[sdiy] Micro as a Linear to Exponential converter?

[Colin f]

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> cheater cheater
> Sent: 14 August 2009 11:04
> To: synth-diy
> Subject: Re: [sdiy] Micro as a Linear to Exponential converter?
> 
> The best and simplest way to alleviate what Antti mentioned is to have
> a very slow lag processor/LPF after the DAC output. It would have to
> be reset (by for example quickly setting the time constant to very
> short) every time you want your output to change. Then you can start
> LPF'ing the signal from the DAC and turn the time constant up again,
> and then 100 (dithered) samples later you can get a nice voltage
> level. That still allows 2 ms latency at 48 kHz.
> 
> On the other hand you have amazing DACs nowadays like the ESS Sabre
> which will provide 8 outputs at -132 dB DNR. That should be pretty
> damn good for your expo. Because of the huge headroom they even
> started making one that accepts 32 bit floating point input now.

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