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

[Dave Magnuson]

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom Wiltshire
Sent: Friday, November 3, 2017 1:34 PM
To: paula at synth.net
Cc: SYNTH DIY
Subject: Re: [sdiy] Log/Expo or Lin VCAs for digital control?


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


Is it because the envelopes are usually expo shaped? (not sure myself actually)

Dave


> 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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