AW: Re: optocouplers

[Haible_Juergen#Tel2743]

>  > With OTAs you always have to apply a feedback loop to get
>  > "RC-type" exponential slopes.
>[...]
>  When you say "add feedback" - do you mean running the output of the adsr
>  to the VC time inputs?
>  I personally find this to be a feature, and, therefore, prefer envelope
>  generators with linear slopes  - as it allows for;
>  1. Linear slopes (no feedback)
>  2. Expo slopes (normal feedback)
>  3. Reverse-expo slopes (invert the signal, then feed it back)

 In the following, I will only speak about OTA-based ENVs.
(With Optocouplers, I have no experience, and I think Gene
was the very first who suggested this ... so we will see if they
are similar or not).

Now, OTAs have an almost ideal current-source-output, which
causes linear slopes if you charge/discharge a capacitor.
All the old analogue ENVs have (more or less truncated) RC-
type envelopes. They could also be described as "exponential",
but the shape is far different from a linear ENV that controls an 
exponential
input (due to different asymptodes for Attack, Decay and Release),
so I prefer to call them "RC-type".

You can get the same shape if you use an OTA with a feedback network
to its negative input (*not* to the time control input - that's another 
story, and
useful as well), just as you would add a feedback loop to an opamp.
With very strong feedback you can achieve almost linear slopes
(by overdriving the OTA and using its input stage as a comparator),
or perfect RC-type slopes (if you make sure that your feedback path
doesn't overdrive the OTA). And of course everything in between.

>  I personally find this to be a feature, and, therefore, prefer envelope
>  generators with linear slopes  - as it allows for;

Are you sure? I mean almost every analogue synth I know has RC-shaped
slopes. I use a Slew limiter (=AR envelope with linear slopes) in my
Modular from time to time, but for general use I highly prefer the
*traditional* shape.

I remember when I used to program this D-50 back then. Even with the
programmer and its sliders, I had a hard time to create *traditional*
slow attack behaviour for strings/pads, and it usually ate up 3 segments
of the envelope section ! Now I don't know what exactly the shape
is in the D-50, but I am sure it wasn't the traditional shape.

So I love to have some extra courves any time, but not without the
option of RC-type ! (Just my opinion, of course)

JH.