THE ANALOG FOUR MKI/MKII, AND ANALOG KEYS FILTERS
4-POLE LADDER FILTER
There is no way to turn the ladder filter completely off, but turning up the cutoff frequency will pass all frequencies through. It should be noted that zero resonance does not give the flattest frequency response – instead, it drops some decibels both in the treble and in the bass. The flattest response is reached when resonance is somewhere around 25, which is also the default value.
2-POLE MULTI MODE FILTER
There is no way to turn the multimode filter completely off, but if no filtering is desired there are a number of good ways to let all audio pass through unaffected by the filter:
HP2 at min frequency, no resonance.
BS at min frequency, no resonance.
LP2 at max frequency, no resonance.
PK at max frequency, no resonance.
PK at min frequency, no resonance. Note: this gives an inverted signal.
LP2 This 12 dB/octave lowpass mode attenuates frequencies above the cutoff frequency with twelve decibels per octave, similarly to most other classic 2-pole VCFs. The amount of resonance determines how much the frequencies around the cutoff frequency will be boosted. The sound of this filter is cleaner than the ladder filter, and their different sonic characters are both worth trying out. If combined, they can add up to an extraordinarily steep 6-pole filter.
LP1 This unorthodox 6 dB/octave lowpass mode has a less steep attenuation slope above the cutoff frequency than the LP2 mode. More high-frequency content will thus be retained, making the filtering less aggressive. The slope is gentle like a 1-pole filter – which normally cannot have resonance – but works in essence like a 2-pole filter with a resonance peak. The amount of resonance determines how much the frequencies around the cutoff frequency will be boosted. The LP1 mode is useful for equalizer duties without resonance, or for adding a resonance with just a hint of lowpass softness.
BP The bandpass filter gradually attenuates frequencies above and below the cutoff frequency with a slope of 6 dB/octave. The resonance setting controls how much the frequencies around the cutoff frequency will be boosted. The bandpass filter is good for isolating a sound in the frequency spectrum, making it easier to blend with other sounds in a mix.
HP1 This unusual 6 dB/octave highpass filter attenuates frequencies below the cutoff frequency with a less steep slope than the HP2 mode. The amount of resonance determines how much the frequencies around the cutoff point will be boosted. Besides ordinary highpass filter effects, the HP1 mode without resonance is useful for removing some of the bass of a sound, which in turn is helpful when you want to avoid a muddy mix.
HP2 This is a classic 12 dB/octave highpass filter, attenuating frequencies below the cutoff frequency. The filter slope is steeper compared to the highpass 1 filter, thus filtering out more low-frequency content. The amount of resonance determines how much the frequencies around the cutoff point will be boosted. A highpass filter is suitable when creating lead sounds or hi-hats.
BS The band-stop filter, also known as a band-reject or notch filter, works as an inverted bandpass filter. Signals around the cutoff frequency will be attenuated the most, while frequencies above and below the cutoff frequency gradually will become less and less affected. The higher the resonance, the narrower the filter notch will be. This means that in contrast to the other filter modes, the effect of the resonance parameter is greatest at a low setting, giving a wider notch. This filter shape is useful for both basic equalizer duties and unique filter effects. For example: sweeping the cutoff frequency with an LFO gives a phaser-sounding filter effect. Controlling it with an envelope while at the same time using the ladder filter gives a more complex filter sound. Static filter settings can give a somewhat acoustic resonant quality to sounds, especially transient-heavy ones.
PK The peak filter passes all the sound, boosting frequencies around the cutoff frequency. The higher the resonance, the more they will be boosted. A peak filter comes in handy when you want to highlight a certain characteristic of a sound, by for example adding more low end or more mid end. It can be used either statically, as the peak of a parametric equalizer, or dynamically, controlled by for example an LFO or envelope.