Dechreuad: A technical breakdown

     A sawtooth core DCO with no DAC - How does it work?

    The original synths that used this quirky oscillator core are both from Roland, the Jupiter 4 and ProMars (for some reason, Sound On Sound claim that the SPV355 rackmount guitar synth also uses this architecture, but a look at the schematic disproves this).

Roland ProMars/JP-4 VCO operating principles. (Source: Roland ProMars service notes)

    The above block diagram gives a rough overview of how the VCO actually works, but it's a little obtuse, so I'll break it down.

    The timing pulses that are presented to the oscillator core need to meet two conditions for the circuit to work properly - They must be of fixed pulse width, independent of input frequency, and must be a power of two higher than the target frequency.

    The bottom half of the diagram illustrates how this conditioned clock is transformed into a sawtooth. The fixed pulse width clock is fed into a simple passive frequency-to-voltage converter, essentially little more than a lowpass filter, and the resultant voltage is used to charge an op-amp integrator. As frequency increases, charging rate increases, and vice-versa. Simultaneously, the clock is divided by the same power of two that the timing pulse frequency is multiplied by, thus recovering the target frequency. This is used to discharge the integrator, and due to the fact that the charging voltage is proportional to frequency, the resultant sawtooth amplitude is constant regardless of input frequency. Roland specifies a 4052 dual four-to-one multiplexer before the integrator and reset circuit, to allow for a front panel switch that selects between four octaves. This works by changing both the frequency divisor and resistor value before the integrator in tandem.

    Of course, this system is not without its flaws - The waveform starts to deviate away from a sawtooth at low frequencies, which the frequency-to-voltage converter cannot utilise properly, and at high frequencies the waveform collapses in volume as the fixed pulse width approaches the frequency at which it is re-triggered. But as long as you keep the frequency in the 'Goldilocks zone', all is well and the oscillator performs admirably.

    The below diagram illustrates the changes made to this scheme to digitise the frequency generation for use in Dechreuad.

Dechreuad frequency generation and oscillator core block diagram.

    This is, of course, highly simplified. In order to increase the oscillator's range as much as possible, the integrator trigger divider and charge rate will be dynamically switched, and a lot of other necessary circuitry and tweaks have been omitted. But the central thesis is there, and so far it works fantastically on breadboard.

    For testing purposes so far, the CPU has been a Parallax BASIC Stamp 2p24, but this will be substituted in due course for a microcontroller or microprocessor more suitable for integrating into an actual product. Currently I'm experimenting with a simple 8051-based chip, programmed in assembly.

    This is a much more involved design process than Pas-Isel ever was, so I shall try and post more intermediate updates as and when I can. I'm looking forward to seeing where this module takes me, and to sharing more about it in the near future.

Introducing Dechreuad: A minimally digital DCO

     I'm very glad to report that Pas-Isel launched to great success back in February, and has provided a solid foundation for Dwyfor Tech to flourish from! But to do that, we need some new products to bring out. Two are currently in the works. Cryfder was intended to be the first of these, a dual VCA with a mixer/distortion section based around a vacuum fluorescent indicator lamp coaxed into audio valve duties, but is currently undergoing a fundamental re-design due to my unhappiness with the first physical prototype. It's just not there yet.

    What's currently showing a lot more promise on both breadboard and LTspice is Dechreuad. The name means 'the beginning' in Welsh, as since it's an oscillator, it's the start of your signal chain. This module comes from discussions I've been having with some like-minded individuals about creating a Juno-esque polysynth that uses Pas-Isel as the filters. I liked the DCO design I came up with for it so much I'm going to release it as a standalone module before anything else. The fundamental crux of Dechreuad is that it's intended to be a 'minimally digital' DCO, insofar as the CPU is doing as little of the task as possible. As much of the core duties of the oscillator as reasonably can be will be offloaded to standalone CMOS ICs and analogue circuitry.

    Perhaps the most unique aspect of it is that the analogue portion of the circuit creates a very close approximation of a sawtooth using nothing but a clock timing pulse, with no DAC, no digital stepping and using little more than a 555 timer, an op-amp integrator, a frequency divider and a handful of assorted discrete/passive components. This particular oscillator core is exceptionally rare, despite having been used in a Roland Jupiter-series synth, and even with the inherent and obvious design advantage in pairing digital timing circuitry with an oscillator core that requires only a timing pulse to generate a sawtooth, it has only ever been applied to a VCO and never a DCO, as far as I'm aware.

    In my eternal quest to not make straightforward clones or replicate what others have done before me, I intend to not only DCO-ify a previously VCO-only core, but add waveshaping and suboctave circuitry of my own unique design, to make a truly idiosyncratic, yet flexible and characterful oscillator.

