVXbYRfWk
INTRODUCTION • Each guitar string vibrates at a unique fundamental frequency. • The fundamental frequency of the string directly relates to the musical note being played via the formula: • n is the number of steps away from middle A (which is exactly 440Hz)
PROBLEM STATEMENT • Frequency measurement is typically done by custom made Digital Signal Processors (DSP’s). • DSP’s take a lot of time and money to develop into a working product.
PROPOSED SOLUTION • Analyze and tune the guitar using general purpose devices that are easily acquired and very inexpensive. • Perform the signal processing and motor control in software instead of a hardware DSP device.
Frequency measurement • Measure the voltage of the guitar signal using the Atmel AVR built in Analog-to-Digital convertor (ADC). • Sample the change in voltage at a very high rate (200uS) to detect each time the waveform crosses the zero point or bias. • After a predetermined number of zero crossings have been detected, determine frequency using the formula 1/T where T is the amount of time that passed during the sample.
Frequency measurement
Frequency measurement • Compare the measured readings to those known to be in the correct range (+/- .05% of intended frequency) and tune string accordingly with a stepper motor. Standard Guitar Tuning: Frequency (Hz) Period Samples @ 200uS E = 82.4069 0.0121 606.75 A = 110.0000 0.0091 454.5 D = 146.8324 0.0068 340.5 G = 195.9978 0.0051 255 B = 246.9417 0.0040 202.48 e = 329.62775 0.0030 151.69
Problem #1 • The output of the guitar is very weak. 3mV peak-to-peak is typical. • The 8-bit ADC of the AVR is setup to use a 5V reference voltage. • We have 255 usable steps over the 5 volt range which is equal to 19mV per step. • This will not allow us enough precision to detect the zero crossings of the wave form. • The bias or baseline of the guitar signal needs to be at 2.56 volts, so we can detect both the positive and negative part of the waveform.
Solution #1 • Amplify and bias the signal using a simple op-amp circuit.
Problem #2 • Due to the varying stiffness of steel guitar strings, along with the fundamental frequency, harmonic frequencies are also generated, which can be quite powerful as seen below:
Problem #2 • The 2nd harmonic is especially powerful. • Since we are only measuring zero crossings of the waveform, the harmonics end up being measured also, producing incorrect results.
Solution #2 • To prevent false readings, we will filter the harmonics from the signal in software, with the filter customized for each string. • I use a bitshift in this filter (>>) to do the division in order to save CPU cycles. • The value of filter is set according to current string selection. Lower values create a more narrow band-pass. signal = ((filter * last_sample_value + (16 – filter) * sample_value)>>4);
RESULTS • The circuit and program are successful at reading determining the input frequency accurately. • The stepper motor assembly is successful at tuning the strings automatically, although slow.
AVR uC String Selection Darlington Array BIAS OP-AMP Guitar Input
Stepper Motor Gear Reduction unit Guitar Interface
media eGuitar tuner in action: http://www.youtube.com/watch?v=5ctUjII-M6Y http://www.youtube.com/watch?v=LXHzzM-_Uao eGuitar put to the test against commercial tuner: http://www.youtube.com/watch?v=rRlvrPupO9o eGuitar C code: http://www.stockmarker.org/AVR/eguitar/eguitar_code.html
CONCLUSIONS • Considering commercial guitar tuning devices use specialized DSP’s, I was quite surprised at how well the AVR functioned. • I learned many new things during the project, such as filtering signals in software, the use of AVR ADC and Timers, and stepper motor control.
Cost Device Cost Source AVR uC $3 Mouser LM324 op-amp $0.50 Mouser Components $2 Mouser 4x20 LCD Display $10 EBay Stepper assembly $0 Junk FB Scanner Power Supply $0 Junk Box Total Cost: $15.50 Commercial Tuner: $80 Savings: $64.50
FUTURE WORK • Use a shielded enclosure to prevent outside interference. • Fine tune the OP-AMP circuit and implement hardware filters for faster and more accurate tuning. • Streamline the program by writing it in pure assembly code.
Software Only free / open source software was used in the making of this project, except for Microsoft PowerPoint. Operating System: Ubuntu Linux running KDE Development Environment: KATE Compiler: AVR-GCC AVR Programmer: AVRdude Signal Generation: Siggen
REFERENCES Atmel Corporation, ATtiny261/461/861 Datasheet, November 7th 2006, Retrieved from: http://www.atmel.com/dyn/resources/prod_documents/doc2588.pdf Atmel Corporation, Digital Filters with AVR, July 16th 2008, Retrieved from: www.atmel.com/atmel/acrobat/doc2527.pdf NonGNU.org, AVR Libc Online Manual, Retrieved from: http://www.nongnu.org/avr-libc/user-manual/index.html
