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Know Your Filters

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Does a Steiner-Parker filter sound different from a Moog filter? What are the various filter types and designs anyway? MARK JENKINS investigates…

Arturia’s new PolyBrute and the earlier MatrixBrute synth both feature a “Steiner-Parker Filter” as well as a more conventional lowpass filter. And just what that means, we’ll explain in a moment.

But what’s the significance in terms of sound, of these and other different filter types?

“Steiner-Parker Filter” is a term invented by Arturia for the Brute instruments, and refers to a rare early synth design, the Steiner-Parker Synthacon. The instrument is expensive when found, and distinguished by having a filter design that was a little different from the norm at the time (it appeared in 1975).

Steiner-Parker’s Synthacon remains a rare and sought after instrument

Steiner-Parker, based in Salt Lake City, offered a modular system, the SynthaSystem, but also built several hundred Synthacons. Designer Nyle Steiner later invented the EWI (electronic wind instrument) and EVI (electronic valve instrument), subsequently taken up by Akai. (He is also a very accomplished and well-known performer on EVI and trumpet.)

Common filters such as those on Moog, ARP, and Oberheim synths in the 1970s worked in more or less the same way. They are lowpass designs – meaning that as the filter’s cutoff point setting is reduced, low frequencies continue to pass, while higher frequencies are removed from the output, tending towards a smoother, softer sound (this sound creation technique is also known as subtractive synthesis).

The extent to which this happens is known as the filter’s “slope.” And this is where things become a little technical. If reducing a filter’s cutoff point by an octave (say down from 10,000Hz to 5,000Hz) cuts the output audio power to a quarter, it’s operating at a 6dB/octave cut (sorry, but decibels work on a logarithmic scale). Twice that power cut, and they’re rated at 12dB/octave. Four times, and they’re rated at 24dB/octave.

The basic Sallen-Key filter design (Wikipedia)

Basic single electronic filter designs typically achieve a 6dB/octave cut, so dual circuits achieving 12dB/octave cut are often known as 2-Pole, while circuits achieving 24dB/octave cut are often known as 4-Pole.

What’s the audible difference between these? Well, the Moog “transistor ladder” filter design (so called because it typically arranges pairs of transistors in a row on the circuit board) achieves a very steep 24dB/octave cut. If you manipulate it either by hand or under the control of an envelope, the change in sound is very dramatic, potentially from very bright to very smooth, very quickly. Sequential Prophet filters (in the 1970’s and today) were similar, although using a single-chip design.

The 1970s Oberheim synth filters, on the other hand, were typically 12dB/octave, so in some ways more subtle, offering sounds which remained brighter and more punchy. Think Van Halen’s “Jump,” which used the archetypal brassy Oberheim filter sound.

That all got very technical very quickly, didn’t it? And we’re not even touching on the way different filters resonate – that is, how they behave when the frequencies around the cutoff point are boosted, resulting either in a very squirty, “peaky” sound, or in fact in a loud whistle as the filter begins to act as a sine wave oscillator (a useful technique in itself).

To get back to the point about the Steiner-Parker filter, this wasn’t the same as Moog, ARP, and Oberheim designs of the time. In fact it’s what’s known as a Sallen-Key design, after R.P. Sallen and E.L. Key, who worked at the MIT Lincoln Laboratory in 1955. This was a relatively simple filter design with one major advantage: changing a couple of components switched the way the filter worked from lowpass to highpass to bandpass.

Waves V-EQ3 audio plug-in

Well, we already explained what lowpass means. Highpass means that the low frequencies are removed and the high frequencies retained as you adjust the filter. Use this option if you want a high, thin sound like an oboe, or high strings that don’t have a deep end to muddy up your mix.

And as for Bandpass – well, both high and low frequencies disappear, with only a band towards the center remaining. Use this maybe for human voice sounds, with nothing too bassy or ultra-high frequency, or use the setting for a very dramatic, “middy” sweeps like tuning a radio receiver.

For practical design purposes, the Sallen-Key filter usually operates only at 12db/octave (choosing component values poorly can also result in too much random noise), so it’s a good partner for a Moog-style filter, the latter fixed at 24dB/octave lowpass, the former variable from 12dB/octave lowpass to highpass to bandpass – a highly versatile pairing. Also, if you push a lot of volume through the Sallen-Key filter it starts to overdrive subtly and then more distinctly, which is another useful sonic option.

Arturia’s Matrixbrute – here in its new Noir Edition – offers both “Steiner-Parker” and “ladder” fiters

On the Brute synths, Arturia arranges the “Steiner-Parker” style (Sallen-Key) and “Moog” style (transistor ladder) filters so they can be used either in series – one operating on the basic oscillator sound after the other – or in parallel, with the outputs from both equally audible. A Master Cutoff knob is also provided to change the basic setting of both simultaneously.

So that’s the filter offering as found on Arturia’s Brute synths. What other filtering options are available on the market?

In the past, a few synth modules have appeared with all these and many more filter types, including some that model the human vocal mechanism known as Vocal Formant filters, which appear to “talk” as you manipulate them. This model and many more can be found on the E-Mu Morpheus module (1993), which boasts no fewer than 193 different filter types, and several other E-Mu modules later also featured this “Z-Plane” multiple filter design.

E-Mu’s Morpheus offered dozens of different filter designs (pic from Matrixsynth)

Modern digital synthesizers such as those from Waldorf can also typically offer multiple filter type options, since these don’t have to be implemented in hardware but simply modelled in software.

In terms of Eurorack, there are many filter modules that either offer the highpass/bandpass/lowpass settings of the Sallen-Key design – these are often referred to as SVF or State Variable Filters – or some other specialized filter function. Zlob has one simply called SVF, and there’s also an SVF-1 from Tenderfoot Electronics.

Koma’s SVF-201 is an SVF design working in a completely different way (it’s based on a Vactrol, a light dependent resistor packaged in with a variable Light Emitting Diode), while GRP had a (now discontinued) model called 12dBSVF. In addition to lowpass, bandpass, and highpass at 12dB, it offered notch, which is the opposite of bandpass – cutting out a selected range of frequencies from the middle of the audio spectrum.

Befaco’s BF22 Eurorack module is an easy way to find the Sallen-Key filter design

For very specific Sallen Key designs available now or in the past in Eurorack, Spanish company Befaco offers the BF-22, which is probably the most readily available currently, Livewire the Frequensteiner, and Elby Designs the CGS735 Synthacon Filter, for example.

I hope this quick tour through some of the obscure and not so obscure corners of filter design gives you some ideas about how to manipulate and keep control over your sounds, particularly with reference to removing frequencies that could muddy up your mix, or boosting those such as bass frequencies, which may be sounding a little weak.

And remember, the best filter for keeping you going through those long studio sessions still remains a coffee filter.

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