    In my next blog post, I'll go into the technical details of Dechreuad and explain how the oscillator core manages to create analogue waveforms from digital timing pulses.

Pas-Isel: A technical breakdown

    So now we've established why I designed Pas-Isel, here's how it happened.

    We begin, as all good things do, with the timeless patent for the transistor ladder filter by Dr. Bob Moog. However, we look at the not-so-legendary half - the horizonal highpass.

Bob Moog's highpass, from the seminal 1969 transistor ladder patent. (Source: Google Patents)

    This is indeed a highpass, not a lowpass... But that's easily changed. Simply put the highpass in a negative feedback loop! This trick was used by Don Buchla in his 100 series modules, with the 191 Sharp Cutoff Filter. Each filter pole of the Moog design, consisting of a capacitor, a pair of transistors and buffering amplifier, is put into a negative feedback loop, for a total of four stages of inverted single-pole highpass filters, resulting in a four-pole lowpass response at the end.

    A breakdown of the Buchla 191's operating principle, written and illustrated by the brilliant Osamu Hoshuyama, can be found here. 

    The Buchla 191 is a bit of a complicated and unconventional beast, though - It's highly discrete, incorporating differential transistor pairs for the negative feedback sections and JFETs for buffering, and as a result needs a lot of trimpots to get it to work. It's also non-resonant, although in recent years Verbos Electronics made a resonant (but not self-oscillating) version with their equally discrete take on it, the Verbos Dual Four Pole.

    Buchla is not the only company to try and take on the inverted highpass approach to making a lowpass filter. The Hammond Organ Co. patented a filter that used the principle, utilising a quad JFET array as the voltage control element, and went on to put it in their catchily-named 102100 and 102200 preset monosynths. It's a minimal parts count take on the idea, putting the entire four-pole highpass in a single op-amp negative feedback loop and removing the ability to adjust resonance.

Hammond's take on the inverted highpass lowpass. (Source: Google Patents)

    Pas-Isel takes the best of both the Buchla and Hammond designs, with a few twists of our own. We took the independent buffering/feedback per pole of the Buchla, the use of monolithic IC op-amps as buffering/feedback elements of the Hammond, and as a nod to previous attempts to get around Moog's patents, diodes as the voltage control elements. The control voltage processing circuitry is also derived from the diode ring CV driving circuit of the Korg MS50 filter, and the resonance feedback loop incorporates soft-clipping diodes like most classic Korg MS-series filters.

    Pas-Isel cleanly self-oscillates at maximum resonance. At attenuated input gain, it behaves politely and transparently like an 80s Roland filter, yet livens right up into an ARP-like bite at stronger input levels. Altogether, it's a very different beast from any VCF we've heard before. We hope you'll like using it too!

Introducing Pas-Isel: A classic, done in a new way

     Everyone knows and loves the voltage-controlled resonant lowpass filter. It's the great staple of popular electronic music, from the dawn of the Moog modular to the present. It's certainly the component of a synth that gets the most love.

    It's pretty easy to understand why - The filter is the 'heart and soul' of a synth more than any other part. It's what imparts a particular synth's 'colour' onto the sound. There's a reason nobody's celebrating the Minimoog's envelope generator or the MS20's LFOs quite as much as their filters.

    However, looking at the synth market as it currently stands, there seems to be maybe only a handful of distinct VCF cores that get recycled a lot, usually derived from the original giants of the industry back in the 60s and 70s. Most 'new' VCFs are still variations on classic OTA-based designs, transistor ladders, Sallen-Key topologies, or other tried and tested technologies. Either that, or the innovation happened decades ago (such as the EDP Wasp or Polivoks filters), and have only recently been brought to the attention of the synth community. With so much innovation going on particularly in the Eurorack scene, why has there been relatively little by way of filter cores?

    Well, that's why I came up with Pas-Isel.

    Pas-Isel (Welsh for 'low-pass') is my attempt at creating a filter core that isn't clone of a classic. My intention was to implement a classic lowpass VCF in a way that hadn't quite been done before, experimenting with underused, forgotten and obscure filter topology, and hopefully achieving a whole new character of sound in the process. I think I managed it!

    Being my first ever synth module, I've opted to keep it simple - No voltage-controlled resonance, no resonance insert loop, no pole mixing... That's what future revisions are for. For now, it's a straightforward four-pole lowpass, that will give a life and colour to your signal like no other filter can.

    In my next blog post, I'll go into the technical details of how and why it works, and how the topology of the filter is derived.