[sdiy] Buchla 257 is an extremely strange circuit

[Oren Leavitt]

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.
>>
>>
>>
>>
>