2-Pole Vs 4-Pole Filters
Filters are an essential component within any subtractive synth and paramount to achieving a given sound is a thorough understanding of the guiding principles behind this complex topic. In this article we take a deep dive into the wonderful world of filters, arming you with a plethora of knowledge to set you off with your best foot forward on your sonic journey.
So, What is a Filter?
At its core, a filter is a timbre/tone-shaping device that alters the audio signal from any sonic source, i.e. an Oscillator – think a broad-stroke equaliser set by the sound designer and you are on the right path. There are numerous filter types to help sculpt your perfect sound; common variants include the Low Pass Filter, High Pass Filter, Band Pass Filter and Notch Filter, although it is important to note you will likely encounter more on your sound design odyssey. The good news is, the theory outlined in this article is directly applicable to virtually any filter you may encounter.
Most types of filter specialise in attenuating or boosting a certain range of frequencies set by the user using two principal controls – Cutoff and Resonance.
In a Low Pass Filter, high frequencies in the source audio are attenuated at a position set by the user using the cutoff control. Low frequencies are able to pass through, hence we speak about a Low Pass Filter.
In a High Pass Filter, low frequencies are attenuated at a position set by the user using the cutoff control.
In a Band Pass Filter, high and low frequencies are removed around a frequency set using the cutoff control. High frequencies are able to pass through, hence we speak about a High Pass Filter.
In a Notch Filter, frequencies within a specific frequency range are attenuated, allowing frequencies outside that range to pass through.
Cutoff – pretty straight forward right? But what does its sister control Resonance do? In a nutshell, resonance (sometimes called Q) creates a boost at and around the cutoff point, emphasising the frequencies in a manner that adds brightness and clarity to the sound. Although this isn’t always the case – for example with a lot of Notch filter types, increasing the resonance parameter has the effect of making the notch width smaller, without really boosting anything.
Some synths are bound to one filter type, thus limiting the scope of the sounds that can be created; the good news is, in most modern synthesisers multiple filters are often at your disposal, and their structure within the synth’s architecture can be characterised by one of four broad categories:
- Series – in this format, the filters are organised sequentially with the source signal passing through one filter first and then the other. You can therefore tweak your sound using both filters, however this is often considered limiting in terms of the sound output that can be achieved.
- Parallel – in contrast to filters routed in series, Parallel routing allows the signal to be independently altered by two independent filters, with the signal summed from both filters after the processing has taken place.
- Multimode Filters – in the instance of Multimode filters, users can select the filter type desired and adjust accordingly.
- Morphable Filters – A very expressive filter type where a morph control allows the user to seamlessly adjust between filter types. This allows for a greater degree of accuracy and significantly more sonic options when seamlessly morphing through different filter types and harnessing filters that are otherwise unobtainable with static, non-morphing filters.
What about Slopes and Poles?
No, this is not Skiing terminology! Infact slopes and poles are integral to one another and vital in your understanding of your subtractive synth and how to get the best outcomes from it.
Let’s look again at the filter type illustration above; you will notice a gradient bordered by attenuated frequencies on one side and the un affected frequency content on the other – this gradient is referred to as a slope of the filter and indicates how steeply frequencies designated by the cutoff position are attenuated.
Within synth filters, the scale of the attenuation at the cutoff point and therefore the steepness of the filter’s slope is designated by the number of poles the filter exhibits – you might be thinking this is a mechanical or electrical component. Unfortunately not. The term Poles (in the context of filters) comes from mathematical analysis of a filter’s behaviour. The general trend with synthesiser filters is that the more poles a filter has, the steeper its roll-off / slope will be. As a simple case with a low pass filter, frequencies above the cutoff in a two pole filter will be attenuated less than that of a four pole filter, leading to a two pole lowpass sounding brighter. In this way the pole count is often used as shorthand to let you know how steep the filter roll-off, with each pole commonly adding an extra 6dB of attenuation per octave.
There are some legendary filters out there, and an understanding of their sonic characteristics is fundamental to crafting the perfect synth sound in tandem with the filter theory discussed above.
First let’s take a look at the ladder filter, originally developed by Bob Moog in the mid-1960s. The ladder filter, aptly named as a result of the shape of the circuit, is a low-pass filter with a steep 24dB per octave slope complete with an adjustable resonance control. When adjusting this resonance around the cutoff point, fused with the nature of the input gain, the ladder filter exhibits a sonically gratifying overdriven tone as all the transistor stages clip gradually – a key factor which helped to popularise this filter at its inception, and to this very day.
