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[Gordonjcp]

Yeah, you don't use a resistor ladder with a matrix keyboard.  You use a microcontroller to scan the keys, then generate the control voltages with a DAC hooked to the microcontroller.  I suspect - but haven't tried it - that the PWM output of an AVR or similar MCU might be suitable for filtering and turning into a VCO control voltage.

[jamiewoody]

i want the keyboard to be a short one...one octave with one note below and one note above...i think that would make 14 keys....C to C (i think...).

i want to make a few of these. i have bass pedals, which i will build a box big enough to add things, yet thinner than traditional "taurus" pedals.

there are these momentary n/o toggle switches i have. i bought them before i knew what "momentary" was...lol! or the description on bg micro was probably poor...anyhow, the aluminum keys are annodized silver and other switches are more of a warm gray. this would make for a cool "black and white keys" layout! ;-)

i also want to make another with a real keyset.

[Gordonjcp]

Righty-ho, grab the Moog Taurus circuit from retrosound.de and make a cup of tea.

Got tea?  Got circuit?  Good.

Start at the top left.  Notice how everything is labelled with what it is?  I'm going to attempt to explain how it does it.

The "Oct Divide Chain" is a set of very precisely-matched resistors wired from 15V to ground - 12 in total (they are probably multi-pinned "resistor packs" like you used to get for terminating disk drive chains.  Anyone who has never used Windows 3/Linux kernel 2.0 will be too young to remember them ;-)

Okay, so 15V at the top, 0V at the bottom, first tap is 2/3 of the way in (four up, eight down) - this gives us 10V.  The next one is tapped 1/3 of the way up giving 5V, one at 1/6 up giving 2.5V and one at 1/12 up giving 1.25V.  Don't take my word for it, use Ohm's Law and work it out for yourself.  Step away from the computer, pick up a notepad and pencil, and go do some arithmetic.  Oh, and make another cup of tea.  Mine's black, no sugar.

Right, we've got our precise reference voltage to be fed to the resistor ladder.  The 4016 is a quad analogue switch, which selects which reference voltage we choose.  The opamp forms a little buffer amplifier so that the reference voltage isn't "pulled" by the load when you press keys.  What's the oscillator thing below it?  Well, that injects a tone onto the control voltage, which is detected by the amplifier and filter just to the right of the keyboard assembly.  It's far too high a frequency to affect the control voltage for the oscillator (and indeed it's easy to filter off) but it's possible to detect it at a low level and use that to generate the note on/off trigger.

I can't find any information on what the values of the keyboard voltage dividers are.

[jamiewoody]

when i build a keyboard, i would like to add filters (resonance, envelope, etc...).

how do those hook together? should i plan to make my synths (keyboards, bass pedals, etc) as vcos, and make a sort of rack of filters, and hook the synths into the filters then output...sort of like a guitar into an effects chain? is this how filters work? or is there a different logic? so far, as i gather, the logic seems to be VCO>LFO>VCF>OUT...am i close at all?

[Circuitbenders]

how do those hook together?

Hmmm, thats kind of like having all the separate parts to a car engine and asking 'how do these hook together'. Theres probably a hundred different answers and all of them would be impossibly difficult to explain on a forum to someone that didn't have a fundamental understanding of how engines work. I know i'd be lost in about 5 minutes!

I'd probably recommend taking a looking at modular analogue synths and working out how the individual modules work together with CV's and triggers etc. As an example, the LFO will be a discrete module that generate low frequency modulated control voltages, just like ones you might build to modulate the pitch of a bent toy. The output of the LFO can be applied to the pitch of the VCO, the filter cutoff of the VCF, retriggering the ADSR, or any number of other uses.

Audio output wise you probably want to go VCO-VCF-VCA-OUT but theres any number of other processes that can effect each of these stages.

[jamiewoody]

this comes close to answering my other thread "synth logic". ;-)