Crazy days. The Big Last-Minute Update at work required about three more layers of updates, but we’re getting through it. (Update ADOS so we can update the build agents (requiring new VMs with updated OS) so we can update so they support VSTest 2022 so we can update everyone to Visual Studio 2022 and Fortran 2024 so we can update the three critical third-party libraries, which required writing some code to cover for their mutual incompatibilities, so we can pass a cybersecurity certification.)

The plumbing stuff at my parents’ new home is done except for final inspection and we’ve been doing some final fixes and cleaning. We’re going down this weekend in a rental SUV to pick up their cats and some stuff.

Last weekend though I managed to record 10 more minutes for the next album, putting it at one continuous 30-minute mix. Since it’s an album rather than a live set, I’m gonna keep going… though probably will switch to a “side B” as I did for Parallax. As a thank-you gift for the house work, my parents sent me a small steel tongue drum, like a mini version of my HAPI Origin. This sounds pretty great through Wingie 2 (I heard you like resonators, so I put a resonator in your resonator…)

Scottish Eurorack company Instrūo recently released a dual allpass filter, called DAPF (the only non-Gaelic module name they have). DivKid did a great demo video of it, and I was convinced. It’s 4HP, the size of the last remaining gap in my case. Perfect.

Allpass filters are a bit weird. If you have a basic concept of lowpass, highpass, bandpass filters, the idea of a “filter” that doesn’t filter anything sounds like a joke. But a side effect of those other filters is a phase shift around the cutoff frequency, and indeed, an allpass filter does the phase shift without changing amplitudes. What that means is, if you put a particular waveshape into it, you get a different shape out that sounds exactly the same, having the same harmonic content, just shifted ever so slightly in time according to its center frequency. This turns out to be a surprisingly useful tool:

  • mixing the affected and unaffected signal in various ways creates other filter types.
  • moving the filter’s center frequency induces phase shifts; do it at audio rate and it sounds like FM synthesis.
  • move that center frequency more slowly, and with a feedback loop through the allpass filter (typically with several APFs in series) and you’ve got a phaser effect.
  • altering the waveshape means that nonlinear waveshaping (e.g. distortion, wavefolding) and analog logic operations/ring modulation/etc. will affect the signal differently. This allows you to fold a “square wave” (since it’s not square anymore, it just sounds like one). Shifting that center frequency changes the waveshaping in interesting ways.
  • allpass filters somewhat resemble the diffusion of sound waves in physical spaces. Digital reverbs tend to use combinations of delays, allpass filters and comb filters to do their thing. While you’d need an armload of DAPF modules to patch a reverb this way, putting it into feedback loops of various kinds does have neat results.

I found a particularly cool, unexpected application I found for it last night. Running a unipolar envelope through it, while also patching that envelope to the CV input, lets you get a bit of bipolar ripple/wobble. This reminded me of New Systems Instruments Inertia… so I added a feedback path through the allpass and got resonant wobbles like Inertia. Maybe this is how that module is implemented.

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