[sdiy] Buchla 257 is an extremely strange circuit

[Derek Holzer]

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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