Synth Studies: WMD Synchrodyne

WMD Synchrodyne is one of the most unusual and experimental filters in Eurorack. It’s a switched-capacitor filter, which requires a high rate clock to determine the cutoff frequency. This clock comes from an internal VCO, through a PLL which can multiply or divide the frequency, and the VCO is normalled to the filter input. The module also features a VCA and optional Serge-style wavefolder on the audio input, and 2- and 4- pole filter outputs, each with a separate single-stage wavefolder output.

The main sonic characteristics that separate Synchrodyne from other filters are:

  • Aliasing at lower clock rates. This is not inherently a digital audio phenomenon, but happens any time you sample a continuous signal at discrete time intervals, which is what is happening here with the filter’s capacitors. The lower the clock rate, the lower the filter cutoff frequency but the lower the Nyquist limit as well.
  • Analog PLLs do not track perfectly. You can adjust tracking to find a balance between fast response but instability, and slower response but more stability. In general, higher multiplier settings, faster Tracking Speed settings and lower Damping settings result in more instability.

Filters are usually simple to use, but Synchrodyne can be challenging, especially when you don’t have a solid grasp on its behavior. I realized I needed to take the time to really study its behavior and make some notes to fully appreciate it. Patch ideas will be highlighted with yellow backgrounds.

Some basics:

  • The internal VCO range is .02Hz to over 400kHz, at least according to the manual (I can confirm it’s over 80kHz at least). You don’t actually need to use the PLL multiplier to achieve the highest possible cutoff frequency. The point of the PLL is (A) to allow the VCO to remain in a nice audible range while the filter clock is faster and (B) PLLs are fun.

The high range of the VCO (and/or the PLL output) can be used to clock other modules such as S&H, BBDs or other effects, or Xaoc Drezno. Adding some instability to Drezno’s clocking can introduce some nice lo-fi crackles and noise that I like quite a bit.

If you have a comparator capable of high enough rates, you can patch the VCO’s saw output through it, change the comparator level to produce different pulse widths, and use the result to control a VCA for a Class D VCA patch. It would probably also work well to control the cutoff of other filters.

Even without a comparator, using the VCO or PLL output to control a VCA can be fun if you FM the VCO. And then you could feed that result through Synchrodyne’s filter section.

  • The highest cutoff frequency for the filter seems to be about 3880Hz when the ratio is set to 100:1, or 7760Hz when set to 50:1 — even “wide open” it will roll off the high end.
  • Setting the ratio switch to 50 rather than 100 means, relative to the cutoff frequency, the Nyquist frequency will be an octave lower, thus there is more aliasing. At low cutoff frequencies aliasing is unavoidable.

To reduce aliasing you can combine control of the filter cutoff with the internal VCA and use it in LPG style, or you could use another filter serially after the Synchrodyne. But overall, the aliasing is simply a part of its character.

Generally, using an external audio source — leaving the internal VCO solely to control the filter — gives the most control and best results.

  • The input VCA has plenty of drive. With the folding switch disabled, monitoring the P2 output with the filter wide open, it will easily drive a sine wave into a square.
  • With the wavefolding switch engaged, the character sounds quite a bit like the analog wavefolder in Intellijel Shapeshifter (which is based on the Intellijel uFold 2, which I believe is based on Serge style wavefolders). But the output voltage seems to be a bit more restricted. It sounds smoother than the wavefolder in Mutable Instruments Blades, and I personally prefer its character over Buchla-style or Make Noise wavefolders. It can round a sawtooth to something more sine-like or emphasize harmonics above the fundamental.
  • The “WF” outputs are a single-stage asymmetric wavefolder after the filter. There is no control over the fold amount.
  • the 4P output is phase shifted 180 degrees from the 2P output.
  • the 4P output exhibits some resonance that isn’t noticeable on the 2P output even with the Resonance knob fully counterclockwise.

Mixing the 2P and 4P outputs in an external mixer can give a different sort of filter response/character and is definitely worth trying.

Crossfading between 2P and 4P outputs at audio rates is also satisfying.

  • With the Resonance knob fully clockwise, even attempting to add extra resonance through the CV input, if you monitor the 2P output, it may not reliably oscillate unless there is some input to excite it. The 4P output is more consistent.
  • When pinging the input with triggers, the 2P output works much like a typical filter does. The 4P output has more drive and sustain, even when the resonance and input level are fairly low.

Pinging the filter when the PLL rate is oscillating due to high Track Speed can result in some very lovely FM bell-like tones.

  • PLL tracking is at its least stable when the VCO frequency is low and the multipliers are high, and its most stable when the multipliers are at x1. (This can be observed on the PLL outputs as well as the cutoff frequency.)
  • Setting multipliers and divider to x1 do not bypass the PLL and may still be unstable.
  • The fastest Track Speed settings are always unstable and can devolve into noise. At about 3-4 o’clock it’s more likely to settle down, assuming non-extreme multiplier settings. Lower Track Speed settings result in slower response to changes in frequency.
  • The Damping knob has less influence on stability but can resist fluctuations in the frequency. Low settings generally work well enough.

Dialing in a bit of instability to taste can be a wonderful source of texture for drones. Different mult/div settings as well as the tracking controls, the wavefolder or VCA drive and resonance settings can lead to many different sweet spots. This is now the #1 reason I am keeping the filter in my rack.

You can also use very high settings of Track Speed combined with the multipliers to create new timbres, subharmonics and harmonies for the internal VCO that can vary with pitch.

A patch I really liked was giving V/OCT a pitch sequence (optionally also using an external VCO), using lowpass filter mode with fairly high resonance, and dialing in a good amount of aliasing and instability. Feed the result to other filters for both highpass and lowpass (or strong shelving EQ) and perhaps a touch of retro-sounding delay for a “ghostly” degraded sound.

  • I’ve found that plugging a dummy cable or a 0V DC source into the Influence input will lower the PLL output frequency. About +1.2VDC seems to be the equivalent of no cable plugged in; higher voltages will raise the PLL frequency and lower voltages will lower it, more or less according to the Influence knob. (Apparently this is because the Influence input is affected both by voltage and current; its knob isn’t a potentiometer but a rheostat.)
  • The Influence input seems less affected by Track Speed and Damping than the VCO but is not independent of them.
  • Audio rate signals work very well in the Influence input for filter FM effects (more so than FMing the VCO itself).
  • The pulse width at the PLL output is narrower with higher divider settings.
  • The pulse width at the phase delta output is affected more by the second multiplier knob than the first, and seems to be unaffected by the divider.

In more recently built Synchrodyne units, the CLK IN jack is not as described in the manual. The input signal is logical ORd with the output of the PLL before going to the folder, where the original function of the jack was to replace the VCO input to the PLL.

You can create formant effects by effectively using two clock rates. PWMing the clock signal here can create interesting blends of filter frequencies. Giving it a clock at the same or a multiple of the rate of an input oscillator, while modulating Synchrodyne’s VCO frequency or holding it steady, can produce some neat sounds.

You could also use the input to gate the filter to silence (holding it high blocks the PLL’s clock pulses).

For drone patches using an external VCO, mixing the internal VCO in at a low level with the filtered output can be nice.

Don’t forget the Sync input for the internal VCO as well — try syncing it to an external VCO being processed by the filter.