Sometimes with a modular system made up of modules from different manufacturers, or when integrating with other external analogue gear, it is necessary to invert a signal to get your modules speaking to each other in the desired way. Other times you may want to invert a signal for some creative patch you are building. This is especially true of triggers because there is no standard to use a rising edge trigger versus a falling edge trigger and sometimes we can be very particular about which kind of edge we want.
The other day I wired up the trigger and CV outputs of a Yamaha CS-10 to respectively the trigger input of a Maths (being used as a simple AD envelope to open a VCA) and the 1V/Oct input of a Z3000 VCO. It didn’t take long to realize that when I was playing the keyboard the VCA wasn’t opening until I lifted my finger off the keys.
This told me two important things. First of all, clearly the CS-10 was not sending a trigger, rather (not surprisingly for a keyboard) it was sending a gate. Second, the gate signal it was sending was normally high and would go low when the key was depressed and then go back to high when the key was released. This meant that the rising edge needed by the Maths happened on the release of the key. This obviously would not do when the goal was to play the modular with a keyboard.
There are specific modules that will do inversion for you that would fix this in a snap but I don’t have one of those. What I do have is an Intellijel Plog which is a digital logic module that has two, 2 or 3 input boolean logic operation blocks, a toggle flip flop and a data flip flop. For our uses we need just one of the boolean blocks and we only need to be concerned with the 2-input logic because the third (Z) input for each boolean block is normalled to the second (Y) input unless it is patched
We want a logic function that will take our trigger signal (actually a gate in this case) in one input and when the other input remains a logic 0, (we are not plugging anything in to the other input so it will be zero) we want to get the opposite output. Looking at the truth table there are two operations that would give us this, NOR or XNOR though the NOR is a better choice because it will always work. If we used XNOR and for some reason the second input ever got a high signal on it (logic 1) then the XNOR wouldn’t flip the output correctly for us.
Here is the Plog wired up so that output A outputs the inverse of input X:Anyway, this solved my problem of needing an inverter when I didn’t have a dedicated module. I’m not sure what other logic modules are out there but any logic module that can do a NOR operation should be up to this task. Hope this is useful for some patchers out there.