[sdiy] FM with two oscillators

[Tom May]

Christian <Christian at actor.freeuk.com> writes:

> Hi everyone
> 
> A bit of a newbie question this -
>  
> I want to build my synth with two oscillators so VCO2 will modulate
> VCO1. I need this as I really love the rich Electric Guitar type
> sounds you can get - One of my favourites is 'Clarion Call'.
> I was wondering if it would be simply a question of feeding a
> proportion of the output of VCO2 back into the CV input of VCO1, or
> would there be other steps necessary.

To do that you'll need ac-coupled linear FM inputs as opposed to
exponential (v/oct) inputs so your FM doesn't shift the pitch of the
carrier.  You can see linear FM inputs on the ASM-1 VCO.  They work.

Or maybe modulating vco2 from vco1 and syncing vco2 from vco1 and
tuning vco2 above vco1 would work.

> Also, is FM only possible (or is it best) with sine waves - My VCO's
> currently only output Square with PCM, saw and triangle.
> BTW both the VCO's I have built have SYNC inputs.

Any waves work, but the thing you might miss is through-zero FM wich
allows for more extreme FM like a DX7 etc.  There should be a lot of
discussion of through-zero FM in the archives.

fTom.