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

[Tom Wiltshire]

True, for modulation depth control, liners probably always going to be best. I was thinking more of audio. For some reason (availability?) the final VCA in most classic synths is linear. Doesn’t make sense to me.

Tom

> On 3 Nov 2017, at 15:51, paula at synth.net wrote:
> 
> 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
>> ==================
>>       Electric Druid
>> Synth & Stompbox DIY
>> ==================
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