[sdiy] function generation

[James Patchell]

At 09:57 AM 11/9/2003 -0600, Grant Richter wrote:
> > The Terry
> > Michael's (ASM-1) vco core is much more suitable for pitch
> > generations.
>
>Not to quibble with Jim, who is giving good advice about linear VCOs being
>quite difficult to control exponentially.

         Actually, exponentially controller linear VCOs is not 
difficult....the difficulty comes into play when the linear VCO's are just 
not very linear.  The Terry Michaels VCO is so linear I connot measure it's 
non-linearity with the equipment I have here at home....whereas something 
like the XR2207 is very easy to see.


>As a note of historical interest. The EML 101 used uA726 exponential voltage
>to voltage converters driving linear VCOs. While not preferred now, it can
>be made to work, as some thousand EML 101s were produced and sold.
>
>The problem is the tiny voltage differences at the low end of the scale
>accentuate any offset or gain problems, along with temperature stability
>issues and noise issues which are worse for the linear VCO design.
>
>The approach might work OK for a microprocessor controlled system with a
>high resolution DAC. The DAC could use a lookup table to correct for tiny
>linearity flaws in the VCO. It would be a whole lot of extra work for only a
>minor savings in cost.
>
>John Simonton did pull it off OK in the PAIA Fatman using a low resolution
>DAC, and some of his own brand of magic.

         -Jim
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