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Modular Diary

Here’s my take on an Audulus version of Rob Hordijk’s Triple LF-VCO.

The first of the three VCOs is pretty much the same as oscillator A in the Rungler, and so I used @stschoen’s Audulus recreation of it as a starting point. Oscillator A, taking the 3rd oscillator of the EMS Synthi as a point of inspiration, has a range from around a minute extending all the way up to 5 kHz, well into the audio range. Its special feature is a rounded triangle ‘parabol’ waveform (very close to a sine) and the fluctuation modulation1 that can be applied to it with LFO C as the modulation source. Fluctuation, a combination of frequency and amplitude modulation, makes for a gentler, more natural sounding vibrato than is possible with regular frequency modulation, and since the amplitude varies, opens up for interesting modulations when combined with a waveshaper.

LFO C is closer to a more standard LFO with a range from around 60 seconds to 100 Hz. Both oscillators A and C have bipolar audio outputs and this, in combination with the wide range of oscillator A, makes them useful in combination with the modulation inputs on Hordijk’s other modules, many of which can be used with bipolar signals well into the audio range.

In the version of the Triple LF-VCO demonstrated in both the NOVARS tutorial and Mallorca demo, LFO C has a selfmodulation feature in which the downward part of the triangle wave can be modulated with the pulse wave that runs parallel with it. This means that the upward slope maintains the set frequency while the downward slope can be slowed down allowing for the creation of an something approaching an inverted sawtooth wave. The pulse width of the accompanying pulse wave is coupled to the upward slope of the triangle, and maintains its width, while the time between the pulses (the downward slope) can be stretched to as much as half an hour! 2

Hordijk has apparently replaced this selfmodulation feature with a sample and hold circuit in his most recent version of the Triple LF-VCO. The S&H is triggered at both the upper and lower peaks of the triangle wave making it possible to create slopes that rise and fall at different rates, again with potentially large differences between them, as was possible with the selfmodulation version. In this case the modulation input for LFO C, i.e. the input for the S&H, is normalized to the output of LFO A, which can in turn be modulated by LFO C, allowing for complex cross-modulations.

LFO B differs in that it generates a unipolar signal, making it suitable for generating envelope-like modulations. Inspired by the modulation generator on the KORG MS-20 it has a range from around 10 seconds to 100 Hz, and its triangle wave can be skewed to gradually approach an inverted or regular sawtooth shape. As with LFO C the accompanying pulse wave follows the rising slope of the triangle wave, meaning that skewing the triangle also adjusts the width of the pulse wave. I used the skewed triangle LFO in the Audulus library as my starting point, but ended up having to rethink the equation for the triangle wave so that it syncs from the bottom point of the triangle rather than in the middle of the wave (as it does in the Audulus library/docs examples) – in this way both the transient of the inverted saw (when the triangle is skewed that way) and the pulse wave correspond to the beginning of a new (sync) cycle.

The syncing is an important aspect since LFO B can be synced via a switch with LFO C (which can in turn be synced externally). This makes it possible to easily generate irregular or shuffle-like rhythms. The switch can also be set to a ‘hold’ position so that on each downward slope of LFO C the pitch of LFO B is frozen until the point at which LFO C changes to an upward slope, at which point LFO B continues from the point at which it left off, making it possible to create interesting step-like modulations. Hordijk provides at good demonstration of the possibilities in his Mallorca demo as well as at the close of the NOVARS tutorial video.3

I’ve uploaded both the S&H and selfmodulation versions to the Audulus forum.

For schematics and further information see: thehordijkmodular.blogspot.com


  1. See also more on fluctuation on the Audulus old forum

  2. In the hardware version the LFO C pulse wave is normalized to the modulation input of LFO C – in my Audulus version this ‘internal’ connection needs to be re-connected manually if broken when connecting an external modulation source.)  

  3. The switch in my Audulus version doesn’t maintain its state when closing the patch, and so I’ve chose to have the free, ‘mode’ position at the top, followed by the sync and hold settings.)  

