This project has been my focus for design and building since January, 2013. The Quantisise (a composite name of quantize and six) will be a self-contained modular synthesizer designed for live performance. The core consists of six identical modular blocks comprised of a VCO, VCA and two Sample and Holds. Originally I conceived this as a collection of four of blocks or sections, which I called Quartussy. I expanded the number to six and added a VCA to each. Here’s the block diagram of one section, which I will describe.
The architecture is based on the Quantussy section of the Ciat-Lonbarde Cocoquantus, which has five sections, each also having a VCO, VCA, and two Sample and Holds. If you first get a grasp of the Quantussy, you’ll catch on to this design much faster. While the architecture is similar, mine differs a lot from the Quantussy. I wanted to connect the sections by patching, instead of electronic switching. So, the Quantisise design separates the six blocks, with the exception of a common control over the VCA level. Let me go over the above block diagram in detail.
The VCO
The VCO produces a triangle wave in the region of about -0.5 to +4.5 volts with an extremely wide frequency range from very, very slow up to about 3 Khz. Each VCO has its own panel knob for initial frequency. Its triangle output is available on a jack. Things start to get interesting when we consider the control of frequency (i.e. FM). Each VCO has an attenuator knob to set the final FM level. If the level is zero, then the VCO is unaffected by any goings on in the other parts of the block. The source for FM comes internally and is selectable between the direct output of the first S&H and the level-controlled output that same S&H, going through the VCA. (One reason to want to bypass the VCA is to exclude this block from being impacted by the global ‘chaos’ knob. But I’m getting ahead of myself.) The main point here is that FM for the VCO comes via a Sample & Hold. Let’s look at that.
FM is via a Sample and Hold
The first S&H takes an external input, optionally sampling it at the rate of the VCO in the block, and passes this sample over to the FM section of its own VCO. A three-way toggle switch affects the sample. When in the center, the S&H is in tracking (i.e. following) mode. It just follows its input. The maximum slew rate at which it can follow is the same rate as the maximum VCO frequency, so it tracks pretty fast. In this mode, with the VCA bypassed, the CV is effectively passed right to the VCO. Or, with the VCA in the loop, the FM level can be dynamically controlled. The other two S&H modes are Track/Hold and Sample/Hold. Track/Hold is done by clocking the Hold input with the square wave of the VCO (labelled ‘Cycle’). The CV will be tracked for one half of the cycle and then held for the other half of the cycle. In Sample/Hold mode, a one-shot causes the CV to be sampled and held twice during one clock cycle of the VCO. This mode imitates what happens in the Quantussy. (Quantussy also has a Track/Hold mode, which is indicated by the blue LED in its lozenge. But Quantussy does not have a pure following mode.)
The VCA
It takes the S&H output and applies a multiplication factor from zero to one (unity gain), based on the sum of the initial setting and its own CV input. The initial setting I called ‘Master Chaos’ after the similar function of the Quantussy. One master knob controls the initial gain level of all six Quantisise sections. But, unlike the Quantussy, each VCA also has its own CV input, allowing independent FM depth control. And the VCA output is available on a jack for patching.
The Castle S&H
Finally, there’s a second S&H, with a three-way mode switch, also clocked by the VCO. This one has a patchable output. I call this the ‘Castle’ S&H, to use Quantussy terminology. A difference from Quantussy is that the Castle input is patchable instead of being internally selected.
How to patch?
Patching will need another post. A Quantussy emulation patch is straightforward, using five sections. But, due to the additional patch points, Quantisise will have more flexibility. A Fourses emulation patch can also be done, by patching the VCO outs to the CV ins in a loop and driving the VCA CV inputs from some combination of outputs.
What can I do with it that I can’t do already with Cocoquantus?
I was asked this question and had to think a while before answering. Functionally, the feedback complexity is similar, and I expect Quantisise to be capable of very interesting, self-evolving patches. The ‘playability’ will differ in that I’ll have a lot more manual controls. I also believe that, due to the different circuit implementation, it will have a different character. For example, the way the VCOs drift.
There’s more…
Quantisise will include four wave shaping modules to liven the timbre of the triangle waves. More on those in a forthcoming post. It will also contain a four-channel stereo output mixer.
