[sdiy] Buchla 257 is an extremely strange circuit

[mark verbos]

Aaron,

The Roland VP-330 uses PWM VCAs in the vocoder section. One triangle  
oscillator provides the bias for all the channels. I hear the  
CraneSong compressors use PWM, at a VERY high frequency.
Also check out Grant Richter's Tracking Generator. http:// 
musicsynthesizer.com/DIY/Grant/CVtwister.html

I believe the reason Don used these VCAs is that they track very  
predictably, making a good crossfader. It's a great Buchla module  
with no hard to find parts! (just use whatever OP amps instead of the  
RC4136) You could probably crank up the frequency of the oscillator  
and filtering to get more range in the CVs. I'm building one with  
high end analog switches and I'll try to make it much faster.


Mark





On Jun 11, 2008, at 4:32 AM, 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.
>
> I've seen tricks like this before: you can do AM modulation use  
> this kind of trick - there's even a section on it in the Signal  
> Processing First textbook we use in the sophomore ECE class I'm  
> teaching now; it is also reminicent of switched capacitor filters,  
> and the PWM style of D/A conversion that some microcontrollers use,  
> etc.
>
> However, I don't think I've seen it used quite this way before.
>
> Can anyone think of other instances in synthdom, or anywhere  
> really, where this approach has been used? Advantages  
> disadvantages, using any of the myriad of more traditional ways  
> crossfading between two signals?
>
> - Aaron
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