Introducing the WMD Synchrodyne, a multi part and multi use eurorack module for the creation and manipulation of audio. Containing several pieces to a traditional synthesizer voice, the Synchrodyne is a powerful addition to any subtractive oriented system. However, it is designed primarily as an experimental sound source/filter, intended to push the limits of modular synthesis...WMD style.
The Synchrodyne is designed as a way to implement a switched capacitor filter (SCF) chip without the use of a microcontroller to run the clock. We developed a fast analog VCO that can drive the SCF with a broad range of frequency control. The problem with this would be that the VCO is then dedicated to only running the filter. So we designed a frequency multiplier based around a Phase Locked Loop IC (PLL) to take the VCO and multiply its frequency up so that the VCO could be used as an audio source, while still controlling the filter. The SCF has a frequency ratio of 50:1 or 100:1 clock frequency to output cutoff frequency. The PLL allows for many ratios of input to output frequencies to be selected, making the VCO a simultaneous audio and control source.
The PLL introduced its own challenges. Taming the PLL to stabilize at high multiplication ratios required us to develop a damping circuit. The Damping and Track Speed controls cause the PLL to have its own sound, sort of a slew limiter and self Frequency Modulation. We wanted the Synchrodyne to distort nicely with audio input, and also to self-oscillate. We added single stage rectifiers to the -12dB and -24dB filter outputs for a more aggressive sound. We opted to develop a VCA and wavefolder for the front end of the filter to further expand the palate of distortion and processing of the Synchrodyne. We had more features to add, but space was limited and we didn't want to make an excessively expensive module, so we added two 14pin expansion headers for future development.
The Synchrodyne's VCO is a traditional Sawtooth Core. It uses very modern parts to achieve the stability and speed necessary for driving the Synchrodyne.
1V/Oct Input - Calibrated input for pitch control. The VCO will track at least 5 octaves using this input.
Sync Input - Rising edge sawtooth reset/sync input. Does not affect slope of sawtooth.
Coarse & Fine Knobs - Coarse knob adjust the frequency of the VCO throughout its full range. Use the Fine knob to tune the oscillator within approximately 1/3 octave range.
Exp FM - This input jack and bipolar attenuator provide exponential FM to the VCO. This input is DC coupled.
Lin FM - This input jack and bipolar attenuator provide linear FM to the VCO. This input can be DC or AC coupled. A jumper on the back enables DC coupling when in place. We recommend AC coupling to prevent frequency shift caused by the bipolar attenuator's offset voltage.
Saw Output - The Sawtooth output is available here. Its level is approximately +-4.5 volts.
Pulse Output - This output presents the rectangle wave necessary for driving the PLL. It is not a 50% duty cycle square, but approximately 30%/70% as that provided the best performance for driving PLL.
The PLL takes the rectangle wave from the VCO and drives the filter.
CLK Input - The Pulse Output from the VCO is normaled here. Plug in an external clock source to drive the PLL with something else.
Multiply & Divide Switch Knobs - These rotary switches adjust the frequency of the clock multiplier & divider inside the PLL. Adjust these to set the frequency ratio. Higher multiplcation factors cause the PLL to react slower to changes, especially at low input frequencies.
Phase Delta Output (ΦΔ) - This output is the result of the multiplication factor. It produces some self PWM as the PLL stabilizes and will equal the input clock frequency if the PLL is locked. This output provides an interesting self modulating audio or clock source.
PLL Output - This is the output of the PLL stage. It is the input frequency multiplied and divided by the set ratios.
Track Speed Knob - This controls the reaction speed of the PLL in trying to keep up with changes to the input frequency. Fully CCW it will introduce lots of glissando in the filter reaction. Fully CW and it will over-track, causing the PLL to respond faster than the input frequency. Overtracking produces bursts of high frequency output and low frequency output.
Damping Knob - This control reduces the rate of fluctuation while overtracking, damping the overtracking and increasing stability. More damping will be needed as higher ratios of Frequency Multiplication are selected.
Influence Input and Knob - This control acts directly with the Track Speed and Damping controls in stabilizing the frequency of the PLL. Use a voltage source to modulate the PLL signal. Very complex modulations are possible by using Influence with the internal VCO and other controls.
The core of the Synchrodyne is a 4 pole (-24dB/octave) Switched Capacitor Filter circuit. The 2 pole output is tapped and provides a mellower filter sound. The Synchrodyne will self-oscillate at high resonances on both outputs, however it is more prominent on the 4 Pole output.
The Switched Capacitor Filter topology has a few interesting features. It is a sampled data construct, so at low cutoff frequencies, the filter will produce aliasing noise, similar to the sample rate reduction on the Geiger Counter. This topology is also very stable and accurate, allowing for predictable cutoff frequencies to be selected. Across its entire range (.02 Hz to 16kHz) the filter will remain within +-0.2% of where it should be.
In Jack - Insert signal to be filtered here.
VCA CV Input and Knob - When there is no input to the VCA CV jack, the knob acts as an attenuator for the input signal. Plugging into the VCA CV jack causes the knob to act as an attenuator for the CV signal controlling the input VCA.
Fold Toggle - Flipping this up causes the input VCA to drive a four stage wavefolder. The output of the wavefolder then goes o the filter. The level of the input signal going into the filter from the wave-folder is adjustable with the blue trim-pot on the back of the Synchrodyne. This will be broken out to the expansion.
Resonance Knob - Directly Controls the resonance of the Switched Capacitor Filter.
Resonance CV Input and Attenuator - This input will add to the primary Resonance knob through the attenuator. Use for animating the Filter's resonance.
100:1 - 50:1 Ratio Toggle - This controls the filter core's frequency ratio. There is one octave of difference between the two settings. At low frequencies, the 50:1 ratio will produce more aliasing/sampling distortion.
LowPass / BandPass / HighPass Toggle - Selects the output mode of both the 2 Pole and 4 Pole outputs.
Filter Outputs - Jacks for the 2 Pole (-12dB/oct) and 4 Pole (-24dB/oct) are available. Please note that they are 180 degrees out of phase from each other. Additionally, a single stage wave-folded output is available for both the 2 Pole and 4 Pole outputs. The wave-folded outputs provide another level of sonic depth to experiment with.
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