Transcript theremin - Courses - Mississippi State University

Theremi
n
THEREMIN
Department of Electrical and Computer Engineering
Theremin
Theremin Team
Douglas Beard
[email protected]
Way Beng Koay
[email protected]
Dr. Raymond Winton
Faculty Advisor
Jeffrey Jun-Fey Wong
[email protected]
Micah Caudle
[email protected]
Theremi
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Responsibilities
Micah Caudle:
Way Beng Koay:
Oscillators.
Frequency to Voltage
Conversion
Beat frequency detector.
Volume Circuit
Voltage to Frequency
Douglas Beard:
Jeffrey Jun-Fey Wong:
Analog to Digital
Output Stage
Digital to Analog
Footswitch Circuit
Microprocessor
Tuner Out
Theremi
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Motivation
Theremins are hard to play because they are
continuous frequency instruments like violin or
trombone.
Limited playing style currently prevents broad use. A
more versatile theremin will expand use.
Quality theremins can be pricy.
•$350-3,500
Theremin
Problems
• Continuous Pitch: The theremin is a continuous
pitch instrument like trombone or violin which makes
staying in tune difficult. This fact makes theremin difficult
to learn, but it also produces some desired effects.
• Continuous Volume: Staccato playing or quick
stops and starts are difficult with the theremin because of
continuous volume.
• Lack of Reference: Since the thereminist does not
actually touch the theremin, the thereminist has no point of
reference for notes and nothing to steady his or her hand.
Theremin
Design Requirements
•Discrete Frequency Accuracy
–Switchable between playing the traditional continuous range and playing
only distinct frequencies in selectable scales with error < 0.1%.
•Frequency Range
–A frequency range of four octaves with a center frequency at 440Hz.
•Precise Articulation
–A footswitch will connect to the theremin to enable the performer to
quickly and easily articulate notes.
•Tuning
–A small amplitude signal will always be present at the 1/4" tuner out
jack to enable the performer to locate starting pitches and for pitch
verification during practice.
Theremin
Theremin Modular Design
Variable
Oscillator
Fixed
Oscillator
Detector
Pitch Control
Variable
Oscillator
Freq-Voltage
Converter
PIC controller
with A/D
14-bit D/A
V/F
Converter
Discrete Frequency Controller
Freq
Switch
Voltage Controlled
Amplifier
Audio Out
Tuner Out Signal
Volume
Tuning
VCA
Processor
Volume
Control
Output Control
Footswitch
Theremin
VPO and FPO Circuits
Theremin
VPO and FPO Outputs
Theremi
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Detector Circuit
Theremin
Beat Frequency Output Range
CMAX = 6 pF gives
fMAX of 1765.4 Hz
f = 1 / T = 1765.4 Hz
T = 0.566 msec
CMIN = 2 pF gives
f = 1 / T = 109.9 Hz
fMIN of 109.9 Hz
T = 9.1 msec
Theremi
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Lab Results
290 – 288 kHz
Audio:
80 – 850 Hz
Variable
Oscillator
Detector
Fixed
Oscillator
292 – 288 kHz
Left:
Left:
• VPOmax = 291 kHz
• FPOmax
= 292 kHz
Detector
Output
Right:
Waveform
Right:
• VPOmin = 288 kHz
• FPOmin = 288 kHz
Theremi
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Beat Frequency Oscillator
290 – 288 kHz
Audio:
80 – 850 Hz
Variable
Oscillator
Detector
Fixed
Oscillator
292 – 288 kHz
•Beat frequency oscillator functions fairly well.
•Audio range falls short of design requirement.
Theremi
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Plan of Action
Step 1:
• Increase audio range by using larger antenna.
• Lab tests showed that a larger antenna gives increased
hand capacitance.
• Greater hand capacitance should cause a greater change in
VPO frequency and increase our audio range.
Step 2:
• Experiment with physical placement of the oscillator
circuits on the breadboard to vary their tendency to
synchronize.
Theremi
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Footswitch
Footswitch
Ring
Normally
Open
Tip
Control
Signal
Audio In
• Stepping on footswitch enables Audio Out.
