2 waveforms from a single oscillator - how?

[Eddy Deegan]

Having played with the Mother-32 a bit (and by golly it's very playable-with) I've been experimenting with various patch panel combinations and two very useful outputs are the saw and pulse wave outputs from the VCO.

This got me thinking ... the Mother-32 has a single VCO which has a switch on the panel to select saw or pulse outputs. Given that using the patch panel outputs you can mix the two together and hear them combined, the question I'm pondering is ... how is this done using just a single oscillator? I find it hard to accept that there are actually 2 oscillators in there, as to only expose them in this limited form makes little sense to me, and it is a single oscillator synth after all.

I'm intrigued, how is this implemented under the hood?!

[muzines]

"Both Sawtooth and Pulse waveforms have dedicated output jacks on the patchbay."

This suggests that it has circuitry to always generate both waveforms at the same time (eg, it always generates a pulse, but also has some circuitry to derive the saw from that pulse wave (or vice versa).

One oscillator, two different waveforms generated from it simultaneously.

[Eddy Deegan]

... some circuitry ...

One oscillator, two different waveforms generated from it simultaneously.

I figured as much ... but I'm curious as to the how of that 'some circuitry'

[muzines]

... some circuitry ...

One oscillator, two different waveforms generated from it simultaneously.

I figured as much ... but I'm curious as to the how of that 'some circuitry'

Well, my electronics is a lot more rudimentary than my other skills.
I did make a spring reverb in GCSE Electronics back in the late eighties!

Someone else will have to come up with circuit design topologies, or just google analog oscillator design for some ideas if you're that interested.

As a *really* basic thing, let's say your oscillator outputs a pulse waveform. And you can tap it there for the pulse. But you can also feed that pulse into a capacitor, which discharges over the duration of the pulse to get recharged by the next pulse - now you've got your saw waveform, from the output of the capacitor - and you tap that as your saw output.

(Not saying it works like this, but you get the conceptual idea...)

[Eddy Deegan]

Ah, that makes some kind of intuitive sense, thanks. Yes, so the frequency of the two waveforms will always be locked relative to the fundamental frequency of the oscillator (barring some kind of external treatment to one or other of them via the patch panel).

Cheers!

[Hugh Robjohns]

So the frequency of the two waveforms will always be locked relative to the fundamental frequency of the oscillator

Yes, it's the same oscillator, so the same pitch/frequency. The only difference will be the harmonic content of the two outputs (square being only odd-harmonics, and saw having both odd and even harmonics)

H

[Folderol]

Pulse discharging a cap that has a constant current recharge is the classic way to get a linear sawtooth (first seen in ancient oscilloscopes), make the pulse generator level sensitive and you've got your free running oscillator.

However, for the greatest possible range of waveshapes start with a sinewave oscillator· push it through a trigger circuit to get a square wave, pass that through and RC filter to get a triangle wave.

Feed said square wave into a narrow pulse generator, and again to CR and you've got a sawtooth again.

And that's just for starters

P.S.
Fun fact:
In a square wave all the harmonics are in the same phase. Alternate them and you've got a triangle wave!

[nathanscribe]

Fun fact:
In a square wave all the harmonics are in the same phase. Alternate them and you've got a triangle wave!

Mind blown!

Also, yes, if the M32 is anything like the Werkstatt internally (which I am hazarding it is, going by the spec) the oscillator core is the usual cap-charging-plus-reset ramp generator, which is then fed to a comparator to get a pulse wave.

Moog have the Werkstatt schematics online for purposes of experiment and modification. Not the plainest circuit around, but I suspect the M32 is not a million miles away from it.

[Eddy Deegan]

Great info - thanks all!

[Adam Inglis]

However, for the greatest possible range of waveshapes start with a sinewave oscillator

Not often seen in voltage controlled synths though, are they? Why is that?

[Folderol]

However, for the greatest possible range of waveshapes start with a sinewave oscillator

Not often seen in voltage controlled synths though, are they? Why is that?

Because they are much harder to make the frequency voltage controlled. Also, if you don't do so in a properly balanced way you also change the amplitude.

Sawtooth is the easiest. For a cheap-n-chearful design you can just use a transistor as a constant current charger for the cap. Change the transistor bias and you change the frequency. Amplitude stays constant.

[Andy Cobley]

I thought you started with a sawtooth. Then attach a level detector, voltages below that level are zero, higher are "ful voltage". At 50% of the sawtooth level you get a square, higher and lower gives you different pulse widths.

BTW I thought a Square was odd Harmonics only, Saw all harmonics only.

Geometric Wavesweb page

[nathanscribe]

That's one way to do it, and a common and easy one. There are more complex ways used too, such as starting with a triangle (eg Maplin 5600 etc), or layering multiple-octave squares to approximate a sawtooth (eg Korg Lambda, Elka Synthex, Bit 99).

[Folderol]

BTW I thought a Square was odd Harmonics only, Saw all harmonics only.

Ummm. Yes, exactly as Hugh said at the beginning

P.S.
That link only tells half the story. Not only do the amplitudes have to be correct but the starting phase must be as well. If, say, your fundamental is positive going at the zero point, while your third harmonic is negative going, you'll never see a square wave, but you'll be well on the way to a triangle wave