Custom waveform synth/LFO idea

[nexekho]

I imagine I'm infamous for having bright ideas that don't work by now but 

I stumbled upon the 4017 decade counter and realised that if I could use variable resistors to vary each pin's output, I could probably build a very flexible low-fi synthesizer.  Ram 10x the frequency you want into the CLOCK pin and it should output the wave requested.  I put a rough layout together, just the rough outline with many of the components simplified or missing; can anyone say if it would/would not work or if something similar's been built in the past?



I can't imagine it'd be great for LFO work though as the low resolution of the waveform would be very obvious.

[frequencycentral]

I've seen similar in a copy of E&MM from the 80's.

[slacker]

Yeah that basic idea will work, it's essentially a sequencer running at audio speeds. There's a lot of CD4017 based sequencer projects out there so you could have a look at some of those for ideas.

One of the synth gurus over at electro-music has a project based on the same idea, so you're in good company http://www.electro-music.com/forum/viewtopic.php?t=28571

[nexekho]

Ah, thanks!  I've just built it in Falstrad and it seems to work, though I might look into some form of waveform smoothing because it's currently VERY rough, though I picture the real components are probably a bit less precise and have some "give".

[nexekho]

Here's my "completed" Falstrad sim of the circuit.  It took some tweaking to get it to work nicely without any "magic grounds".  An LFO switch allows you to choose a capacitor, for a range of 6-102Hz in LFO mode (.6-10.2Hz at output) and a range of 673Hz-10.2kHz (67.3-1020Hz at output) in synth mode.  Volume and pitch control included.  Would probably include a fine pitch pot in series with the current pitch pot on the actual build.  I can't find an easy way to make it AC, so I've just used a resistor to drop the voltage to a line out safe 1.44v.

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Anyone see any problems?  Will it work in the real world?  I had to add two diodes on each wave fragment to stop them biasing each other, is there a better way or other stuff I can get rid of?

[nexekho]

Sorry to bump a third time, but I've designed another synth based on a very similar principle.  I looked up how much sliders were.  Youch.  Even if I substitute for pots, it's nearly what I spend on an entire project in waveform generator alone.  So I considered what might make a lower cost, yet still fairly flexible replacement.  Two 4017s are used.  One feeds the 555 to oscillate the frequency to make saw waves (adjustable using a single pot so you can saw either way or leave it in the middle for no sawtoothing) and the other goes through a nice pile of resistors to produce a sine and triangle approximation, blending between the two using a pot.  Then, to solve the nasty rough wave problem, there is an adjustable bandpass filter.  I need to clean up the layout, remove superflous parts and so on, but any comments?

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[PRR]

> it's currently VERY rough, though I picture the real components are probably a bit less precise and have some "give".

No.

You are sure to have a huge 10th harmonic. The "cheap cheat" would be a tracking high-cut slaved to the clock speed.

I suspect it will be difficult to get a strong 3rd harmonic (10/3= not a round number). Since the 3rd is musically useful/essential, this may not have all the tones you could want.

You can chain several 4017s to get 20-step, 30-step; or short-cycle for odd counts.

The ARP modular had this exact scheme. It was intended for a melody or arpeggio sequence, but we could rev it up to musical rates. I don't recall any great joy. But that was a long time ago......

[frequencycentral]

The ARP modular had this exact scheme. It was intended for a melody or arpeggio sequence, but we could rev it up to musical rates. I don't recall any great joy. But that was a long time ago......

You've used/touched an ARP 2500........ ........in my dreams........

[merlinb]

This is a DIY waveform interpolator I came across, just for research:
http://homepage.ntlworld.com/henry01/waveform_synth/waveform_synth.htm