[sdiy] Buchla 257 is an extremely strange circuit

[Aaron Lanterman]

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