# Moog Ladder Filter

Yesterday I implemented a Reaktor model of the Moog Ladder filter, using the techniques outlined in the second half of this paper. It consists of a 4 one-pole low pass filters in serial, and has some more flexibility than the Reaktor factory ladder filter – allowing for bandpass, highpass, and notch filters as well as the low pass outputs (although the paper is unfortunately a little vague on how to achieve this, I’ll point the way as best as I can).

The P, Res and Audio inputs can be used as in any other filter module. A, B, C, D, and E are gain coefficients that can used to determine the filter type. There are a few simple setups that can be used for some basic filter types:

Type A B C D E

1-Pole Low Pass 0 1 0 0 0

2-Pole Low Pass 0 0 1 0 0

3-Pole Low Pass 0 0 0 1 0

4-Pole Low Pass 0 0 0 0 1

Bandpass 0 0 0 -1 1

High Pass 1 1 0 -1 -1

The LP filters are easy to derive because of the structure of the filter. The BP and HP filters were found by trial and error using the ezFFT Filter Analyzer and may not be ideal. I was able to find some notch filters but their values seemed dependent on the frequency cutoff.

One intriguing possibility is the ability to morph between different filter types in real time.

I hope you all find this useful. Tomorrow I’ll be posting the bandwidth limited sawtooth oscillator from the first half of the paper, and later this week I’ll post a drawable wavetable as requested in the comments. Until then,

Great stuff ! I was wondering (because I am trying to use a bi-polar knob that cross-fades between LP and HP) what are the gain coefficients for bypassing the filter. This is so -1 would be LP, 0 would bypass and 1 would be HP.

Cheers for the great work !