About
FN BLOCK is pretty much the only module in the CrapRack system that is able to provide smooth voltages and not squarewaves or stepped waveforms. Its design is also pretty straightforward, but it gets way more complex than the previous module of this system, OSC CORE, is. FN BLOCK consists of 3 sections: a dual slope generator (called FUNCTION on the faceplate), a voltage comparator and a vactrol slew limiter. They all work independently and provide different functions. The function (or slope generator) is simple and non-VC'able, but is still a load of fun to play - can be an envelope, an LFO, a frequency divider, a lot of other things really. The voltage comparator is designed to compare the two functions and derive some polyrhythmic stuff out of them to use with functions' own ceil/floor outputs, but really is a very general processor which just tells if one voltage is higher than the other or not. The slew features a switch to change from audio range to subaudio range, and it acts like a VCF-sorta thing for audio and a slew limiter sorta thing for CVs.
Schematic
There's not much to describe about the comparator and the slew limiter. The comparator is just an op-amp with a passively summed pot offset and external input at the + input and a pull-down'd input on the - input. The slew is just a classic "buffer - passive LPF with a vactrol instead of a resistor - another buffer" combo.
The functions though deserve some attention: first there's the rise input with a selector between continuous or spike input. Thing is, the CD4013's SET input has more priority than the RESET input, so while there's a logic high on SET, the RESET won't trigger. So i added a possibility to convert RISE input (the one triggering SET of 4013) to a short spike by passing it through a capacitor with some add'l stuff around it to form a HPF "decaying" spike, and then comparing it with some positive voltage to form a rectangular out of it. Then, the heart of the function is what you can find on the internet by a "simple attack-release envelope circuit" kind of a request, basically it's a slew limiter with separate rise and fall controls. The 4013's Q out is passed through it, so when you trigger SET, the output of this slew limiter slowly glides up to +8 instead of jumping right up like the 4013 Q output. Same with RESET and gliding down to 0. The time of gliding up and down can be adjusted with separate potentiometers, but not CV'd. Finally, there's a ceil and floor comparator section, which compares the slew limiter's output with preset 'edge' voltages. If the output is at its highest voltage (for example, a high voltage was passed to SET, the rise time was set to minimal, and nothing else is happening), the ceil comparator outputs a high voltage. Instead, if the output is at its lowest voltage, the floor comparator outputs a high voltage. That is handy for determining what should happen if the function finished rising or falling: you can patch the ceil output to fall input, and use the rise input as a gate input - you now have an attack-decay envelope! Patch ceil to fall and floor to rise - and it will start telling itself to rise when it's at lowest and fall when it's at highest - you get a variable shape LFO. For those ceil and floor outputs are very handy, the trimm potentiometers handling them should be trimmed very carefully. Turn on the unit and make sure the function is at 0 (input a high voltage to fall and wait a bit), then turn the floor trimmer JUST until the floor output lights up. Same with ceil trimmer, but you have to bring the function's output to the top - just input a high voltage to rise input and also wait for a few seconds. (NB! by 'high voltage' here and everywhere else logic related i mean the logic high voltage, which in this system's case is +8v. Please don't input actual high voltages like 220VAC to the system.)
You can get a more in-depth info on this module from the video down below - i was making a show called SYNTHCRAP about constructing this system, but it kinda got abandoned for reasons.