Similar to the Moog Ladder filter design is the diode ladder filter popularised by the Roland TB-303. Another 24dB per octave filter that replaces the transistor-centric design of the Moog ladder filter, with a diode-based circuit. The results? The iconic squelchy bass lines we all know and love, synonymous with acid-house.
Another filter design that cannot be avoided is the Steiner-Parker filter. Whereas the aforementioned transistor- and diode-based ladder filters functioned as a 24dB per octave filter, the Steiner Parker filter is a comparatively subtle affair slope-wise, with a gradual 12dB per octave slope functioning as a resonant band-pass, high-pass or low-pass filter. A key characteristic of this filter lies in its use of positive feedback where increasing the resonance does not cause the audio to lose amplitude.
From the comparatively tame to the down right aggressive, the next famous filer in our investigation is that of the KORG MS-20. Taking its basis in the legendary Sallen-Key filter design, the MS-20 fused a resonant 12dB per octave low pass filter with a resonant 6dB per octave high-pass filter, routed in series. These 2-pole filters commonly use positive feedback to implement resonance and as such, they rarely lose gain in the passband of the filter – unlike a 4-pole low-pass transistor ladder filter. Self-resonant? Yes! Unmerciful? Absolutely! With the innovative layout and routing, coupled with the filters themselves, a veritable smorgasbord of unique tones could be crafted!
It wouldn’t be fair to omit Oberheim from our famous filter models, specifically their Synthesiser Expander Module (SEM) synths heralding from the 1970’s. These SEM synths utilized a state variable filter featuring a 12dB per octave multimode filter design complete with band-pass, low-pass, high-pass and notch modes effected as a continual morph between the filter types. Characterised by a smooth, transparent sound, this filter couldn’t self resonate but the benefits of the multimode architecture made up for this in true convenience and the expansive scope of sounds that could be crafted.
Next in our highlights are filter circuits incorporating operational transconductance amplifier chips as processing and control components – or OTA based 4-pole filters for short. Think the Roland Jupiter 8, Juno-60 and Jupiter 4 and you should have a good basis for the sonic characteristics of this chip that defined a generation of mainstay synths including the CEM3320 designed by Doug Curtis. When compared to the ladder filters outlined above, the OTA filter is often described as being smoother with less internal distortion and limited excessive noise.
Penultimately, we cant help but mention the Lowpass Gate filter-type commonly found in West Coast synthesiser including the Buchla Musical Easel. It is best thought of as a combination of a Lowpass filter and amplifier, exhibiting the effect of both the cutoff and output amplitude being linked. In this configuration when the cutoff decreases, the overall amplitude lowers and vice versa. Part of the control of the lowpass gate is done using a device called a resistive opto-isolator, which consists of a light source such as an LED and a light dependant resistor linked to the cutoff control. This device when “pinged” with a quick pulse will make the lowpass gate “open” quickly but take a longer time to decay back down. This combined with the dual action of filtering and attenuation can produce fairly naturalistic timbres.
Last but not least, we turn our attention to probably the most outlandishly designed synths – The Electronic Dream Planet Wasp. Sporting an unmissable yellow and black aesthetic, the super affordable (in its day) Wasp featured a resonant 12dB per octave multimode filter; with subtle use, it exemplifies a musical flavour and when pushed adds unmistakable dirt and sizzle to sounds – this is one filter model that has stood the test of time, used in mainstream synths to this very day.
What About Modal Electronics’ Filters?
Turning to our own filters, Modal Electronics takes a multimode stance for both our ARGON and COBALT lines offering creators the most versatile options for shaping the perfect patch. In our wavetable-centric ARGON lines, 2-pole state variable filters take centre stage with 4 Filter types available to select from for extended creative capability – ‘Standard’ filters are based on the resonant filter found in other Modal synthesisers and ‘Classic’ exhibits a more rounded character and a softer resonance response. Turning to the COBALT Series, the discerning synth aficionado is provided with a newly designed, self-oscillating 4-Pole Morphable Ladder Filter with Resonance – perfect for injecting warmth and bite to any sound. You guessed it! You can switch between the four different filter modes, get creative with the Notch filter and form unusual synth textures with the Phaser mode.
And that concludes our discourse into the wonderful and expansive world of filters! You now have all the knowledge at your disposal to harness the full power of any filter you are presented with including those included in our groundbreaking ARGON and COBALT lines!
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