The Automatic Guitar Tuner AGT

1
The Automatic Guitar Tuner (AGT)

• Presented by
• Paul Bradley
• Cody Campbell
• Paul Schwartz
• Matt Whitehead

2
Motivation for the AGT

• Tuning an instrument is time-consuming and
  difficult.
• Tuning is a continuous procedure due to
• Varying temperature
• Varying humidity
• Aggressive playing styles
• Fact Accomplished musicians find it necessary
  to retune their instruments several times per
  hour during moderate use.
• Why the Guitar?

3
Market Research

• Everyone we have talked to has had only
  positive responses to the concept of an automatic
  guitar tuner.
• Current Market
• Semi-automatic guitar tuners
• Display offset direction
• May indicate pitch deviation severity
• Past Grove City College Attempt at AGT
• Short-comings of project
• One commercially available fully automatic
  guitar tuner is available from TransPerformance.

4
Market Research (cont.)

• In addition, there are a few patents for
  automatic guitar tuners.
• One was designed by a student in England (2001)
  and was found through internet searching.
• U.S. Patent 6,184,452
• Another is from 1977, so the AGT concept is not
  new.
• U.S. Patent 4,018,124

5
Original Objectives

• Objectives are broken down into three levels of
  achievement.
• Level 1 represents the minimum required
  objectives
• Levels 2 and 3 are additional objectives
• The initial implementation of the project may
  be mounted separate from a guitar, such as on a
  wooden platform.
• If time allows, the AGT will be physically
  mounted onto a guitar.

6
Level 1 Objectives Specifications

• Tune a guitar string to within a 10 centitone
  frequency tolerance.
• Corresponds to about /- 0.58 of the nominal
  frequency
• (/- 0.5 Hz) for the lowest guitar string (Low E
  String).
• Require less than 10 seconds to successfully
  tune the string.
• An adult with minimal musical training should
  be able to use the AGT easily after reading a
  brief set of instructions.

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7
Level 2 Objectives Specifications

• Tune all six guitar strings, one at a time, to
  within a 10 centitone frequency tolerance for
  each string.
• Verified with oscilloscope
• Total weight under 10 pounds.
• The AGT must not impede the playing area of the
  guitar.
• Power consumption while tuning each string
  should be kept under 6 watts.

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8
Level 3 Objectives Specifications

• Tune all six guitars strings simultaneously to
  within same tolerance
• Design the AGT to be battery powered and ensure
  than it is capable of running for at least 100
  tunings per battery pack.
• Robust against user misuse
• Compatibility with over 50 of guitars (both
  acoustic and electric)
• Consumer cost should be under 500

3
NV
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9
Brief Overview of Design Solution

• Well talk about the design of the AGT

10
Top-Level Block Diagram

11
Second-Level Block Diagram

12
Control Unit
Central Control Unit
interface control signals
interface control signals
6
desired frequencies
6
Signal Processor
Motor Control
raw pickup signal
6
6
measured frequency
motor actuation (PWM)
13
Pickup

• The pickup block detects the frequencies of the
  guitar strings and sends the signals representing
  the string frequencies to the signal processing
  units
• The AGT utilizes the magnetic coil portion of the
  Roland GK-2AH
• Price 230
• Reasons
• This pickup greatly reduces the amount of
  crosstalk between signals
• It also minimizes the amount of filtering and
  signal conditioning required
• Outputs six individual signals representing the
  frequencies of each string

14
Roland GK-2AH Divided Guitar Pickup
The AGT utilizes only the magnetic coil portion
of the pickup.
15
Pickup Conclusions

• To summarize
• The divided pickup enables the AGT to tune all
  six string simultaneously
• May lower cost of system since it will
• Reduce the amount of filtering
• Erase the need for an expensive DSP or personal
  computer

16
Signal Processing

• The AGT utilizes six individual signal processing
  circuits, one for each string
• Each signal processing unit consists of the
  following subunits
• Amplifier stage consisting of a differential
  amplifier followed by a non-inverting amplifier
• 4th order Chebyshev filter
• Amplifier circuit
• Envelope detector
• Comparator
• PIC 16F684 microprocessor

17
Pre-Amplifier Stage

• The individual string signals from the pickup are
  too small for the low-pass filter to effectively
  handle.
• Each signal is input to a pre-amplifier stage
  consisting of a differential amplifier followed
  by a non-inverting operational amplifier circuit.
• This stage increases the amplitude of the signal
  and prepares it for the filtering stage.

