A mysterious forest wizard shot an arrow into a nearby tree trunk with a message wrapped around the shaft. It said "Wouldst thou deign to design a PCB for an exotic precious gem of a microchyppe which one canst obtaineth not except by exxtraction from a Casio FZ-1 or symilar keyboarde?" so I stuffed a message in a bottle saying "heck yes" and threw it on the water.
I had never heard of this chip or the fact that you can build it into a Eurorack module, but it's quite a unique thing. The chip is a "digitally" controlled filter and amplifier, which garnered some praise back when it was new for sounding analog. Turns out it IS more-or-less analog, with a switched-capacitor filter and hmm, I'm not sure how the audio amplifier is managed.
What is a switched-capacitor filter?????? Glad you asked. A filter is almost always a capacitor-resistor situation. A resistor reduces how much current can flow, and a capacitor can be thought of as a kind of balloon which "fills up" when a current flows into or out of it. Most synthesizers vary the resistance (sort of) to adjust the frequency at which a filter takes effect. You can do this with a potentiometer, which you need to turn with your hand like a cave man (ugh) or you can use a voltage-controlled resistive part like a Vactrol or an OTA which stands for operational transconductance amplifier obviously but just understand that it's MUCH easier to use a voltage to adjust the "resistance" section of a resistor/capacitor filter than adjusting the capacitor.
Well! It is possible, turns out, to make an adjustable capacitor except not exactly. You can use a microchip called an analog switch (CD4069 is a legacy microchip that does this) to take little microsecond chunks of a signal and allow it to be exposed to a capacitor for tiny snippets of time. Make the "on" time longer, compared to the "off" time, and you'll get more filtering effect, and vice-versa. You need the switching speed to be fast, at least a couple times higher than the highest frequency you'll be working with, and the resulting signal will be quite choppy, with the on-off switching creating jumps in the signal. The Casio chip from this keyboard has a clock of 3.58MHz, and it's doing four different things to the incoming signals, but even if the switching is divided among the four tasks, that still 879KHz switching speed. I think that's enough for our 20KHz frequency ceiling.
Fortunately, somebody more clever than I figured out how to address this microchip digitally, and wrote code for a microcontroller so we can tell the chip what to do with the knob positions and incoming control voltage signals. I'm adding some JuanitoPots (my clear-shafted potentiometers) (which Tayda kinda sells an inferior version of if you need some and can't get hold of me), I fixed a possible problem with one of the op amps, I used the "either way" power connection, and used careful grounding and power managing techniques to keep possible microcontroller noise out of the audio section of the module.
So anyway, I'm designing a PCB for this exotic unobtanium chip, and the minimum order quantity is FIVE. And each keyboard contains FOUR of the chips. So there will be one extra PCB and panel if you have a broken keyboard that has one of these precious gems?
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