Modular Diary

Despite Hordijk’s warnings on the difficulties of implementing his Harmonic Oscillator algorithm digitally, I was keen to try out some of the ideas in Audulus. One of the aspects of the feedback loop involves using a cosine waveform to avoid the DC offset that would occur when feeding the sine back on itself. In the Hordijk’s diagrams he indicates a crossfade between the sine and cosine signals as part of the feedback loop, but I kept on running into the pitch drop problem that he explains is a result of the DC offset.1 I eventually gave up on the linear FM route and decided to try implementing it with phase modulation instead – with some success.

The morph from sine to saw has a different character than making a crossfade between the two waveforms since the saw edge gradually tilts rather than appearing as an abrupt vertical,2 and the sawtooth and square waves also have a rounder edge than the characteristic forms of these waves. I haven’t been able to achieve quite the same sharpness in the shapes that Hordijk does with his analogue implementation (compare the waveforms at the beginning of the second video) since the phase modulation begins to distort, but the somewhat more mellow quality that results also has a charm of its own. 3

I’ve put it together as a simplified µModule, with the addition of a control that adjusts the level of both the odd and all spectra simultaneously in relation to the sine.

uRM Drone Demo
audio version of this post

  1. There’s more from Hordijk on FM synthesis on the old Clavia Nord Modular website, fortunately still available via the Wayback Machine.  

  2. I’m also curious as to why the inverted form of the waveform appears to be slightly lower in pitch. (In the case of this oscillator shifting the “All” knob in the positive direction results in what is commonly know as a reverse (or inverse) sawtooth. This is the default result of the pitch being fed back on itself.)  

  3. It is possible to increase the definition of the square wave a little more than I have, but that results in distortion when combining the square and sawtooth spectra.  

Modular Diary

I’ve also put together a simplified, single input, micro version of the TwinPeak filter. The modulation input is set to only alter peak 2 by default, but this can easily be changed by opening up the hood and changing it to peak 1 or adding it to both.

The Hordijk Modular Blog suggests using multiple TwinPeak filters in order to achieve a complex filter with multiple resonant peaks, so I’ve tried something of that out in a little demo with the filters being triggered at sub-audio rates.

audio version of this post

Modular Diary – 072

I made a little more progress on my Audulus Blippoo today, with a pulse-width signal generated from a comparator between the two oscillators and, perhaps most importantly, a form of Twinpeak filter.

Once again I found it very useful to take a look at James Cigler’s demo and overview of the Epoch Modular TwinPeak (despite its somewhat shaky start).

The filter adds a lot of character to the chaotic core of the Blippoo, and its ingenious (again cross-modulating) design makes it a lot of fun to play with. I can imagine it being well worthwhile putting together a standalone version of it for use in other contexts.

A standalone Audulus (µ)module would require a little more thought into how the controls are linked up and presented – for the moment I’ve simply kept everything loosely connected since I’m still in a process of learning and discovering how the different parts affect the whole.

The next step is to implement the sample and hold.

Modular Diary – 071

With the Hordijk Putney-Revox patch in place I spent some time taking a fresh look at the schematics of the Blippoo, Benjolin, and Rungler circuits, slowly getting to know them a little better and understand the differences between them.

I also found James Cigler’s video tutorial on the Epoch Modular Benjolin useful in that it starts with a demonstration of the most straightforward aspects of the module before diving into the more chaotic modulations it is know for.

With that in mind I went back to my Putney-Revox patch, replacing the delays with Robert Syrett’s µRungler and starting to fill in the other connections. It’s a small beginning, I still need to add the S&H and comparator, and take a look at what could be done with the filter, but it’s fun to learn about what’s possible with the few elements that are in place, each step of the way.

Modular Diary – 068

I’ve taken another shot at a micro version of my Audulus scale bender. I decided to do away with the knobs and only provide the 1-volt-per-octave inputs on the front panel, along with a knob on the side (easily duplicated) that can be connected to the particular step(s) one would like to bend. The knob is calibrated so that one can set precise deviations by hand within a range of + or - 50 cents, with its output scaled to 1/o so that it can be connected directly to the inputs. One can still open up the patch and set ratios or fractions under the hood as well as see a numerical display of the precise deviations for each step. It feels a little more elegant than my previous attempt.

Modular Diary – 064

The unlabelled look of the ever growing Audulus µModular collection got me thinking of the early Serge modular synthesizers.

Originally, the module configuration for Serge systems could be selected by the user. …These were originally arranged by applying paper graphics to the metal panel… Early systems could have custom graphics—or no graphics—depending upon the whims of the artist.

Serge adopted a series of geometric designs denoting signal types, input, outputs, and triggers. Colored 4 mm sockets were used for most connections - blue, black, and red jacks for (unipolar) control voltages, bipolar signals (NOT necessarily AC coupled) and pulse/gate signals respectively, although these were not rigidly enforced. Later, other colors were introduced, e.g. yellow for triggers.

Ben Edwards (aka Benge) used one of these early ‘paper’ systems for his Works On Paper, the second track of which is a favourite among the modular pieces that I’ve come across during the last few months. Here are are some images of his system. And here some images of someone getting creative with theirs.

Modular Diary – 063

I finally got round to a first take on creating a micro version of my Audulus Scale Bender. The main idea was to have the set of ratio inputs readily available under the hood – that one could open up the module and quickly change the configuration should one want to.

Choosing what to include on the front panel and how to set it up in a way that’s both compact and clear turns out to be quite a challenge though and I’m not sure if I’ve found the most elegant solution, but at least it’s a start.

The general absence of text in the µModule UIs paradoxically means that there’s a need for some explanatory text along with (or inside) them. A clear set of general conventions for the collection helps balance that out though.