18
Signal Following the Pre-Amp Stage
The A-string signal following the pre-amplifier
stage. Note the presence of higher frequency
harmonics.
19
Low-Pass Filter Stage

• The amplified signal is then input into a 4th
  order, low-pass Chebyshev filter
• This filter attenuates the harmonic frequencies
  that are present in the amplified guitar signal.
• The filter also places a small gain on the input
  signal.

20
Signal Following the Filtering Stage
The A-string signal following the filtering stage.
21
To buffer
4th Order Chebyshev Filter
Differential Amplifier
Non-Inverting Amplifier
From Pickup
22
Amplifier Circuit

• The low-pass filter output is sent to an
  amplifier circuit.
• This circuit increases the amplitude of the
  signal for the following stages.
• The gain of this amplifier is large to enable the
  tuning process to occur for a long period of
  time.
• The output of the buffer is directly input to
  both an envelope detector circuit and a
  comparator circuit.

23
Signal Following the Amplifier Stage
The A-string signal following the amplifier stage.
24
Envelope Detector

• The envelope detector is a half-wave rectifier
  circuit.
• It rectifies and smoothes the buffer signal to
  represent the amplitude of the buffer output.
• This signal is then sent to the A/D converter pin
  of the PIC 16F684
• The PIC uses this signal to determine if the
  buffer signal is above a certain threshold.

25
Envelope Detector Output Signal
The A-string envelope detector output signal.
26
From Filter
Amplifier
Envelope Detector
27
Comparator

• The comparator converts the signal from a
  sinusoidal wave into a digital square wave (0 to
  5V) of the same frequency.
• The digital output signal is input to the
  microprocessor.
• The microprocessor uses this signal to determine
  the frequency of the guitar string.

28
Comparator Output Signal
The A-string comparator output signal.
29
Comparator Circuit Diagram
30
Microprocessor Programming

• Six PIC 16F684 microprocessors for signal
  processing.
• The PIC 16F684 has the following capabilities
• Comparator
• PWM Output
• Input Interrupts
• A/D Conversion

31
Microprocessor Programming A/D Conversion

• Receives signal from envelope detector
• Device state (on/off) determined by level of
  signal.
• Hysteresis
• Eliminates small-voltage spike (noise) problems
• Maximize tuning time of each string.

32
Motor System
Power Electronics
6
6
6
Motors
motor actuation (logic level PWM)
string frequencies
motor actuation (drive level PWM)
33
Motor Control

• Microprocessor controlled PWM.
• Our PIC can make 20kHz PWM output on two pins
  for forward and reverse.

34
Power Electronics

• Two Half-bridge drivers
• No bypass diodes!
• IRF530 MOSFETs
• IRF2104 drivers

reverse
Vcc
forward
M
35
Power Electronics
36
Motors

• Sears Screwdrivers, 6V
• No-load speed around 130 rpm
• PWM at 20 duty cycle gives about 20 rpm
• with 181 tuning pegs gives a wide speed range
• roughly 30 oz-in of torque

37
User Interface

• Allows user to communicate with the AGT
• Includes
• Begin Tuning button
• Cancel Tuning button
• LEDs to signify current status of the AGT

38
Central Control Unit

• More a conceptual system block than a real
  component
• Takes input from the user interface
• Signals the motor controllers to begin the tuning
  process
• Monitors the results and signals the user when
  tuning is complete.

39
Power Supply

• Linear Power Supply
• powers most components
• 5, -5 and 12 V
• Motors powered by 6V from four D cell
  batteries
• Not the optimal solution

40
Conclusions

• Being an Electrical Engineer is fun
• When designing analog electronics, watch
  out for noise
• Always test with the parts you plan to use
• Integrate systems as soon as possible
• Work with tools that give you maximum
  diagnostic ability

41
Any Questions?
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42
Movie!

• AGT 4000 in action

• The tuning of the D string.
• Desired frequency 146.83 Hz.

43
Budget Analysis

• 6 motors - 30
• 6 drivers - 18
• Pickup - 200
• Electronics - 20
• Device Packaging - 5
• Printed Circuit Boards - 2
• Documentation - 5
• Assembly per Unit - 20
• Total Per Unit - 300