[sdiy] best soundign vco design?

[Scott Gravenhorst]

<delta_316 at yahoo.co.uk> wrote:
>thanks for the response
>i suppose what i mean by "pure analogue fatness" refers to the 
>waveform shape, when viewed on a scope. i want the oscillator to 
>produce waveforms that have a high quality shape, that is 
>consistent over different pitches. by high quality i mean as near 
>as possible to the perfect mathematical shape i.e. only straight 
>lines for a sawtooth (although it is unlikely to acheive an 
>insant reset time) and a smooth sine shape at all pitches. same 
>goes for other waveforms. i have an esq-1 synth, which is great 
>for some sounds, but the analog waveforms don't have the full 
>analog flavour. and lo and behold, when viewed on a scope, the 
>sawtooth has curves, and the square and sawtooth get all wiggly 
>at lower frequencies. 

The structure of any decent sawtooth VCO will have a straight ramp
due to the fact that the integrating capacitor is charged using
a constant current source (the current value being determined by
the control voltage.)  Reset time is never going to be instantaneous,
that's just physically impossible.  There are faster and slower
ones and this determines the range over which the VCO is linear.
And this can be compensated to make the VCO musically useful over
many octaves.  Even the FatMan VCO isn't as bad as you describe, 
and that's not the best VCO out there.  Probably unanimously agreed 
here or at least nearly so is the MOTM modules are the most accurate 
this way.  However, you do get what you pay for and vice-versa.  As 
far as I understand, the ASM-1 is a very good VCO.

Out of curiosity, why do you feel that you need or want mathematically
perfect VCOs?  Even MOTM is not *perfect*.  Really damned good, but
not perfect.  No matter what you do, there's always at least a bit 
of noise that causes waveforms to be slightly imperfect.

Also, be aware that some pretty ugly looking waveforms can sound very 
analogue and very fat.  What you see does not always mean much with 
respect to sound.
 
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