[sdiy] Log/Expo or Lin VCAs for digital control?

[paula at synth.net]

I guess it depends.
I mean, if you're controlling an audio level (say final VCA on a synth), 
then wouldn't it be best to use log. But if you're controlling the depth 
of say LFO modulation, then wouldn't linear be better?

Paula

On 2017-11-03 14:56, Tom Wiltshire wrote:
> The recent arrival of AS3360 VCA chips set me thinking - given that
> the chip can be operated with either a linear CV or a logarithmic/expo
> CV, which is best if I want to control it from a DAC?
> The question doesn’t only relate to that chip specifically - you could
> ask the same thing about LM13700 vs SSM2164. It was simply that having
> both options on the one chip made the question arise...
> 
> It seems to me that using linear control wastes a lot of the available
> DAC resolution. After all, approximately half of the available DAC
> output will be used up reducing the level to -6dB. Another quarter of
> the DAC’s range is required to get us down another -6dB. So having
> used 3/4 of our resolution, we’re still only 12dB away from where we
> started!
> 
> Using log/expo control makes more sense since this doesn’t occur. If
> you’re trying to cover (say) a 100dB range from 0dB to -100dB, even an
> 8-bit DAC gives you a resolution of 0.4dB/bit, which is easily small
> enough to be an undetectable step. By the time you’re looking at
> 10-bit DACs, you’ve got steps around 1/10th of a dB, which is a *tiny*
> fraction.
> 
> One other advantage of using log/expo control is that since your
> control voltage is in a logarithmic domain, you can just *add* CVs to
> get the effect of multiplying them. Thus if you wanted to control the
> level of an envelope, you can add the Level CV to the Envelope CV and
> you’re done. Or think about adding tremolo to an envelope - you just
> add on the LFO output that’s providing the tremolo. Easy.
> About the only downside is the need to adjust the envelope generation
> to reflect the altered control response. But this is not too
> difficult.
> 
> Is there anything I’ve missed here? Is this a fair summary? Have I got
> the stick by the sticky end?
> 
> Thanks,
> Tom
> 
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> Synth & Stompbox DIY
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