•Releasing footswitch disables Audio Out.
Amplified
Audio Out
Theremi
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Discrete Mode Overview
Frequency/Voltage
110-1760 Hz
continuous beat
frequency from
detector
Binary
representation of
selected output
level
PIC controller
with 10 bit A/D
Converter
0.3-5 V
continuous
voltage range
14 bit
D/A Converter
Voltage/Frequency
Resulting discrete
voltage level
Converter
Desired note
within 0.1%
error
Theremi
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Frequency-to-Voltage Converter
Current Status
Requirement
Current Status
Input frequency
110 – 1760 Hz
110 – 1760
Output voltage
0.3 – 5.0 V
1.44 – 5.12 V
Problem solved:
--- A comparator circuit was implemented to convert the audio signal into
a pulse wave for proper detection by the V/F converter.
Problems:
--- Output voltage range does not swing across the whole range of 5V.
Possible solutions:
--- Apply amplification to the output before microcontroller.
Theremi
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Percentage Error
Percentage Error of Output Frequency Compare to Desire
0.25
Tuner Requirement
Percentage Error %
0.2
0.15
Design Requirement
Max. Error = 0.045%
0.1
0.05
0
100
300
500
700
900
1100
Frequency (Hz)
1300
12 bits
1500
1700
14 bits
Design Requirement
Comman Tuner Acceptable Error
Theremi
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Digital to Analog
0.2
0.18
Output Error (%)
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
200
250
300
350
400
450
500
550
Input Number
In a test of chromatic scale intervals, the largest error was
0.17% which needs to be worked back within our desired
error of <.1% .
Theremi
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Voltage-to-Frequency Converter
Current Status
Requirement
Current Status
Input Voltage
0 – 12 V
0.1 – 7.2
Output Frequency
110 – 1760 Hz
160 - 1760 Hz
Problems Solved:
(1)
Capacitance value of CT (control chip trigger time) has
increased by factor of 10 to make trigger time of the chip
Problems:
Possible
solution:
increase by factor of 10. Response range increase.
(1)ChangeThe
lowest
frequency
reachable
is around
160 Hz.
CT to
higher
value (10x)
to increase
the trigger
time. Test
5.0-7.4V swing
Again,
thelinear
circuit
(2)run, butFrequency
outputsacross
from 110-1760Hz.
voltage are not
totally
in is
too sensitive
to to
thevoltage.
input voltage..
relation
Theremi
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Voltage-to-Frequency Output
Vin vs. Fout (for Voltage to Frequency Converter)
1400
1200
Fout (Hz)
1000
800
600
400
200
0
0
1
2
3
4
5
6
7
8
Percent variance from linearity
Vin (V)
18
16
14
12
10
8
6
4
2
0
0
200
400
600
800
Fre que ncy
1000
1200
1400
1600
Theremi
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Voltage-to-Frequency Converter
Output Waveform
Lower
Range
Upper
Range
Theremi
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Summary
Beat Frequency Oscillator:
Frequency/Voltage Converter:
 Functions well
 Functions but needed pulse wave input
 Need to increase audio frequency range
 Addition of comparator fixed the problem
Volume Control Circuit:
Micro Controller:
 Debugging
 Programming not complete
 Hardwiring control presently
Footswitch Circuit:
 Works properly
Tuner Out Circuit:
 Not yet implemented
D/A Converter:
 Functioning linearly and accurately
Voltage/Frequency Converter:
 Functioning but gives pulse wave output
Theremin
Conclusions and Future Work
• To get the desired discrete frequency accuracy within a
four octave range, we need 14 bits, and accurately
converting this to analog is expensive in relation to other
operations.
• Our .1% pitch error limit may be more stringent than
needed. Some common tuners accept up to .2% pitch error.
• The discrete frequency output may have a different timbre
than the continuous frequency output. A wave-shaping
circuit could be added to give them similar quality.
Theremi
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Questions?
•
•
Mississippi State University
Department of Electrical and Computer Engineering