The CGS757 Bi-N-Tic Filter is a strange mixture of a VCO and a switched capacitor filter based on the filter presented by Jan Hall in Electronotes. If you are after something different, this fits the bill. After all, how many filters have a sync input?
A little on how it works:
This filter is based on the original idea by Jan Hall, as presented in Electronotes EN92 P14- 15, which was in turn based on an idea in a 1974 issue of Electronic Design on a Biquart (twice the order) filter.
A VCO drives a pair of analog switches, switching two banks of eight 47nF capacitors across op-amps (effectively creating multiple integrators), and this a multi-passband or comb filter is created at f0, f1, f2 etc.. The overall filter configuration itself should be easily recognized. Two controls are provided within the structure of the filter - damping and bandwidth. Not all combinations of these two are actually valid, some resulting in silence, but none the less, quite an array of variations is possible.
The VCO section uses the well known ASM1-VCO as originally described in Electronotes ENS-76 VCO Option 1. This core was an obvious choice due to the amount of information, lists of suitable substitutes and so on that are available on the web. In this case, a cutdown version is used as its purpose was simply to drive the analog switches. The VCO is connected via a signal processor to the input of a 4024 binary counter. The first three divisions are used to drive the address lines of the 4051 analogue switches.