portamento circuit

[Haible_Juergen#Tel2743]

>     Yes publish it - will it fit as an ASCIImatic? Let's compare notes. I
>     have made linear/exponential switched glide circuits. Usually what I
>     make is two op-amps, in a row, with the R/C in between them. The
>     second op-amp is always a buffer, but the first is switchable between
>     a buffer (for exponential) or a comparator (for linear). The (-) input 

>     of the first op-amp has a SPDT switch between the outputs of either
>     the first or second op-amp. Works great.

Mine is very similar, only that the potentiometer is shortened in the
3rd position of the switch, and I think I have defused the problem
of different time constants ...

>     However one thing I've always found is that the glide time in linear
>     mode seems subjectively shorter than the glide time in exponential
>     mode. In other words, if you set the linear glide time to some medium
>     value, then flick the switch to exponential mode, the glide becomes
>     very long. This of course is due to the fact that linear glide times
>     depend on how far apart the notes are, whereas expo glide takes the
>     same amount of time no matter how far apart the notes are. Therefore
>     in expo mode it can be thought of that the notes are always the
>     maximum distance apart, no matter how close they are to each other.
>
>     The only way around this I can think of is to use a DPDT switch and
>     switch in a larger capacitor when linear mode is selected. any
>     thoughts?

... by inserting two diodes that limit the maximum charge voltage difference 

to 0.6V in linear mode. It works great; the time constant in linear mode is 
the same
as in expo mode if the notes are 6/10 of an octave apart, which is a
very good compromise, IMO (I started with two diodes in series, but
I like the 0.6V better).
Well, I haven't invented the trick, but I borrowed it from some Sample
and Hold circuits, but  it works fine for Portamento!

ASCII-Drawings: Those which I receive from others are always fucked
up and barely readable (wich is my text editor's fault, I think), so I guess
others would have problems with mine as well.
But I could draw it in Spice and post the netlist in ASCII format (;->).
Or send a GIF to those who are interested. Or send the GIF to you, Gene,
and if it's different enough from yours, we could put it to Music Machines.

JH.