[sdiy] VCA response - continously variable between linear and exponential

rsdio at sounds.wa.com rsdio at sounds.wa.com
Sun Mar 3 03:11:22 CET 2013

```Why would you need continuously variable curves? Are you just talking
about shifting away from the standard 1 V per octave, but still being
based on the octave standard?

In my experience, a VCO either responds to CV with a linear
relationship between voltage and frequency (simpler circuits, easier
to get precise), or it responds to CV with a logarithmic relationship
between voltage and frequency (allowing a pitch bend CV to apply the
same relative pitch change regardless of octave). The latter happens
to have a linear relationship between voltage and octave or
semitones. I cannot think of any reason to have a response other than
one of those two. Note that the relationship between pitch (Hz or
period) and octave is not linear.

As I mentioned, the linear response is more basic, and because it is
linear you can make it as precise as you want. Unfortunately, a pitch
bend CV applied in one octave will have a different relative pitch
offset than in any other octave, making it impossible to pitch bend a
chord on a polyphonic keyboard (unless you specifically desire
inharmonious results). Thus, the only reason for non-linear CV of
pitch is to allow mixing multiple CV sources (e.g. from keyboard,
pitch bend, octave/footage selector, etc), and in that case the only
logical choice is the specific curve of an octave relationship. The
only disadvantage of the latter is that it is very difficult to
create this non-linear curve. It's easiest to use, but hardest to
create. Thus, the only two logical choices seem to be a Hz scale or
an octave scale, e.g. V/Hz, Hz/V, V/octave, octave/V, V/period,
period/V. There are lots of variations, but they still basically fall
into just two categories in terms of the exponential or linear
aspect, and I can't see the utility of anything "between" linear and
exponential.

I imagine that the only useful variation would be the value for "n"
in the expression, "n V per octave." The industry has mostly settled
on 1 V per octave, with some using 1.2 V per octave, but my point is
that it's always something per octave unless it's totally linear.

By the way, I'm sure everyone here understands the above, and I don't
intend to insult anyone by spelling it all out, but I really want to
make sure there's not something I'm missing. It seems like there was
an implication that a cubic or quadratic or higher power exponential
curves might be desirable, but I can't see why. I can see why 2 V per
octave or 0.5 V per octave might be incredibly useful, because it
would allow a different range of pitch for the same power supply. But
unless I'm using the terminology incorrectly, that's not continuously
variable between linear and exponential.

Brian Willoughby
Sound Consulting

----- "neil harper" <metadata at gmx.com>
> hey all,
> i'm looking for some advice/pointers on how to get a VCA response
> curve
> to be continuously variable between linear and exponential. i can do
> switchable between the two, but is there any trick to getting
> everything
> inbetween?
>
> if it helps, my VCA is lm13700 based but i don't mind starting with
> something else.

```