[sdiy] outputing max of several signals

[scottnoanh at peoplepc.com]

Back when I was building my dual four channel mixer module, my idea was to
put in a "min/max" feature into one of the mixers and provide peak, trough,
inverted peak and inverted trough signals out of dedicated outputs of the
module.

I chose Ken Stone's Analogic design as the basis for this function.  When I
breadboarded the idea, I was at first quite surprised to find that, with one
input, all I could manage was half wave rectification of the input signal.
I'd forgotten to take into account that, for the peak output, ground was a
higher potential than -V and for trough, ground was a lower potential than
+V!  So, when only one input was present, the three other inputs to the
"min/max" were at ground (or 0V) potential.  Of course, this was true even
if one had up to three signal inputs with one input unused - that one
uncomitted 'ground' level assured the "min/max" would neither go below 0V
(peak)  or above 0V (trough) as long as that unused input was there.

I preserved the original idea by using stereo input jacks for that mixer.
The inputs to the peak portion of the circuit were normalized at -V well
below any expected signal, and the inputs to the trough portion of the
circuit were normalized to +V well above any expected signal.  When a plug
is inserted into a jack, the stereo lug grounds this extra input, centering
the input signal around 0V.   This works really well, and I have this very,
very useful peak/trough function built into the module.

I recall getting into a tussle about the usefulness of the function back in
the days I was subscribed to AH - the person on the other end declared that
"min/max" was a fairly useless feature, didn't provide much more than a
standard mixer, and was only available as function because "it could be
done".  I find it is much, much more useful than that.

One of the many nice functions for it is mixing two EG's together - you
don't have to worry about the two signals adding together in amplitude -
it's more like superimposing one over the other.  Same goes for mixing
waveshapes in it.  Combining LFO's with it is a gas.  I use it for
rectification, and have even created hyperbolic triangles out of sines with
it in a pinch (with a little bit of processing from the other functions of
the mixer).   However, I may have to stop doing that and build something
that some oddball engineer designed and put in EDN recently.....

Cheers,
Scott



----- Original Message -----
From: "Paul Perry" <pfperry at melbpc.org.au>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, June 21, 2006 7:08 PM
Subject: Re: [sdiy] outputing max of several signals


> Back in the analog computer days, one simply ran
> the two signals via a diode each to the point where you
> wanted the maximum.
> (reverse diodes for minimum).
> The thing to watch for audio, is that it is the max of the
> two voltages *relative to ground* that you are getting, so if
> you are running audio in, you need some way to centre
> the signals about ground (again, if that is what you want).
>
> paul perry melbourne australia