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synaesthesia
Joined: May 27, 2014 Posts: 291 Location: Germany
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Posted: Wed Jan 07, 2015 2:40 pm Post subject:
The Lunetta Voice Subject description: A simple digitally controlled oscillator with a cool sound |
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I am working on new modules for a semi-modular Lunetta right now and wanted to have a simple (as in low part count) digitally controlled sawtooth oscillator. At first I looked into a design using op-amps, but then remembered a version from Scott Gravenhorst that uses 4069 gates instead. That one wasn't controllable by a CV as easy as I wanted it to, but I managed to find a solution for that. The circuit is by no means exact, nor 1V/octave, nor generates note frequencies, but it does the job good enough for a Lunetta. Just before sharing I tried adding the PWM effect I mentioned in another thread (Pimp my square wave) and it works even better here. So I went back to experimentation mode for a while. The result is that I found a simple way to add even more punch to the sound it generates. So I consider it interesting enough to share the circuit as-is now.
In the left part there is your tried-and-trusted 4049 sawtooth generator (the original used 4069 gates). Adding the diode and the pull-up for the simple resistor network did the trick to make it digitally controllable. It feeds the signal into a passive resistor mixing stage. The result is turned into a square wave by an inverter and feeds a counter that takes the sub-octave and the sub-sub-octave using one half of a 4520, smoothes it with a cap, and feeds a small part of it back into the mixer stage. The result is a kind of resonance or tremolo that makes a really interesting sound. In my interpretation it comes from changing the amplitude height of the sawtooth wave, which in turn varies the pulse width of the final pulse wave. You can see that clearly as a jitter of the blue curve in the video. Nice sound, considered the few parts that it took. I tried to turn the sub-octave part of the circuit into a separate module, but that didn't work well. It seems that it really must have a sawtooth or triangle wave as an input. It doesn't affect square waves much.
I have added two control lines that can be used to disable the "resonant" and the "percussive" effects in the output, just in case. They are optional. Also, the percussive part using the LDR/LED vactrol is optional. As described in the schematic, the DCO generates frequencies between 190 and 880 Hz in approximately 40 Hz steps. The video shows the waveform captured from the sawtooth oscillator above and the waveform after the 4.7nF cap below. The final output is a square wave (pulse wave) again. Mind that the oscillator and mixer need a 4049UBE or 4069UBE, it won't work with 40106 inverters. Next step is a pattern generator to feed that thing
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PHOBoS
Joined: Jan 14, 2010 Posts: 5581 Location: Moon Base
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synaesthesia
Joined: May 27, 2014 Posts: 291 Location: Germany
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synaesthesia
Joined: May 27, 2014 Posts: 291 Location: Germany
Audio files: 85
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