North Coast Synthesis Ltd.

Great Fugue in G minor/MSK 007 Leapfrog filter demo

Fugue from "Prelude and Fugue in G minor" (BWV 542) by J. S. Bach, stripped down to four voices multitracked to demo the first prototype of the MSK 007 Leapfrog filter.

First thing to listen for: the treble. There's very little treble, creating a tone that might be described as "warm" or even "fuzzy" (though it's totally different from a guitar fuzz tone). That's one of the distinguishing features of this filter: it has an extremely sharp cutoff, much sharper than other synth filters, giving a unique sound. On the technical side, that comes both from the fact it's a five-pole filter (two is the most common number of poles in synthesizer VCFs) and that the poles are arranged for a near-elliptic response curve, with a main slope near the cutoff much sharper than would normally be expected for the more common Butterworth response even in the five-pole case. Frequencies above the cutoff, especially in the audio range, are almost completely blocked. Some digital filters are even sharper (such as the "Bat" card for the TipTop Z-DSP), but I don't know of another analog filter comparable to the MSK 007.

For demo purposes I've applied the Leapfrog to every voice in this recording, which really highlights the sharp-cutoff effect, but in a real finished track, you'd probably want to use it on only some voices, allowing others to stand out in the treble. This filter allows building up fat analog sounds in the midrange without creating the indistinct mud that you sometimes get when mixing too many voices with other filters.

Another point worth noticing: timbre of each voice remains consistent across a wide pitch range. Especially listen for the rising bass line from about 2:15 in the recording; as the pitch goes up, the general sound of the voice doesn't change. That's an effect of this filter's V/oct tracking. Of course it's not the only synth filter that can track, but I like to think the tracking on this one is more accurate than most.

The patch is pretty conventional, with a single sawtooth wave going into the filter and separate ADSR envelopes for amplitude and filter cutoff. The Leapfrog's built-in VCA is being used to apply the amplitude envelope. On one voice I inverted the filter-cutoff envelope through the inverting channel of an MSK 008, and on another, I used an MSK 010 sine wave through the quantizer in the MSK 008 to create a bit of stepped, textured modulation for the filter cutoff, just barely audible in the longer bass notes.