[sdiy] What's going on in the M12 filter circuit?

[DJ Marmalade]

Hi All,

Well, I tried to have this discussion on AH but it seems AH is more
interested in arguing with one another than discussing technical issues.

Playing the devils advocate, perhaps it's not worthy of discussion,
nevertheless, if you know what's going on, I'd appreciate some input, reply
privately if you don't think it should be on list.

In the M12 voice schematic, seen here :

http://www.planetp.org/xpvoice.jpg

The 3372 LPF is used as a multimode filter.  The circuit on the right
apparantly sums at different levels, the signals available from the filter
caps.

I'm interested to know what is going on here, why does this work?  I also
want to know if there is something unique about the 3372 or could this idea
be applied to any number of filter chips, say for example, the 2044?

Thanks
Daryl