[sdiy] Buchla 257 is an extremely strange circuit

[John Loffink]

Link to Wavemakers PWM filter docs:

http://www.wavemakers-synth.com/822pg3.html

John Loffink wrote:
> 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