Modular Diary

I’ve taken a few extra steps with my Audulus version of the Hordijk OSC SYNC. As much as I’d enjoyed the wide range that came with not having any tracking between the syncing and synced oscillators, I realized of course that as soon as sequencing is introduced, tracking becomes desirable/necessary. Along with a tracking switch allowing selection between full, 2/3, or 1/3 (around ‘c.v.’ = zero as a reference point), I also added an external input for the sample and hold. 1

Using external modulation with the S&H allows for some interesting effects. I’ve uploaded two demo patches to the Audulus forum: In the first the TwinPeak filter is pinged with low pulses creating a resonant percussive sounds. It’s fun to explore the different timbral results that can be obtained by simply changing the tracking setting. The second patch creates an arpeggiator like effect – with this one it can be fun to turn up the modulation dial and/or add some subsquare modulation.

It’s also possible to achieve some granular-like effects with external modulation on the S&H – as Rob Hordijk demonstrates in this video.


  1. In the absence of normalized inputs in Audulus there’s also the output that needs to be connected to the S&H input when using the internal signal. 

Modular Diary

I’ve put together a full version of the Hordijk Sync Oscillator. As usual Hordijk has taken things a step further and turned the irregular transients that were smoothed out with the masking technique, to good use. The transient heights are captured with a sample and hold circuit, and can then be fed back to either the synced oscillator (which generates the well known random S&H pitch effect) or the syncing oscillator (which will change the rate at which the pitches are generated). The combination of the recursive pitch and tempo modulations means that Krell or Rungler-like textures (venturing into noise territory) can easily be generated.

Rob Hordijk provides a nice explanation of how the (true) randomness is generated in his NOVARS tutorial on the oscillator.

An additional aspect is the inclusion of a subsquare divider which can be used to modulate the synced oscillator on every alternate sync cycle. This can be used for the creation of vowel like effects, as well as adding a little bottom end to the synced sounds.

The sample and hold signal can also be applied to the subsquare modulation, and the sync sawtooth, subsquare, and S&H signals also have their own outputs.

I’ve uploaded two demo patches to the Audulus forum.

Modular Diary

I’ve put together a first tryout patch exploring Rob Hordijk’s sync oscillator, in particular the masking feature he describes in his NOVARS video tutorial on the module.

The first step in syncing the two oscillators was to invert the syncing sawtooth so that the synced waveform shares its transient with the sawtooth. If the two Audulus oscillator nodes are synced directly, the transient of the synced oscillator will appear in the middle of the syncing oscillator’s sawtooth. Since the synced oscillator is reset whenever the input crosses zero from below, the rising slope of the sawtooth resets the synced waveform when it (the sawtooth) reaches its zero midpoint. With the inverted sawtooth the slope crosses zero from above and doesn’t reset the oscillator until is crosses from below at the point of the transient.

I was particularly intrigued by the masking feature Hordijk describes in the video. In this case the inverted sawtooth is converted to a unipolar, positive only, waveform and used as an envelope applied to the synced oscillator. This means that the transient points of the synced waveform correspond to the zero point of the envelope and, importantly, are always the same size – effectively reducing that unwanted side-effect of the introduced transients.

Some more information on the masking technique can be found in one of Hordijk’s tutorials for the Nord Modular, as well as on his own site. As the Nord tutorial points out, the masking, while smoothing out the transients, also introduces a distortion of it’s own, and suggests crossfading between two versions of the synced waveform slightly out of phase with each other in order to minimize this side-effect.

In the case of this tryout I’ve kept things simple. I find the masking particularly effective in the case of the triangle wave – it’s interesting to compare it with a version in which the inverted envelope is replaced by setting the amplitude to full on the oscillator. With the sawtooth the mask side-effects are more noticeable, and since there are so many transients in the wave already I find it somewhat smoother to simply set a fixed amplitude for the synced sawtooth. Using a mask for the amplitude does however make for a smoother crossfade between the triangle and sawtooth waves and also allows for some interesting effects at low frequencies where the envelope is more apparent as an envelope.

I’ve uploaded the patch to the Audulus forum.

Modular Diary

I’ve put together a new version of the Hordijk Dual Envelope with updated internals – this time with envelope A placed on the right. Since the ADBDR modulation inputs will probably mostly be run via the S&H, it seemed more practical to avoid having the internal patching cross over the knobs too much. I found myself sometimes inadvertently attaching the S&H output to the knobs themselves as I attempted to detach it from the modulation inputs with the previous UI.

The patch is available on the Audulus forum.

Modular Diary

Here’s envelope B from the Hordijk Dual Envelope generator, updated as with envelope A, as a little standalone module.

