I spent the bits of time I had today further exploring the details of STS’s Audulus Rungler. The next step was to take his new shift register design and implement it in the Audulus Blippoo I’ve been putting together. I was wondering how to best implement the looping feature of the Rungler (even though it’s not actually featured in the Blippoo) when I noticed that STS had posted his take on recreating the Benjolin – in which he implements that feature beautifully!

Audulus helpfully pointed out to me that using the Audulus z-1 UnitDelay node in my Blippoo FM feedback loops would help improve the sound by processing them at audio rate. I first came across this node in the fascinating thread on non-linear filters and perhaps for that reason it stuck in my mind as a filter thing. The tip on using the UnitDelay was a good reminder that the Hordijk cicuits make good use of the blurred line between both oscillators and filters as (potentially chaotic) feedback systems.

“A chaotic signal is not a random signal.” I got round to taking a look at Rob Hordijk’s recent Rungler Demo at Modular Meets Leeds 2017, a nice complement to the Mallorca demonstration from 2012.

One detail that I found very interesting was his demonstration of the patterns that can appear within a chaotic signal – the bifurcations or period doublings that can create harmonic partials that are lower in frequency than the signal fed into the filter.¹ I’d noticed something like this in James Cigler’s TwinPeak demo yesterday, and had some fun trying it out myself with the filters in my Audulus Blippoo patch.

In essence, since a filter is a feedback system, the moment you apply non-linear feedback, the system automatically becomes potentially chaotic.

I also managed to implement a sample & hold in the Blippoo patch. Hordijk’s inclusion of a S&H in the design is interesting since the Rungler modulations already have something of a S&H character. I take it that including it had something to do with his observation that frequency modulation has a stronger effect when the modulating signal is lower in frequency than the one it is modulating. Using a S&H provided him with a useful device for effectively modulating low frequency periods such as envelopes, for example. In the Blippoo context the S&H definitely seems to add to the chaotic character of the patch rather than simply adding an extra FM modulation band when the modulating oscillator is higher in frequency.