Download Designing an Optical Theremin

Designing an Optical Theremin
EE 300W
Lab2
Section 002
(Team Vee)
Fei Bu, Zakkere McCartney, Matt Baranoski
March, 7, 2014
Abstract
The Theremin allows a user to play music without physically touching the
instrument. Most Theremin’s use antennas to measure the distance from the players
hand to the instrument which controls the frequency and gain of the sound played.
Our design will be an optical Theremin which is controlled by how much light is
exposed to two photodiodes. A typical Theremin uses hardware to condition the inputs
and generate an output but our Theremin will use a simple physical circuit and
extensive LabView code to generate the sound.
Our design is two parts; the simple physical circuit and the more complex virtual
LabView circuit. The physical circuit consists of two photodiodes which generate a
current based on the amount of light they are exposed to. We will use a current to
voltage op-amp circuit to generate voltage that the MyDAQ can sample. The MyDAQ
functions as the bridge between our physical and virtual circuits. The virtual circuit
samples the voltage waveforms from the physical circuit and uses them to control the
amplitude and frequency of an audio waveform that becomes our output. Both signals
are conditioned and scaled with user defined limits as well as an auto-tune feature
with optional key select.
Introduction
The optical Theremin we designed needed to use the amount of light exposed to
two photodiodes to control the amplitude or gain as well as the frequency of an output
audio signal. Our design will include a physical circuit that transforms the current
generated by the photodiodes to a voltage the myDAQ can read and sample. The
myDAQ will sample the voltage from the physical circuit and then provide that to the
virtual circuit as an input. The virtual circuit will be multipurpose; it will need to
sample the signal from the myDAQ, condition that signal to remove noise, scale and
coerce the signal based on user inputs, rescale the frequency output to a user defined
range, auto-tune the frequency, provide a key select feature, generate a wave based on
the conditioned frequency and amplitude signals and output that wave to the myDAQ
where it can be played through speakers.
The complex virtual circuit allows us to make the circuit much more complex and
offer the user many more features to choose from while playing.
Rationale
The optical Theremin design we created uses a simple physical circuit to gather
the input and make it readable for a more complex virtual circuit. A photodiode (P/N
365-1084-ND) is used to sense the light intensity which is our input and will directly
control the frequency and amplitude of the sound played. The photodiode generates a
current that corresponds to the light intensity exposed to it. We will use an op-amp
current to voltage converter to transform the current produced by the photodiode to
voltage signals that the myDAQ can read. We will use the TL074 Op-Amp in the
configuration pictured below to convert the current to voltage.
Op-Amp schematic and equations:
Ie = RL
Ie being the output current in photodiode, L being the optical power.
Ua = -IR
Ua is the output voltage of Op-Amp.
The myDAQ will read the voltage from the physical circuit and provide a digitized
signal to the virtual circuit. Our LabView code will condition the signals and use them
to create an audio signal. The input signals will both be scaled to limits the user
defines on the front panel and it will be coerced to a value between zero and one. This
value corresponds to the output that will create the audio signal. At this point the
amplitude can be used to control the amplitude of the output audio signal. The
frequency signal will be scaled again based on limits the user defines for the output
audio signal. We will also auto-tune the frequency signal to ensure it corresponds to a
musical note. The tuned and conditioned frequency will control the frequency of the
audio output. We will generate the audio signal using a wave generation vi and output
it to the my DAQ where it can be played.
Implementation
The physical circuit is built by TL074 Op-Amp chip and two photodiodes
(P/N 365-1084-ND). The Op-Amp chip is to convert the current through
photodiodes to voltage that inputs into My-DAQ. Two outputs of Op-Amp chip is
connected to Analog I/O ports 0+ and 1+ on My-DAQ. One side of audio wire is
plugged into My-DAQ Audio port and the other is plugged into the speaker.
Circuit Schematic:
I Physical Circuit Schematic
Amplitude and frequency sampling data are separated and processed in
LABVIEW to generate the tune. We used a DAQ assistant vi to sample the input
signal from the myDAQ and create a waveform we can condition in the virtual circuit.
The signals were unbundled and filtered to remove noise using the mean function.
This turned our input array into a variable which is much easier for us to condition.
For amplitude, we unbundled the data from the user controlled slider and used
that to scale the input signal. We used the In Range and Coerce function to scale
the output of amplitude from 0 to 1. For frequency, we have to scale both input
and output frequency limits. The input frequency limit is scaled from 0 to 1. The
output frequency limit is scaled from 0 to 20k Hz. The scaled frequency can be
written directly to the wave generator now but if the user wishes to use the
auto-tune feature we must tune the signal. Our auto-tune feature builds a two
dimensional array of the music notes and octaves we were supplied. The array is
transposed into a one dimensional array which we compare our signal to using
the thresholding vi. From this we can output the note our signal is closest to and
control the frequency of the audio wave using it. The audio wave is generated
with a sine wave generator vi. We use our conditioned amplitude and frequency
signals as inputs to the vi. The output waveform is read by the myDAQ using a
DAQ assistant vi which generates the wave on the audio out port. Our design also
includes a key select feature the user can choose to use. The user selects the key
they would like to play in and the key select vi picks out the corresponding notes
from the two dimensional array of all possible notes and removes the rest. Our
frequency signal is then auto-tuned to only the notes in that key. To generate a
consistent audio signal we had to tune the sampling rates of each of the DAQ
assistant vi’s and the wave generation vi. We based most of our settings off the
Nyquist sampling theorem and made sure our sampling rate was at least twice
that of the number of samples to write in a second. The settings we found to
work best are listed in the table below.
