Future Music

Filtering, resonance and delay

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How can the modern producer or sound designer go further with filters? Cytomic’s Andrew Simper gives us an overview of the various filter types, and how they can be used practicall­y. “It’s standard practice in mixing to keep different elements of a mix apart from each other in terms of overlappin­g frequencie­s, as this keeps them sounding clear. A clean, EQ-type high-pass and low-pass filter are especially useful for this, since this is exactly what they’re designed for. A new dimension can be added by using non-linear filters as exciters. Nonlinear, synth-type filters tend to add the most distortion at the cutoff frequency, so you can use them to not only keep frequencie­s bounded, but also add some sparkle at the same time.

“For mixing, I find the most useful filter is a non-linear resonant high pass filter. You get some drive at the cutoff, but you can also add some bass boost with resonance to keep the low end sounding big. This is in addition to cleaning things up by removing frequencie­s lower than the cutoff like any high pass filter. A subtlety of the non-linear nature of the filter means that if the signal is low in amplitude then more boost is added by the resonance as there is more headroom. When the signal is louder it starts to fight against the resonance, which does two things: firstly it limits the actual bass boost gain that can happen at the cutoff frequency, and secondly it distorts the signal more which makes it brighter. These both add up to keeping the things sounding big in the mix, while also keeping the dynamic range and mud down.

“A non-linear low-pass filter is also very useful, and can be used on thin-sounding material to make it warmer and more intimate. Set the cutoff up quite high, around 14-16 kHz usually works, as this cuts the fizz but isn’t too intrusive. Adjust the drive of the filter so it gently adds some overtones to fill back in the top end of the spectrum. Since there is most distortion at the cutoff frequency this works wonders to get things sounding cohesive and rich.

“Band-pass filters are pretty severe a lot of the time, but are good for special effects like turning noise into big whooshing sounds, or adding motion to high hats or leads. Another good use is to make something previously big in the mix sound thin, which adds contrast - not everything needs to dominate the mix! The core of a bell EQ is actually a band-pass filter which is added to the input signal with some gain, the Q is the resonance.

“Notch filters cut the signal at the cutoff, so are mainly used in engineerin­g circles for removing unwanted hum, but can also be used to create motion in a much more subtle way than a band-pass. Multiple notches with their cutoffs spaced apart sound similar to a phasers, which are actually another form of filter called an all-pass. “Remember that talk at the start about the phase of a filter? This comes into play here, because all-pass filters only change the phase, but do not attenuate any of the amplitudes of the input frequencie­s. Changing the phase is just adding delay to each of the frequencie­s of the input, but each frequency gets delayed by a different amount. A regular delay line delays all frequencie­s by the same amount, which is called “linear phase”. An all-pass filter delays each frequency by a different amount, which is called “non-linear phase” and in some cases “minimum phase”. A phaser is formed when you cascade multiple all-pass filters and then add this delayed signal back to the input. Some of the frequencie­s will be boosted, and some cut. If you also use feedback you can generate resonance, and can make the cuts and boosts more pronounced.

“A delay line is a special case of filter. It stores energy, and if you feed back the output to the input you can generate resonance. The main difference is that IIR filters delay each phase by different amounts, and so form low- and high-pass filters when cancelled. Because delay lines delay each frequency by the same amount, you get a series of evenly spaced notches and boosts, and this type of filter, with its regular spacing, looks a bit like a comb, so it’s called a comb filter. Delay lines combined with all-pass filters are the two core components of reverbs, both in analogue and digital.”

 ??  ?? Andrew has been responsibl­e for some of the most lauded digital filters
Andrew has been responsibl­e for some of the most lauded digital filters

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