[sdiy] Three questions for you Synth Experts

[scottnoanh at peoplepc.com]

Hi Chris,

>   1) On the Synth's control panel between the two VCO sections I intend to
have a SYNC switch (like the ARP Odyssey) for some cool Electric Guitar type
sounds.
>   I would like to experiment with FM sounds too, and I thought also about
putting a control at this point on the panel which would feed a proportion
of the VCO2 waveform output into the VCO1 FM input. Does this sound OK? Do I
need any other controls to get this effect or give more options? (LFO and
ADSR circuits aside, or course - I am just thinking of creating Clangorous
Bell type sounds.)

I would use a pot as the control to adjust how much modulation you want from
VCO2 to VCO1.  A switch to select the modulating waveform might not be a bad
idea, though may find that you'll use a sine or a triangle wave most of the
time.

>   2) On the Mixer section on the control panel, I will have VCO1 level,
VCO2 level and Noise level as is usual on a synth.
>   I wanted to experiment with Ring Modulation, and thought about putting a
control at this point on the panel to add a proportion of the result of the
Ringmod circuit into the Mix. Does this sound OK? Do I need anything else or
any other controls for basic Ringmod?

Sounds like that would work fine.  Which waveforms are you going to use for
the ring modulation (again you may find that sine/triangle may be used more
than others)?

>   3) There is a gap in my knowledge about CV inputs, and FM inputs. My
VCOs that I have built have a CV input and also an FM input. I wonder if
anyone could tell me the difference between the two inputs and why you would
feed (say) an LFO signal into one and not the other. Anything to watch out
for?

A couple of things - the CV input may be the keyboard tracking input, and
normally you would pass this straight to the VCO without any input
attenuation control.  FM inputs themselves often come in two flavors - V/Oct
(expo) response and linear response.  The expo FM input is often used for
vibrato because the pitch deviation will always be even, no matter where in
the scale you are.  Using the linear input for vibrato would result in a
wider swing on lower notes and a narrower swing on higher notes.  Linear is
very good for faster FM (the clangorous stuff you mentioned earlier) and is
often AC coupled.  The Expo input is good for this, too, of course - the
response is noticeably different.

Sounds like you have a nice project in mind.

Cheers,
Scott