Mutator reviewed by David Mellor
The Mutator is basically a two-channel low-pass filter, with LFOs to modulate the cut-off frequency and envelope followers to allow the envelope characteristics of one sound to be superimposed upon another. The filter really is the essence of the Mutator. Basically, it is simply the filter circuitry of a traditional analogue synthesiser brought up to modern standards of noise and distortion performance. As simple as that, and in fact you could think of Mutator as an analogue synth without oscillators just plug in your own sound source. It is a low-pass filter, meaning, as you already know, that high frequencies are attenuated, in this case with a slope of 24dB/octave. Above the cut-off frequency in a 24dB/octave filter, as the frequency doubles, the output voltage is reduced to a sixteenth. This is the first and major difference between this and standard EQ. With conventional equalisers the slope will be a mere 12dB/octave or 18dB/octave, which reduces the levels of higher frequencies but still leaves them dible. A slope of 24dB/octave chops them off with an axe. The result is that you can input a signal with an irritating fizzy high end and reduce the cut-off frequency to leave only the useful components. With a 24dB/octave filter the result can still be sharp and incisive whereas, with a 12dB/octave or 18dB/octave filter, by the time you have eliminated the fizziness, the sound will just be dull. Another difference between the Mutator's filter and the filter you would find on a conventional EQ is that, where the conventional designer would only allow you to filter frequencies down to, say, 2kHz with a low-pass filter, the Mutator goes all the way down to subjectively nothing at all - the cut-off frequency is so low that the only signal left is a vague rumbling in the distance. Don't conventional EQ designers trust us?
Although a simple 24dB/octave filter is a powerful tool, analogue synthesisers commonly have a resonance control too, and so does Mutator. The resonance control sets a certain amount of boost just below the cut-off frequency. This is, in fact, more of a synthesis tool and, even for Extreme EQ, you would only need to use a fraction of the boost that is available. But who knows what people might choose to do with it given the chance. Conventional EQs never have a resonance control like this, and the Q control of a parametric just isn't the same thing. At this point you might be thinking that this is all very well, but why does the Mutator only have a low-pass filter; why doesn't it have high-pass and band-pass too? The answer to this is that low-pass is the function you will probably need most; band-pass is available (hopefully with variable Q) on your console already, and you already know how to flip a low-pass filter into high-pass mode without too much difficulty. Don't you?
Although the LFO and envelope follower functions of the Mutator are not strictly necessary for Extreme EQ, they are still useful to the sound engineer (and even more so to the creative musician). Once you have found a useful low-pass filter setting it is a worthwhile bonus to set a small amount of LFO modulation so that the cut-off frequency isn't static but changes over time. This makes the sound just that little bit more interesting than it would otherwise have been. LFO modulation can be applied to the level too, if you wish. My feeling on the envelope follower is that it is best used with an external trigger that matches the rhythm of the music, a drum track for example. You could then apply the envelope of the drums to a pad or continuously sounding instrument in a similar way to triggering a noise gate from an external source, except that Mutator is more versatile. Those who wish to take Extreme EQ to the ultimate will probably also take advantage of the optional MIDI input which can control a number Mutator's parameters, including the ability to allow the cut-off frequency to follow MIDI note number.
LOW-PASS INTO HIGH-PASS
As mentioned in the main text. it is possible to turn a low-pass filter, such as that of the Mutator, into a high-pass quite easily. Connect the signal to be filtered directly to one channel of the console and parallel it through the filter to another. Mix both together at the same level with the phase of one channel (either one) inverted. With a neutral setting on the filter no signal should be heard since the two identical but opposite phase signals cancel each other out but, if you bring down the cut-off frequency on the filter then, on that channel, no high frequencies will be present therefore they will not cancel. The result allowing for a little unpredictability due to the phase characteristics of the unit as a whole, is a variable cut-off frequency low-pass filter.