EGUITAR TUNER

Mark StockerEastern Kentucky UniversityDepartment of Technology

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OUTLINE Automatic Guitar tuning Amplification of signal Frequency Measurement Harmonic Interference

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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

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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)

Hzn 4402 12/

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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.

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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.

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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.

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FREQUENCY MEASUREMENT

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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

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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.

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SOLUTION #1 Amplify and bias the signal using a simple op-amp

circuit.

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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:

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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.

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signal = ((filter * last_sample_value + (16 – filter) * sample_value)>>4);

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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.

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AVR uCString Selection

BIAS

OP-AMP

Guitar Input

Darlington Array

Stepper Motor Gear Reduction unit Guitar

Interface

MEDIA

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eGuitar tuner in action:http://www.youtube.com/watch?v=5ctUjII-M6Yhttp://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

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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.

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COSTDevice Cost SourceAVR uC $3 MouserLM324 op-amp $0.50 MouserComponents $2 Mouser4x20 LCD Display $10 EBayStepper assembly $0 Junk FB ScannerPower Supply $0 Junk BoxTotal Cost: $15.50Commercial Tuner: $80Savings:$64.50

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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.

SOFTWAREOnly free / open source software was used in the making of this

project, except for Microsoft PowerPoint.

Operating System: Ubuntu Linux running KDEDevelopment Environment: KATECompiler: AVR-GCCAVR Programmer: AVRdudeSignal Generation: Siggen

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REFERENCESAtmel 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

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