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.