The repeat mode makes for some interesting rhythmic possibilities: On its own, without an incoming gate but with modulation on the attack or release, irregularly fluctuating rhythms can easily be set up. Alternatively, when used in conjunction with incoming gate pulses, slight anticipations or ‘off’ beats that don’t exactly fit the clock grid can be created.

I’ve uploaded a demo patch to the Audulus forum.
Toggle the master clock in the patch to get an idea of how it works.

Modular Diary

It’s taken some time, but I’ve managed to get a little closer to the curves I was aiming for with my version of the Hordijk ADBDR envelope generator.

My approach has been to sample the value of the release or decay curve at the moment a new gate pulse arrives, and use that as the starting point for the next attack. (In trigger mode gate pulses arriving before the attack phase has completed are ignored.) In @stschoen’s original uLope modules this was efficiently achieved by using the addition node to add the attack to the release curve, with the disadvantage though of sudden jumps should a new attack arrive during one of the decay phases.

Sampling the value and crossfading between the first and last part of the envelope introduces some tricky timing issues (which I’ve solved by adding an extra timer for the sampled end/start values and introducing a very small delay before the attack phase timer kicks in). One the other hand, having a sampled start value for the attack not only makes it possible to have it pick up on the value of a potential preceding decay phase, but also scale the amount of time allocated to the attack according to the height available, i.e. the higher the attack point starts (depending on the level of the previous decay or release) the shorter the attack time. This also keeps attacks in quick succession that ‘build’ the overall volume from overshooting the top value of 1.

The result is an ADBDR envelope that can be re-triggered at any moment in gate mode, or any moment after the attack in trigger mode. In gate mode the release kicks in as the gate ends, while in trigger mode it’s independent.

Small glitches can still occur from time to time when pushing the envelopes to extremes and I’ve added a low pass filter in an attempt to smooth them out. Generally it seems to be usable though. The next step is to add build it into the Dual Envelope module.

The patch can be found on the Audulus forum.

Modular Diary

I’ve packed the modified uLope sub-stages into a module that more closely resembles the ADBDR envelope in Hordijk’s Dual Envelope generator.

One of the innovations in his design lies in replacing the sustain section with second decay stage that can slowly taper off, thus allowing for more ‘natural’ sounding contours. With the decay knob turned all the way up, a conventional flat sustain can be achieved. The second decay is also preceded by a break setting that adjusts how far the first decay falls. The resulting shapes are perhaps most easily understood by taking a look at the diagrams in Hordijk’s schematic.

Each stage also has a modulation input that adds to the values set by the knobs. Since modulation is most effective when the pitch of the modulating waveform is lower than the one being modulated, a sample & hold has been included. Hordijk provides a nice explanation of this technique in one of his NOVARS tutorial videos.

The patch can be found on the Audulus forum.

Modular Diary

I’ve made a further modification to Stephen Schoen’s uLope so that sustain modules can also be added to the envelope chain with the trigger mode activated.

Before I pack it into a module that more closely resembles Hordijk’s design, it’s fun to have all the controls available to experiment with. For example, by setting the first sustain module in the chain to a lower level than the one that follows, it’s possible to achieve a secondary attack-like phase with a resulting ‘reverse’ effect. It’s also interesting to play with the degree to which the curves are logarithmic or exponential, finding the sweet spots for what sounds ‘natural’ – at least in relation to what we know from acoustic instruments.

I’ve uploaded some patches to the Audulus forum.

Modular Diary

I’ve made a small start on putting together an Audulus version of the Hordijk Dual Envelope generator.

Hordijk mentions in one of his videos that designing an envelope generator is perhaps the most difficult of all the modules in that there are so many possibilities to consider, and that certainly rings true as I’ve begun to explore it.

In this first small patch I’ve adapted some of Stephen Schoen’s uLope modules to include a trigger mode – i.e. the attack time of the envelope is independent of the gate time. This means that a short trigger can result in a long attack swell, or a short attack can be triggered with a broad gate. It’s a simple feature that opens up a number of possibilities. With the attack time set to longer than the clock rate the envelope generator can suddenly starts to function as a kind of clock divider, as Hordijk points out in his NOVARS tutorial video. With in-between settings, e.g. with the gate setting in before the release stage has completed, interesting rhythmic effects can arise.

I’ve uploaded the patch to the Audulus forum.