Project Name: Theremix

Project Description:

The Theramix is a musical instrument played without any direct physical action by the instrumentalist. By moving one’s hand in proximity with the main antenna, the instrumentalist can generate musical tones. In the default configuration, the instrumentalist may also control the dynamic specifications, such as attack, sustain, and decay by moving their second hand near an ultrasonic sensor, with a stretch goal to replace the ultrasonic sensor with a second, looped antenna. The instrumentalist will also be able to apply various digital signal processing techniques to the instrument, including compression, saturation, and a three band equalizer. The final audio signal of the instrument will then be output at the instrument level to a ¼” and RCA audio output.

The main antenna of the instrument will act as one of the parallel plates in an LC oscillator, with the instrumentalist’s hand acting as the other plate. Thus, the instrumentalist completes the circuit, creating a variable rate oscillator with a bandwidth of 3kHz, depending on the position of the instrumentalist’s hand. The primary (pitch) antenna will be positioned on the right side of the instrument.

Upon the proximity of an instrumentalist’s hand, the instrument will generate the variable rate oscillation, which will then be combined with a fixed rate oscillator, generating a beat frequency (also known as a sub frequency) [8]. This beat frequency will then be converted to a digital signal by the microcontroller, and subjected to a number of digital signal processing effects [8].

The instrumentalist will control the musical dynamics of the instrument via the proximity of their hand to the body of the instrument, measured by an ultrasonic sensor. A stretch functionality is to incorporate the more traditional looped antenna instead.

The instrumentalist will be able to adjust the parameters of a built in compressor through various knobs, potentiometers, and sliders along the side of the instrument facing the instrumentalist. These adjustable parameters will include input gain, dry/wet, dB threshold, ratio, attack, release, and gain reduction. Drive, dry/wet, ratio, attack, release, and output gain will be controlled via knobs. Decibel threshold will be controllable via a slider.

Input gain adjusts the gain of the signal as it enters the compressor, allowing for greater control over the amount of compression present [8]. Input gain controls the gain of the signal before it is input to the compressor (allowing for heavier compression of a signal). Dry/wet controls the ratio of the signal being passed through the compressor and the amount of the original signal which passes through from 0 to 100%. Threshold controls the dB level at which compression starts. Ratio determines the level of compression when the signal exceeds the threshold from 1:1 to 18:1. Attack sets how long it takes to reach maximum compression once a signal exceeds the threshold from 0.01 ms to 1 s. Release sets how long it takes for compression to stop once the signal has fallen below the threshold from 1 ms to 3 s. Output gain refers to the amount of gain applied to the signal once it has passed through the compressor from -36 to 36 dB.

The instrumentalist will be able to adjust the parameters of a built in saturator through various knobs, potentiometers, and sliders along the side of the instrument facing the instrumentalist. The adjustable parameters will include dry/wet, drive, curve shaping, and output gain. Dry/wet, drive, and curve will all be controlled via potentiometers. Curve selection will be done on a gradient, adjusting the slope of a sigmoid function which maps input amplitude to output amplitude. Dry/wet adjusts the amount of the original signal present in the output of the saturator from 0-100%. Drive sets the decibel level at which the input signal will be clipped from -36 to 36 dB. Output gain will adjust the gain of the signal as it exits the saturator.

The instrument will be able to output the audible signal through both RCA and ¼” outputs. Both the ¼” output and RCA output will be at line level, meaning no pre-amplifier is needed, but an amplifier will be needed to drive passive speakers or headphones. Typical audio interfaces will be able to operate without an amplifier.

The instrumentalist will be able to view the status of the instrument via an LCD segment display, with a stretch goal of a graphical display. The LCD segment display will allow the instrumentalist to see power status, and the statuses of each of the adjustable parameters of the digital signal processing chain. A stretch goal will be to allow the instrumentalist to adjust the order of the digital signal processing chain (eg saturator -> compressor or compressor -> saturator).

As for the graphical display stretch goal, the graphical display would boast the ability to show more detailed information about each of the digital signal processing chain, such as graphical information about the shape of the compressor and saturator. The graphical display would also be able to display the waveform of the audio signal in real time allowing the instrumentalist to see the overtones being produced live.

Project Specific Design Requirements (PSDRs):

1. PSDR #1 (Hardware) The ability to create a variable rate oscillator circuit with a bandwidth of 3kHz through the variable capacitance between an antenna and the instrumentalist's hand.
2. PSDR #2 (Hardware): The ability to mix the variable rate oscillator signal with a fixed rate oscillator signal through a heterodyning mixer, resulting in an audible range signal.
3. PSDR #3 (Hardware): The ability to create a communication interface between the microcontroller and an LCD display using the SPI protocol, allowing the instrument to display the status of the saturator, compressor, and the instrument in general to the instrumentalist.
4. PSDR #4 (Software): The ability to apply digital signal processing techniques to the audible frequency signal created by the heterodyning mixer (PSDR #2), including compression and saturation, with the ability for the instrumentalist to adjust the DSP parameters in real time.
5. PSDR #5 (Software): The ability to sample analog signals from a frequency range equal to or greater than the bandwidth of the antenna signal and generate an output wave on the DAC output with equal frequency to the input wave.
6. Stretch PSDR #1 (Hardware): The ability to control the dynamics (musical volume) of the instrument via the variable capacitance formed by a human hand and a looped antenna, rather than the non-stretch functionality which uses an ultrasonic sensor array.
7. Stretch PSDR #2 (Software): The ability to display further detailed information about the status of the instrument on a more advanced OLED display, including a sample of the waveform, a visualizer of the saturator, and/or a visualizer of the compressor.