bipolar 303 filter

[Heiko van der Linden]

Hi there,

I read that the filter in the tb303 is bipolar. 
Does anyone know: what exactly this means ? 
                             how this is done in the circuit? 
                             and if this indeed gives the characteristic
squelchyness the 303 has?

Also is there a text that goes into the design of diode ladder filters.
(except for the moog patents).

Thanks a ot in advance,
heiko