Frequency Settings in LABVIEW:
LABVIEW
Module
DAQ Input
Simulate Signal
Value
Samples to read
Sampling rate
Sine
Timing
DAQ Output
Mode
Samples to
write
Sampling rate
Frequency
Amplitude
Samples per
second
Number of
samples
10
200k Hz
5 Hz
1V
80,000 Hz
20,000 Hz
Continuous Samples
20k Hz
80k Hz
Block Diagram Analysis:
The input to the system is the light that is exposed to the photodiodes. Our system
uses this input to output an audible audio signal. The light exposed to the photodiode
is translated into a voltage reading that can be sampled by the MyDaq. We will use the
MyDaq device as our data acquisition device and convert the analog input voltage
signal to a digital signal. The LabView code will scale and coerce the input signal
based on user defined limits. The user will have the option to turn on an auto-tune
function that will tune the frequency to one of the musical pitches. The conditioned
signal will be transformed into a sine wave that is generated with controlled frequency
and amplitude based on our input signals. We will use the MyDaq to sample the sine
wave and output an audio signal.
II Block Diagram
Modifications:
There were three major modifications we made during this project; auto-tune
feature, frequency and sampling rates, and reversion of indicator range.
Originally, we used a for-loop to build the frequency table, and compare our value to
it in order to tune the frequency signal. This design was too complicated and difficult
to adjust so we discarded the whole schematic and used the threshold 1D array
function. We built a 2D array that contained all 121 notes from the table we were
given and used the transpose array function to create a 1D array. Using the “threshold
1D array” function we compared our signal to the array of notes. This generated the
index corresponding with the 1D frequency array. Finally, the index is fed into “index
array” to output the closest note frequency.
Originally, in DAQ Assistant and DAQ Assistant3 module we set the samples to read
to 200 and the rate to 2k Hz. In Simulate Signal module we set the frequency to 2k Hz,
amplitude to 1 V and the samples per second to 5k Hz (5k Hz > 2*2k Hz). We picked
5k Hz for the sampling frequency because it must be twice the signal frequency
according to the Nyquist Sampling Theorem. Under these settings, the tune changed
when we moved hands up and down but the sound was discrete beeps and the volume
was low. To improve the sound quality we decided that we should minimize the
samples to read in DAQ Assistant module to 10 and maximize the rate to 200k, the
max rate of the myDAQ. This decreased the number of data points to be processed
each second allowing the code to execute faster. In the Simulate Signal module, we
set the frequency to 5Hz, samples per second to 80k Hz, and Number of Samples to
20k. In DAQ Assistant3 module, we set the samples to write to 20k, and the rate to
80k. Now the tune is more continuous and the volume is high enough. But the volume
change corresponding to light intensity is not sensitive enough. We cannot achieve
completely continuous sound because of the 200k Hz maximum limit of My-DAQ
sampling rate.
The amplitude and frequency limits were reversed at first. When we would move our
hand closer to the diode the amplitude of the signals would increase. We reversed how
we scaled our values in the main VI so the output decreased as we moved our hand
closer to the photodiode.
Value Statement:
The optical Theremin we designed is unique because it uses a very simple
physical circuit and a very complex virtual LabView circuit. Most Theremin’s use a
complex physical circuit and little to no software component. Our design makes great
use of the inexpensive photodiodes and op-amps that make up the physical circuit but
the cost to implement the physical circuit is high. The necessity of the myDAQ and
LabView program drive the cost up and make this an expensive design. The Theremin
could be implemented much cheaper with more complex physical circuitry and
inexpensive components.
Conclusion:
Through a design and development process, we created a circuit that used the
leakage voltage from a photodiode to input a voltage to LabView code. The virtual
LabView circuit sampled the output of the physical circuit, conditioned the signals,
and used them to generate a single sine wave. The amplitude and frequency of the
sine wave would vary based on the two inputs of the photodiodes. We have also
designed a circuit that could pick specific keys from a short list of common keys.
These keys will auto tune the input to specific frequencies. Through this lab we
have proven you can build an optical Theremin with a physical circuit in combination
with LabView.
Appendicies:
Screen Captures:
III Fron tPanel of Theremin LabView code
IV Block diagram of Theremin LabView code
V Auto-Tune feature block diagram
VI Key Select block diagram