basic synth logic...

jamiewoody · February 03, 2011, 10:53:03 PM

i know this has been covered before, but please tell me if i have this right...

a synth block diagram is basically:

VCO>LFO>VCF>VCA...yes?

and VCFs are hi-pass, lo-pass, band-pass, band-stop?

also, when hooking up VCFs, is there a concrete way to do it? or is it a matter of taste? i mean in terms of series or parallel. should i think of VCFs similar to pedals on a guitar pedal board?

Circuitbenders · February 04, 2011, 12:05:42 AM

The LFO shouldn't be there. The LFO is like an envelope generator in that it produces a signal, in this case probably a control voltage, that modulates parameters of the other sections. Other than that the basic signal flow is correct.

Most synths will have one multimode VCF, as opposed to several separate VCF circuits cascading into each other or running in parallel.

jamiewoody · February 04, 2011, 01:42:17 PM

i feel this would help me to actually LEARN how to build filters, instead of just copying an existing circuit. for some reason, the hands on approach helps me to learn better.

doesn't an lfp basically vibrate the frequencies of the vcf?

Circuitbenders · February 04, 2011, 02:06:41 PM

an LFO can 'vibrate' or modulate the frequency of a VCF cutoff, or a VCF resonance amount, or a VCO pitch, or a VCA level, or a VCO waveform shape, or VCO pulse width, or just about anything else you can set up to have a CV input. Thats the wonderful world of modular synthesis!

Bogus Noise · February 04, 2011, 05:52:13 PM

If anything, the LFO should go above or below the entire VCO>VCF>VCA, and have an arrow pointing to each stage.

Think in terms of synth signal flow, rather than synth logic.

As for parallel vs serial filters, both paths have their benefits and available techniques, I'm always happy to have 2 filters on a synth!

jamiewoody · February 04, 2011, 10:00:05 PM

ah, so the lfo should be parallel!

on the bass pedals, i will have to actually wire switches (heavy stomp style) for each function. but, on my keyboard, since i will be playing it with hands and not feet, a patchbay will prolly be cool!

druzz · February 05, 2011, 03:12:29 PM

no , the LFO is NOT in the signal flow . it applys modulation to the modules that are part of the signal flow (vco vcf vca) or indirectly to other modules that modulates modules of the signal flow (others lfo'S, vco'S used as modulator, enveloppes ... )

the audio path (signal flow)
and the control volage connections are two different things but sometimes you can trow an audio signal in a control voltage input and it can give good results

i suggest you read a couple of the articles on this page , that what i do while i drink my morning coffee

http://www.soundonsound.com/sos/allsynthsecrets.htm

jamiewoody · February 06, 2011, 09:41:25 AM

confusing...that link just had articles on how to make a synth sound like a cowbell....lol

druzz · February 07, 2011, 01:18:35 AM

go way down to the bottom of the page . its a serie of articles that were published monthly. the first 20 articles cover the basics and the later one cover more specific techniques . so go down to part 1 and begin from the start.

i'm somewere around part 26 . i'm not yet to the cowbell chapter

jamiewoody · April 24, 2011, 08:49:07 PM

thanks, i understand now. the LFO is not inseries but parallel...to modulate the VCFs.

Bogus Noise · April 25, 2011, 02:08:48 PM

That's the idea. Bear in mind that it could also modulate the oscillator for siren effects, or the VCA for tremolo effects, or even modulate the speed of another LFO which is in turn controlling the VCF cutoff. It's like an automatic knob twiddler.

jamiewoody · April 26, 2011, 09:56:57 PM

i guess that is the appeal of a patchbay on a keyboard synth, to experiment with all the elements...ah, cool idea, more than one lfo!

while i am still DREAMING, i am thinking of building a basic vco/vcf/lfo (in no order) synth, but also making the box big enough for upgrades....like output tremolo, ring modulator, etc. this way, i can add more as i learn and at my own pace...

jamiewoody · April 26, 2011, 10:02:36 PM

on my keyboard, i have been wondering how many vcos i should build. should i make one? or several, to handle bass, mids, etc? the keyset i want to use is on a pcb strip with reed switches. it would be too hard to separate the keys.

Gordonjcp · April 27, 2011, 07:54:10 AM

Typically the VCOs track the full range of the keyboard. Having more VCOs means you can create more complex sounds, but they should all handle the same frequency range.

One notable exception is the sub oscillators on the Korg Polysix and Roland Junos (and related models) which is a squarewave generated by passing the output of the oscillator into a divide-by-two or divide-by-four circuit.

Bogus Noise · April 27, 2011, 02:10:24 PM

I'd say go for two or three oscillators. Three will give a good amount of sound making scope.

When you say one for the bass and one for the mids, do you mean them both to be components of the same sound? This way would make the most sense to me, for example one oscillator at C3, another a fifth above it, and another acting sub oscillator an octave or two below - the sub osc doesn't necessarily need to be a dedicated oscillator, see Gordon's tip on the frequency divider.

As you say, splitting the keyboard would be a bit harder... worth making a monosynth at this stage. I'd be inclined to say build up the basic monosynth you have planned, but make it all patchable for future expansion.

I'm gradually piecing the components together for my own DIY monosynth, but fitting it around all the other projects, a music career, a girlfriend, and necessities like sleep and food isn't easy!