[sdiy] Micro as a Linear to Exponential converter?

[cheater cheater]

Nobody mentioned aliasing?
Also you ignored everything Antti brought up.


On Fri, Aug 14, 2009 at 11:23 AM, Colin f<colin at colinfraser.com> wrote:
>> -----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
>
>
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