Xpander/Matrix12 filter design

[J.D. McEachin]

On Thu, 27 Mar 1997 jwilmin at hzsbg01.nl.lucent.com wrote:

> In the Oberheim Xpander/Matrix12, the cem3372 vcf/vca chip is used, right?
> This is however a 4-pole low pass vcf. How did uncle Tom manage to make all
> those other filter types with this device (high pass, band pass, combinations)??
> Or did they use other chips (or discrete designs) in addition, which I don't
> know of.
> 
> Anyone who knows how this filter section is designed??

I don't think Tom Oberheim had anything to do w/ the Xpander.  If I
remember correctly, Marcus Reilly did the filter design.  The 3372 is not
a 4 pole lowpass vcf/vca chip, it is a chip that has an exponential
converter linked to four tranconductance stages, and another separate
transconductance stage (or is it two? - my memory on the exact features of
the 3372 is a bit foggy).

The most obvious application is as a 4 pole lowpass VCF with VCA, but I
suppose there are other things you could use it for besides filters. 

In the Xpander/M12, CMOS switches are very cleverly used to switch around
the signal path and component values, which changes the filter
coefficients and thus the filter types.  It's all written up in either the
Xpander or the M12 Service Manual (but not both, I don't think). 

FYI "Uncle Tom" is a derogatory racist term in the US.

JDM