[sdiy] Lin/Log VCAs and envelopes - compensating for log VCAs

[rsdio at audiobanshee.com]

I was confused by this at first, too. After thinking about it for a bit, I believe that the only reason it sounds fine when sustain is zero is because you can't actually hear anything when the transition from decay to sustain occurs. I assume it would sound just as bad, if it weren't for the fact that it's silent.

With an analog ADSR, the voltage is asymptotically approaching the sustain level as soon as the decay starts. It always takes about 5TC to "reach" sustain, no matter where sustain is set.

When a linear ADSR feeds a log VCA, there's no way to recreate the asymptotic curve that ends at sustain. The log VCA has a preset curve that does not change when sustain is altered.

Brian


On Feb 11, 2016, at 11:14 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> Hi Simon,
> 
> Sorry, I don't understand why a linear decay into a exponential VCA only gives an exponential decay when sustain is zero. If the VCA is set up to drop by 6dB for 30 mV, then it drops by 6dB when the linear control voltage drops by 30mV. I don't see how the sustain level can affect that.
> 
> Am I missing something?
> 
> Thanks,
> Tom
> 
> On 11 Feb 2016, at 15:59, Simon Brouwer <simon.o at brousant.nl> wrote:
>> Hi Tom,
>> 
>> To my ears, linear slope envelope generators (as found in many digital synthesizers, as a linear slope is so much easier to calculate) are vastly inferior to exponential slope ones, as at the end of the decay phase the envelope does not gradually evolve into the sustain level, but switches from changing into static all at once. This often results in a sound that is interesting in the beginning, but loses all life in the sustain phase, or is cut off in an unnatural way. Layering and/or lots of reverb may mask this to an extent but that is just stopgap. Real synthesizers have curves!
>> 
>> A linear decay combined with an exponential control/gain VCA will give an exponential decay of amplitude, but only if the sustain level is 0. With nonzero sustain levels the envelope will again switch from changing to static all at once.
>> 
>> 
>> Best regards
>> Simon
>> 
>>> Op 11 februari 2016 om 15:47 schreef Tom Wiltshire <tom at electricdruid.net>:
>>> 
>>> Yeah, I've done the same thing. My PIC VCADSR has linear or exponential envelopes selectable. But that's not exactly what I'm suggesting here.
>>> 
>>> Instead, I was thinking of doing a normal exponential ADSR for use with a linear VCA, and a "log compensated" ADSR for use with a log VCA. This would have a linear decay and release, but an extreme curve to the attack section to give the typical shape even with the log VCA.
>>> 
>>> Tom
>>> 
>>> On 11 Feb 2016, at 09:14, Roman Sowa <modular at go2.pl> wrote:
>>>> It was too easy not to implement it, so I did that on my EG too. One switch determines exponential or almost-linear curve for attack and the other one for 1st decay. It simply changes the voltage charging the timing capacitor.
>>>> Here's example scopeshot of both settings:
>>>> http://www.sowa.synth.net/modular/addsr_punch.jpg
>>>> 
>>>> Roman
>>>> 
>>>> W dniu 2016-02-10 o 23:27, P Maddox pisze:
>>>>> Tom,
>>>>> 
>>>>> On 06/02/2016 19:08, Tom Wiltshire wrote:
>>>>>> Being able to switch between the two curves would enable people to
>>>>>> use either a log or linear VCA and still get the same effect.
>>>>>> 
>>>>>> What do you all think?
>>>>> That's what we do with 001/002/002R, you have the option for "linear" or
>>>>> "exponential" Envelopes.
>>>>> Exponential "sound" quicker than linear but give the typical "synth" sound.
>>>>> 
>>>>> P