AW: Don's interpolating scanner

[jhaible]

> I've *finally* finished the article I was writing about my
> interpolating scanner design.  Enjoy...
> 
> http://www.till.com/articles/scanner/index.html
> 
> (It was heavily inspired by Juergen's work and benefits from
> at least a couple of his suggestions.)
> 
> The basic idea is to build an N-way diff amp circuit to drive the
> current-controlled amps, taking advantage of the symmetrical
> exponential transfer characteristic for a smoother transition between
> the inputs.  It also has a pretty low parts count.
> 
>   -- Don

Congrats to Don for further improoving the Scanner idea !
Now I hope you will soon come up with a ready circuit for the full
Hammond Chorus emulation ! (for the list: Don gave me valuable
hints to simplify a general gyrator circuit for this application, but
I never came to really try it.)

I really like the idea of a smooth, and even variable transition. 
Should have a real advantage in waveform generation.
You wrote about transistor matching. Did you simulate temperature
effects as well ? (If there is a temperature effect on the waveform,
one might compensate it with the shape control voltage ...)

I may have mentioned this before, but I also have worked out a
simplified all-transistor / smooth transition version of the Scanner.
There is not much description, but the schematics can be found
in the JH-4 section of my homepage (The filter scanner of the
JH-4 VCA). 
I use two cascaded transistors
for each channel, one driven from the base, and one driven from
the emitter. The design goals were different from Don's, however.
While I also wanted a simplified version, I needed different width
of the separate triangle windows. I can set both end points and both
ramps of each window separately, and I can even have a trapezoid
shape. The big downside is that it's all set by a handful of resistors
that are very different to calculate. (I only got the desired courves
with spice simulation parameter sweeps.) 

Don's circuit is very elegant - can't await the full Hammond Vibrato 
version !

JH.