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
- Front End Linear VCA
- Switchable Four Stage WaveFolder
- TempCo Sawcore VCO
- Linear (AC/DC) & Expo (DC) FM
- Bipolar Attenuators for FM
- Calibrated 1V/Oct Input
- Hard Sync Input
- VCO Range ~.02Hz - >=400kHz
- Phase Locked Loop
- PLL Frequency Multiplier/Divider
- Variable Slew & Damping in PLL
- Switched Capacitor Filter
- Multimode Filter Output
- -12 & -24 dB/oct Output Taps
- Stable Self-Oscillation at -24dB/oct
- Voltage Control of Filter Resonance
- Single Stage WaveFolder on Outs
- 14HP, ~60mA Power Draw
- 100% Analog/Discrete/CMOS
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.
VCO Controls and I/O
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
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
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.
PLL Controls and I/O
The PLL takes the rectangle wave from the VCO and drives the
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
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.
Filter Controls and I/O
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
In Jack - Insert signal to be filtered
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
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.
Other Notable Things
The Synchrodyne is 14 HP
Current consumption is 60mA for the +12 rail; 50mA for the -12
The majority of the audio circuitry runs at +-5 volts and uses
high-speed rail to rail opamps.
The depth from the back of the panel is roughly 40mm.