"Oscillators" for circuit bent machines?

Rolbista · Joined: Nov 17, 2012 Posts: 23 Location: Poland

Ok, so here's a concept: take the most sinusoidal waveform of your bent keyboard and filter it a little to remove harmonics. Step 2: convert it to a square wave with a Schmitt trigger - here's your first waveform. Step 3: use a 4040 to get one and two octaves below. Step 3: either route the received square wave of chosen octave straight to the output or feed it into this: http://electro-music.com/forum/post-244968.html to get a sawtooth. Build 3 of such units and voila! you have 3 "VCO" on your cheap Casio keyboard! Ok, does that even make sense, I've never seen anyone do it, maybe there's a reason? And what would be the right Schmitt setup to get it to work? I used a TL072 set up as a non-inverting trigger from wikipedia with two 25k trimpots, but no matter how i set the trimpots, it doesn't output any square. Power: +/-5V, I'm viewing the signal on a scope. The signal from the Casio is around 1Vpp.

JingleJoe · Posted: Tue Feb 19, 2013 8:08 am Post subject:

Your casio output is too low, add a pre-amplifier, a simple op amp circuit should do. Incase you don't know and for anyone else who doesn't, to make it work from a single rail supply you can bias up your non inverting input to half supply AKA virtual ground.

a circuit you could use:
http://i.cmpnet.com/audiodesignline/2008/03/2vrms_fig4.jpg
(I'd suggect you make the input capacitor above 1uF to avoid filtration effects on low frequencies, it is effectively a high pass filter to remove any existing DC offset)
real ground and virtual ground:
http://lh3.ggpht.com/_gnU30vMbtYg/Tcsp40CovkI/AAAAAAAAAlo/5B6pBJftET8/dual%20ground_thumb.png

Technically it's not 3 vcos because they are all at the same frequency, inr eality 3 vco's would be entirely separate things which can be at individual pitches.
However some complex waveshaping for your simple keyboard does bring it ever closer to being a true synthesizer Smile

Rolbista · Joined: Nov 17, 2012 Posts: 23 Location: Poland

Thanks for the reply! Yeah, that's why I put "VCO" in quotes Very Happy

. So paired with a preamp stage, the circuit you posted in the first link should output a square wave of the same frequency as the input signal? How much should I amplify the signal? Would 5Vpp be fine? I've been thinking about this for a while now, I recently thought about incorporating a fragment of an analog guitar octaver to extract fundamental frequency, but I don't know where that fragment ends. Such "VCO" paired with an LTC1799 pitch bend mod could give me a wide range of possibilities (for a Casio anyway).