[sdiy] Buchla 257 is an extremely strange circuit

[John Loffink]

PWM was a very early method of amplitude control that, while never 
widespread, seemed to lose favor with the advent of OTAs. You would be 
most likely to find it documented in 1960s to early 1970s era textbooks 
or data sheets.

The Wavemaker 822 Integrated Processing System, which is part of the 
Wavemaker 4 and designed around 1974, details PWM circuitry for its LP, 
HP and Phase Shifter filters.  However, actual units I have examined use 
more conventional OTA based filters.  It is possible that Terry Kincaid 
did initial versions with the PWM method, then moved to OTA versions, 
probably due to superior audio quality.

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
> _______________________________________________
>

-- 
John Loffink
The Microtonal Synthesis Web Site
http://www.microtonal-synthesis.com
The Wavemakers Synthesizer Web Site
http://www.wavemakers-synth.com