[sdiy] OTA VCA

[rsdio at audiobanshee.com]

One catch is that exponential curves come in two flavors, and it's important which one you have for a VCA.

For linear, there's only one straight line, so there's no confusion there.

Human perception of volume is relative. For any given amplitude, the amount of change that is perceived is always a percentage of the previous value. That's why dB is a relative scale, not absolute (unless you give the reference, like dB SPL, dBFS, or dBV, etc.). At quiet volumes, only a small change is necessary. At the loudest volumes, a high change is necessary. Every 6 dB increase in amplitude represents a doubling in the circuit!

Thus, a useful mapping of CV to volume (VCA) is one where the amplitude change is larger for high volumes and smaller for low volumes.

An RC circuit produces an exponential curve on charging and discharging, but each is a different curve for the purposes of volume. The discharge curve of an RC circuit is useful for volume, because it falls very quickly at first (in the high volume section) and then slows down to smaller and smaller changes (in the low volume section).

Unfortunately, the charging curve of an RC circuit is exactly the opposite of what we want for volume (VCA). The circuit charges very quickly at first (in the low volume section where we perceive small changes) and then more slowly charges at the voltage reaches maximum (in the high volume section where it would be better to have large changes). If you want a fast attack without clicking and a slow release, then this mismatch might not actually be so bad.

A linear ADSR (or just AR) would be better in general so that the rise and fall curve would be the same (linear). Then, a linear to exponential converter could be used for circuits like a VCA that need larger changes at higher volumes to produce a roughly decibel response that would be perceived as a smooth volume change.

However, as I mentioned above, it might not be so bad to have an attack that sounds faster than it is combined with a release that sounds slower than it is. You already know that you need an inexpensive circuit because you're repeating it for every key, so a minor imperfection is not a show-stopper if it sounds fine.

Brian Willoughby
Sound Consulting


On Mar 20, 2017, at 1:20 PM, cheater00 cheater00 <cheater00 at gmail.com> wrote:
> Exponential is curved, linear is straight lines. Look on google images for exponential envelope. Look at your CV on an oscilloscope. If you don't have one, get one, it's an important tool.
> 
> On Mon, 20 Mar 2017 21:13 Elain Klopke, <functionofform at gmail.com> wrote:
>> Cheater: I have no idea what linear vs. exponential looks like unless the description tells me so, lol. I'm such a hack and constantly grabbing a piece of this or that circuit to suit my needs....
>>