[sdiy] Micro as a Linear to Exponential converter?

[cheater cheater]

> Nobody mentioned aliasing?
stupid me. Of course they have :^)

Time out for me :)

On Fri, Aug 14, 2009 at 11:48 AM, cheater cheater<cheater00 at gmail.com> wrote:
> 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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>