[sdiy] Micro as a Linear to Exponential converter?

[Tom Wiltshire]

On 14 Aug 2009, at 11:23, Colin f wrote:
> You only need to start worrying about aliasing if you want to do  
> audio rate
> modulation of the generated CV.
> IMO, if you're going to go down that path, you might as well just  
> generate
> the oscillator waveforms digitally and save yourself some circuitry.
>
> If you just want to drive a load of linear VCOs with a high-res DAC,
> multiplexed out to save you the cost of lots of expo conversion,  
> it's easily
> enough done. Just use 16 bits of DAC, and follow it with op-amps with
> exceptionally low offset voltages.
> Then build a lookup table for the frequency value for each MIDI note.
> And make sure you dont use an audio DAC with a large, drifting  
> output offset
> voltage.
> If you want exponential modulation of the output CVs, consider  
> modulating
> the DAC reference, rather than trying to do it in the digital domain.
> That's how the Yamaha linear VCO synths achieve pitch bend.
> I've used a PIC driving an LTC1597 DAC, followed by a 4051 then LT1112
> op-amps to generate 8 linear CV outputs at 0.5ms refresh rate, with
> RC-shaped portamento done in software, with good results.
>
> Cheers,
> Colin f

I'd just like to agree with Colin's outlined scheme above. I've done  
similar things. Antti's comment that "the code part is trivial" is a  
bit misleading, IMO. "Trivial" in the mathematical sense of "not NP  
Hard" perhaps, but programming is rarely trivially simple. A lookup  
table for the exponential conversion is definitely required, and some  
kind of interpolation too, if you want pitch bend or anything other  
than exact MIDI notes.
So, not "trivial", but not "difficult" either.

T.