Digital Theremin

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Digital Theremin
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Team Members
Alex Milevsky (CPE)
Dan Nagle (CPE)
Advisor Bob Reese
Nathan Palmer (CPE)
Jerry McMahan Jr. (EE)
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Responsibilities

• Daniel Nagle PC learning software, grading
  algorithm, lesson format
• Alex Milevsky Team Leader, PIC programming (MIDI
  output), PC learning software, MIDI handler
• Nathan Palmer PIC programming, real world
  interfacing, LCD interfacing
• Jerry McMahan Oscillators, mixer, filters, A/D
  conversion

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Previous Work

• Main Points
• Switchable discrete pitch mode
• Lower cost
• Tuner out
• Problems
• Pitch drift
• Did not complete
• Exceeded budget

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Goals

• Provide an LCD to display the pitch of the
  theremins output
• Interface with a computer using the MIDI
  protocol, allowing information on the playing to
  be gathered
• Develop software to evaluate the performance
  based on the MIDI information gathered to provide
  feedback to the player

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Block Diagram
Volume Knob
Beat Frequency Oscillator
Volume Processor
Mixer/ Filter
Speaker Out
Learning Software
MCU
MIDI Out
LCD Out
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Frequency Detection

• Methods Considered
• Frequency to voltage converter
• Measuring zero crossings
• Other methods avoided to reduce cost

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Frequency/Voltage Chip

• National Semiconductor LM2907
• Strengths
• Ease of use

• Weaknesses
• Accuracy poor for low frequencies
• Output ripple

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Ideal vs. Measured
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Zero Crossings

• Made easier since output is roughly sinusoidal
• A/D converter could be used, but problems with
  noise
• Solution convert sine wave to square wave and
  apply to digital input pin, triggering interrupts

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Zero Crossings (cont.)

• Theoretical accuracy

• 2500 times highest frequency
• Computationally cheap

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Frequency Detection Error
Average Percentage Error .07
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Filter Circuit
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MIDI Output

• Requires volume and pitch detection
• Filter circuit used to make volume detection
  easier
• Volume detection via onboard 10-bit A/D
  converter
• MIDI protocol provides 7 bits of data to specify
  volume, so highest 3 bits ignored

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Firmware Flowchart
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Software Flow Chart
Start
Record button pressed
Open lesson file
Call MIDI Handler
Display music, Description, Etc.
Record playing
Grade recorded file
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Software
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Grading Algorithm

• Scans raw MIDI data for pitch wheel and volume
  messages
• On transition from volume low to volume high,
  software latches the pitch wheel value
• Scoring format
• Missed note 50 points
• Correct note Remainder of pitch wheel value
• Total score 100 (remainder / notes graded)

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PCB Layout
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PCB Hardware
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PCB Problems

• increase dimensions for placement of parts
• space the inductors farther away
• the volume control has an unnecessary resistor in
  series
• The mixer/filter has a capacitor connected to the
  wrong place.

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Testing Plan

• Frequency Detection oscilloscope
• Pitch Drift conductive plate
• MIDI Midimon, MIDI keyboard

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Cost
Component Cost
PIC16F876A Resonator 7.55
LCD 15.33
Resistors 1.00
Capacitors / Inductors 2.00
Antennae 6.00
Op-Amps 4.52
Buttons / Switches / Jacks 8.00
Transistors 6.00
PCB Board (bulk) 5.92
Total 56.32
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Test Results
MIDI compatibile Yes
3 octaves 130-987.77 Hz 5 octaves 187-4000Hz
5 accuracy pitch detection 0.07 accuracy
2V preamp 1.4V preamp
2 pitch drift lt3 pitch drift
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Results (cont.)
Calibration -
Win98 or higher software Tested on Win98, Win2k, Win XP
Market price lt200 56.32
LCD Interface Yes
Speaker jack Yes
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Other Progress

• Prototype case
• MIDI functionality working
• PC software stable
• Volume knob
• RCS simplifies updating documentation

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Acknowledgements

• Dr. Robert Reese
• Dr. Joseph Picone
• Way Beng Koay
• Douglas Beard

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Questions
?