TL;DR:
- Use linear VCAs when controlling them digitally, even when controlling audio. That lets you generate whatever control slope you want in software.
- The VCA response is always exponential, not logarithmic. The only time we're actually talking about something looking more like a logarithmic response is in classic "RC" envelopes where the attack part is more logarithmic. Logarithmic control signals are generally only used to linearise an exponential VCA.
The full text
Through testing the digital envelopes in combination with the v2164 VCA, I realised that all is not good. It was particularly hard to dial in the sustain level as it dropped so fast when turning the pot due to the exponential nature. This made me realise that I had to revisit the topic of envelopes to fully understand how it is done in practice.
My initial confusion stems from the fact that people keep saying that you should use exponential VCAs for audio because they more closely approximate the way our hearing works. While this may be true when using the VCA as a volume control alone, it isn't necessary true elsewhere in the synth.
There is a lot of confusion about the use of the terms log(arithmic) and exp(onential) in the synth world, both when talking about potentiometers, VCAs and envelopes. I'll not go into detail, but just conclude that the slope/response is almost always exponential, not logarithmic. The only time we're actually talking about something looking more like a logarithmic response is in classic "RC" envelopes where the attack part is more logarithmic, and even here it is just an exponential response turned "upside down" (charging a cap instead of uncharging it). Logarithmic control signals are generally only used to linearise the response of an exponential VCA.
Another thing that is repeated is that using an exponential envelope with a linear VCA is the same as using a linear envelope with an exponential VCA. That is almost true, but with an exponential envelope controlling a linear VCA, you get direct control of the sustain level whereas when a linear envelope controls an exponential VCA, we get the "mapped" version of the sustain level which is significantly lower than the control voltage - which may be what you want but I found it hard to actually get the necessary control.
There's another point to be made though. When people talk about exponential envelopes, I suspect they mean the classic ones that is the result of charging and discharging capacitors, "RC" response. Those have a rapid increase upwards at the start of the attack and a rapid decrease at the start of decay. Using a linear envelope with an exponential VCA would give you a slow attack.
A better name for the stages in an "RC" response envelope seems to be concave upwards and concave downwards. This is not what you get with an exponential VCA controlled by a linear envelope.
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| Top: A linear envelope. Middle: The effect of feeding the linear envelope above through an exponential VCA. Note that the attack slopes downward and that the sustain level is significantly lower than the linear input as an exponential VCA drops very fast in the beginning. An exponential envelope would have the same shape but you would of course control the sustain level directly. Bottom: "RC" response, the classic envelope shape you get from charging/discharging a capacitor. |
A lot of other versions exists. When you dive into the realm of digitally generated envelopes you find stuff like the Alpha Juno multi stage envelopes where the slope changes on a per-stage basis.
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| Alpha Juno lets you set envelope by specifying Time and Level for each stage. Slope varies from stage to stage, sometimes being linear (1, 2) and sometimes exponential (3, 4). These envelopes are purely digital. |
One question was still unanswered for me - with an exponential envelope, should one make it control an exponential or a linear VCA? I've concluded that it has to control a linear VCA. The issue with sustain level speaks clearly of this. You want the response of the envelope, not an exponential version of it - especially when doing digital envelopes as you can pretty much do whatever shape you like. Hopefully, the resolution of the CV is high enough to mimic exponential growth with a fairly good quality even at low volumes.
What VCA chip to use in the XM8
Having to use linear VCAs kind of sucks. I was hoping to use the v2164 quad VCA extensively because it offers four VCAs in a compact package and at a reasonable price. If envelopes should control linear VCAs, and if I want to be able to patch envelopes anywhere, I also need to use linear VCAs everywhere. My best option seems to be the AS3330, which is a dual lin/exp VCA costing almost twice that of the V2164 meaning I have to spend four times as much on VCAs. That in itself kind of suck, but it will also take twice the space on the PCB which REALLY sucks.
I will give the V2164 a final try though. If I can generate a logarithmic control signal, the output response will be linear. There is a very standard circuit going around that does this by combining two 2164s, but that sort of defeats the purpose. I have to do it digitally. This may result in very low resolution for the higher volumes, we'll just have to see.
Update: AS3364 is a quad linear VCA, that may be a good option. Unfortunately it's +/- 12V, not 15. I've ordered 10 of them for testing. I also realised that I have both AS3330 and AS3360 chips that I can try if I want. The AS3364 seems to be a dual AS3360 but with the exponential inputs removed, the text and specs in the datasheet is almost identical. This is good as it makes it possible to replace the AS3364 (which is an Alpha Rpar specific chip) with two 3360s on an adapter board later, should one fail and no replacements be available.
Resources
https://www.muffwiggler.com/forum/viewtopic.php?t=217707
https://www.muffwiggler.com/forum/viewtopic.php?t=115675
https://www.gearslutz.com/board/electronic-music-instruments-and-electronic-music-production/984069-embarrassing-question-explain-alpha-junos-envelopes.html
https://musicianonamission.com/adsr/
https://www.muffwiggler.com/forum/viewtopic.php?t=102357
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