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Theremin with Onboard Effects. Patrick Tarantino Shaun Cinnamon Project # 43 ECE 345 - Spring 2001 . Introduction to the Theremin. Invented by Leon Theremin, a Russian physicist Musical signal created from beat frequency of two RF signals

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Theremin with Onboard Effects

Patrick Tarantino

Shaun Cinnamon

Project # 43

ECE 345 - Spring 2001

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Introduction to the Theremin

  • Invented by Leon Theremin, a Russian physicist

  • Musical signal created from beat frequency of two RF signals

  • Pitch and volume controlled by hand capacitance effects

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Basic Design Approach

Basic theremin circuit implemented primarily with IC chips

  • Greatly reduce temperature dependence

  • Reduce overall circuit cost

  • Reduce total circuit area required

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Design Specifications

  • Want at least a three octave musical range

  • Want a good level of sensitivity to hand position change

  • Want unmodified output and also at least one other “instrument sound”

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Tone Control Circuit

  • Uses two 8038 chips, both with RF outputs, an 8” by 5” square aluminum antenna, and an analog mixer to produce a frequency in the audible range that is controlled by the user’s hand position

  • Has a base frequency of about 160 kHz, with a range of about 1 kHz

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Volume Control Circuit

  • Uses two 8038 chips, an 8” by 5” square aluminum antenna, a frequency to voltage converter, and an analog mixer to function as an amplifier whose gain is controlled by the user’s hand position

  • Has a base frequency of about 160 kHz and a range of about 1 kHz

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Volume Control Operation

  • If Vi = Acos(2pft), Vo = k*f

  • Apply tone signal and this new control signal to an analog mixer, and the mixer now functions as a voltage controlled amplifier

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Mixer Operation

  • Multiplies the signals from the fixed and variable oscillators in both sections

  • Apply a LPF to eliminate high frequency components, and get output of cos(Dwt), the difference frequency

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Complete Theremin Circuit

  • Tone now controlled with one antenna, volume by the other, and the output signal is ready to be applied to the DSP for digital effects

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Wavetable Synthesis

Digital Effects for the Theremin

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Overview of Wavetable Synthesis

  • What is a Wavetable?

  • How are they used?

  • What is the purpose?

  • How is Wavetable Synthesis done?

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What is a Wavetable?

  • A Wavetable it a set of samples taken from an actual instrument to simulate the instrument.

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How are they used?

  • The samples are saved into three different Wavetables, along with corresponding amplitude envelopes.

  • Where do these three different Wavetables come from?

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The different Wavetables

  • The first Wavetable comes from the first 4096 samples of the complete Wavetable.

  • The second Wavetable comes from the second 4096 samples and the third from the next 4096.

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Where are the Amplitude envelopes at?

  • The amplitude envelopes are located at the end of the table.

  • They are contained in the next 1500 samples from the end of the wavetables.

  • Each envelope is 500 samples long.

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What happens next?

  • After the Wavetable and Amplitude Envelopes have been saved into their respective tables they are saved as coefficients to be used in the DSP.

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How do we recreate the instrument?

  • The recreation of the instrument is done by table lookup and by determining the increment through the wavetables.

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Table Increment

  • The table increment can be calculated by this equation:

    Table Length = 4096 samples

    Sample Rate = 44100 samples/second

    Frequency = dependent upon signal from the circuit

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What about the amplitude envelopes?

  • The amplitude envelopes are done in the same manner as the wavetables.

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How do we get the frequency of the signal?

  • We get the frequency of the signal through Zero-Crossing Detection.

  • Knowing that a sine (or cosine) crosses zero twice per period we can get the number of samples per period.

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New Table Increment

  • The frequency is in samples per period. In order to get the correct table increment a modification was needed. I needed to divide the sampling frequency by the number of samples per period. With the resulting increment being:

    increment = Table Length / # of Samples

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Completion of Wavetable Synthesis

  • To complete the synthesis we multiply the individual wavetables by the amplitude envelopes and add three tables together to get the synthesized instrument.

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How do we change instruments?

  • A Matlab interface was written to change between the normal Theremin output and the synthesized instrument effects.

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Project Build and Testing

  • DSP configured to amplify input signal

  • Original mixers used drew too much current, switched to analog mixer IC’s

  • Maximum value of hand capacitance measured at about 20 pF

  • Sensing range of antennae begins at about 2 inches from antenna

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Project Build and Testing, cont.

  • Oscillator pairs could not effectively be synched when no hand capacitance was applied

  • Tried potentiometers to do this, but it was still not precise enough

  • This problem creates a constant offset frequency which distorts the desired musical range

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Project Build and Testing, cont.

  • Constant offset frequency of about 4 kHz

  • Volume circuit could not be completed due to problems with the f-to-v converter

  • Tradeoff between sensitivity to hand capacitance and usable output to f-to-v converter

  • With the offset frequency, the volume control circuit will not work properly

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  • Basic volume circuit functional

  • Input to DSP in proper frequency range and magnitude for alteration

  • Zero-Crossing Detection detects difference in frequency down 0.1 Hz.

  • Dividing by number of samples per period

  • Running multiple instruments

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  • 8038 chips limited by a maximum frequency of about 250-300 kHz

  • Volume circuit not completely functional

  • Antennas not sensitive enough

  • Cannot properly synch the oscillator pairs

  • Running multiple instruments due to limitations in memory

  • Dividing by a number obtained at run time

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Other Possible Tests

  • Using passive components with greater precision would make the oscillators easier to synch together

  • Using different materials and shapes for the antennae could make a big difference

  • Using different method to generate frequencies could improve both synching and sensitivity

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  • Use a different frequency generation method

  • Design a circuit that will force the two frequencies to equal each other when no hand capacitance is sensed

  • Consider using an alternative sensing scheme, such as photodetectors