The heart of the VCDO hardware is a DSP microcontroller. This part integrates the processor, memory and peripherals required to sample the control voltages and compute the waveforms. Control voltages are buffered, scaled and summed with the front panel knob settings in the control voltage signal conditioning section. The DAC and output amplifier convert the digital waveform to an audio frequency voltage.
The VCDO signal processing consists of the following stages:
The sum of the various frequency controls is is still in exponential (V/Oct) form, so it must be converted to linear (V/Hz) form. Two operating ranges are supported, selectable by the High/Low front panel range switch. Low range is use for LFO operation and runs from 0.04Hz to 40Hz, while High range is best suited for audio frequencies and runs from 10Hz to 10KHz.
The linear frequency control drives a Numerically Controlled Oscillator (NCO) which generates a phase word used in subsequent processing stages. Rising edges of the Sync input will reset the NCO phase to 0, allowing the resulting waveform phase to be controlled.
FM synthesis is defined by the equation:
where A is called the 'Modulation Index' and B is called the 'Modulation to Carrier Ratio', or simply 'Modulation Ratio'. These two quantities control the harmonic content of the resulting sound. The quantity w(t) is the carrier frequency and represents the fundamental pitch of the oscillator.
In the VCDO's implementation, the Ratio CV is offset by the FM Ratio knob setting and quantized with hysteresis to 32 distinct modulation ratios. These ratios are in the range
[0, 0.25, 0.5, 0.75, 1.0, 1.25, ... 7.75]
Because these ratios are exact multiplies of 0.25, the modulation frequency will always be some multiple of 1/4 the carrier frequency and can be considered to be phase-locked to the carrier frequency. This prevents undesired variations in timber and provides a broad range of harmonic possibilies, with subharmonics of the main oscillator note sounding two octaves below it.
The Index CV is offset by the FM Index knob setting and scaled to create a modulation index in the range of 0 to 314%. This provides a smooth variation from the waveshape as defined by the wave table (see below) to one with increasing harmonic complexity.
The VCDO provides 16 different wave shapes, generally ordered by increasing harmonic content:
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