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The tabulaRasa is a digital wavetable oscillator with voltage control of frequency, waveform selection, and the amount of interpolation between waveforms. The tabulaRasa consists of two components: a hardware synthesis module which can integrate into a modular synth system, and a software application which allows the user to create their own custom waveforms. These waveforms are then transferred from the computer to the tabulaRasa module through a standard SD card slot.

For more information, images, and audio samples, visit http://www.gregsurges.com/circuitry/tabularasa/

A wavetable oscillator is a standard technique in computer music. The oscillator uses a counter (ramp wave) to read a memory array. The array is filled with pre-computed samples of the desired waveform. The memory index being read increases with the counter, and resets when it reaches the end of the array.

The tabulaRasa hardware module has three potentiometers and three control voltage inputs for each of the control parameters (frequency, waveform selection, and interpolation). The control voltage inputs expect a voltage range of +/- 5V, and the board requires a +/- 12V power supply. The potentiometer for each setting selects a midpoint from which the corresponding voltage input diverges.

The tabulaRasa software application allows users to create waveforms in three distinct ways: through manipulating breakpoints to set sample amplitudes, adjusting the strength of various harmonics, or by loading audio samples.

The first method, manipulation of breakpoints, allows the user to adjust 16 breakpoints of a waveform. Each breakpoint is represented by a slider, which can be raised or lowered with the mouse. The points between those breakpoints are interpolated, and there are six user-selectable interpolation types: linear, sigmoid, cosine, circular, exponential, and “Decimated" (a simulated bit reduction algorithm).

The harmonic method uses the same sliders as the breakpoint method, but instead uses the sliders to represent the amplitudes of 16 harmonically-related sine waves. From left to right, the sliders climb the harmonic series, and a harmonic will become more prominent as its slider is raised from the bottom of the screen. The waveform which results from these mixed waveforms is displayed.

Finally, using the “Load Sample” option, the user is presented with a file dialog box from which a short audio file can be selected. The Adventure Kid Waveform pack works perfectly, and is made up of 4300 free single-cycle waveforms.

The vast majority of the design and prototyping stages of this project are completed, but we need to raise funds to produce and assemble the boards on a larger scale.