[sdiy] Buchla 257 is an extremely strange circuit

[Derek Holzer]

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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