[sdiy] Buchla 257 is an extremely strange circuit

[Jerry Gray-Eskue]

A second thought if you do not like the resistive triangle..

Use three slide pots arranged as the triangle with signals on the points and
ground in the center.
Add a plate with 3 slots at 90 degrees to each slide travel with the
matching wiper through each slot.
Add a central control knob to the plate and allow free movement of the plate
but prevent rotation.

- Jerry


-----Original Message-----
From: Jerry Gray-Eskue [mailto:jerryge at cableone.net]
Sent: Wednesday, December 16, 2009 7:49 AM
To: Synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Buchla 257 is an extremely strange circuit



This approach captures the way I visualize a three way mix. See the diagram
labeled

[Barycentric coordinates (?1,?2,?3) on an equilateral triangle.]

What I would be tempted to try is forget about all the math involved and use
the triangle in physical form.
Imagine:
(1) the diagram as a resistive material like used for ribbon controllers.
(2) Each Point (0,0,1) (0,1,0) and (1,0,0) is the signal injection point for
the three signals.
(3) Add a conductive wiper allowed free movement over the entire area of the
triangle.
(4) Feed the signal from the wiper into low impedance voltage follower.

Bingo! a pure analog mechanical solution of very simple order that should
not be too difficult to fabricate.

- Jerry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Oren Leavitt
Sent: Wednesday, December 16, 2009 2:10 AM
To: sdiy
Subject: Re: [sdiy] Buchla 257 is an extremely strange circuit


Hmmmnnn....

It be interesting to try an analog implementation of Barycentric
interpolation. Just an idea.

http://en.wikipedia.org/wiki/Barycentric_coordinates_%28mathematics%29

- Oren

Derek Holzer wrote:
> And to make it a bit more complicated, what if I wanted to cross-fade
> between *three* signals, lets say x, x^2 and x^3? Any ideas out there
> for that one?
>
> D.
>
> Derek Holzer wrote:
>
>> Hi Aaron,
>>
>> my apologies of this thread is dead and buried, but I'm interested in
>> analog computer functions and ways of "interpolating" or cross-fading
>> between them, or between the input signal and the processed signal.
>> This PWM AM sounds rather complicated to implement, would there be a
>> simpler VCA-based solution? Something like the Serge sound processing
>> module with it's voltage controlled cross fader? or am I missing
>> something important in the concept of a CV crossfader/interpolator?
>>
>> Best!
>> Derek
>>
>> Aaron Lanterman wrote:
>>
>>> I know using "Buchla" and "extremely strange" in the same sentence
>>> isn't exactly news, but this is even stranger than usual.
>>>
>>> The 257 is the Dual Control Voltage Processor. It has a means of
>>> crossfading between two CVs by means of a third signal.
>>>
>>> The way it does this is totally odd. There's a fixed frequency
>>> triangle wave oscillator running at 21 kc, and then the controlling
>>> CV is added to that. This is then run through a comparitor, so you
>>> get a pulse wave whose duty cycle is determined by the CV.
>>>
>>> This pulse wave then controls some CMOS switches, set such that you
>>> get a signal that is CV1 part of the time, and CV2 part of the time,
>>> depending on that pulse wave. So depending on the duty cycle, it
>>> spends a particular amount of time at CV1 vs. CV2.
>>>
>>> Then, this goes through a boatload of lowpass filtering to smooth out
>>> things and average the signal.
>>
>>
>>
>>
>
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