Synthesis Essentials: Know Your Filters

The filter is arguably the element of a synthesizer that does the most to define its character. Words like squelchybrassycreamygritty and so on are often used to describe their impact, and reading those adjectives in this context probably made you think of a few specific synths. Perusing the various offerings in both the fixed architecture and eurorack markets presents a perplexing plethora of jargon. To make matters worse, many of these terms overlap, as manufacturers will often use different names to describe the same feature. So what makes all these filters different and how does that affect the way a synthesizer will sound?

The Basics

First, let’s brush up on the basic filter types and attributes. The low pass (sometimes referred to as high cut) filter is the most common type of synthesizer filter. As its name suggests it allows low frequencies to pass through, which can be useful for taming an abrasive top end. High pass (or low cut) filters do the opposite, letting the high frequencies through while cutting out the more rumbly tones. Mix a high pass with a low pass and you get either a band pass or a notch filter, depending on how they are arranged. A band pass allows only a narrow band of the frequency spectrum to pass through while the rest is filtered out. Notch is the opposite, filtering out only a narrow band of the frequency spectrum. A coffee filter, on the other hand, separates the ground beans from your coffee (I threw that one in just to see if you were paying attention).

There is also the more rare and mysterious All Pass filter. If everything is allowed to pass through then what exactly is filtered? In this case the entire signal is passed through but shifted in phase and blended with the original signal. At the filter cutoff point the phase shift is ninety degrees. The resulting phase cancellation causes an effect that sounds similar to a phaser or flanger. Similarly, comb filtering, which is the term for the actual result of a phaser, flanger, or chorus, is caused by blending the original signal with a slightly delayed version of itself which creates a series of notches through phase cancellation. The length of the delay determines where in the frequency spectrum the notches occur. Varying the amount of the delay time is what creates the swishy jet plane effect.

The cutoff frequency is the point in the spectrum where the filtering takes effect. Resonance is an emphasis added through feedback at the cutoff frequency. Sometimes the filter resonance parameter is labelled as Q, standing for Quality. In some cases adding resonance causes a dip surrounding the resonant peak. Some filters are able to create enough feedback that they produce a tone without any input signal at high resonance settings. These are referred to as self oscillating filters because they can effectively function as sine wave oscillators.

The number of poles, or slope of a filter determines how steep or gradually material is filtered beyond the cutoff frequency. Each pole represents a reduction of six decibels per octave, so a four pole filter would have an attenuation of 24db per octave.

While most synth aficionados are familiar with the above types of filters, they are also aware how much a low pass filter can vary from synth to synth, or even between different versions of the same model. Now that we’re on the same page, let’s take a look at some of the different architectures used to create some of our favourite filters.

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Review: Pittsburgh Modular Lifeforms Percussion Sequencer

The Eurorack realm has no shortage of sequencer offerings, and for good reason. They can generate the pulse that drives an entire system, bring a patch to life with modulation, provide the rhythmic foundation to support your patch, or whatever else you can conceive. Because they are so critical to so many systems, and because there is such a diverse range of approach, the market still does not seem saturated. However, I imagine it’s becoming increasingly difficult for manufacturers to make sequencers that stand out by offering something unique and exciting. Pittsburgh Modular adeptly pulls this off by creating a relatively small, hugely functional, performance ready, four channel sequencer.

In the case of the Lifeforms Sequencer, the unique and exciting aspect begins with the four VCAs. The concept is that because modular systems are generally adept at producing noisy signals (both intentionally and unintentionally), why not shape that raw noise into percussion rhythms? Each channel of the LPS has both an input, output, and an envelope output with control over Decay. The envelope out can also work as a basic impulse similar to a kick drum. This arrangement offers a great deal of flexibility because you can rhythmically gate a noise source, trigger and sync with other modules, send percussive envelopes, generate kick tones, or any combination of the above from one place. Combine that with the ability to save 32 patterns and you have a powerful little sequencer indeed.

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Warming the Winter With Belizean Heat

At long last I’m excited to say that I can start sharing Belizean Heat, the follow up to Sonidos de Cuba! I’m really pleased with how this album turned out. It’s quite eclectic, stylistically spanning from ambient and downtempo to progressive house and synthwave, but manages to be cohesive as a whole. It features a wide array of acoustic and electronic instruments, including trumpet, trombone, melodica, xaphoon, shakers, hand percussion, electric guitar and bass, Sonic Forest, Arturia MicroBrute, Elektron Octatrack Analog Four and RYTM, MFB Kraftzwerg, Dave Smith Poly Evolver, Novation Nova, and modular synthesizer.

Belize is a fascinating country. The coast is lined with coral reefs and mangrove islands home to vibrant aquatic life. Inland you can find lush jungles and Mayan ruins. While visiting Belize I captured recordings of birdsong, crickets, underwater reef crackling, a massive colony of grackles, and a persnickety naval radio. These recordings then provided the foundation and inspiration for the nine songs of this album.

The album will be officially released on February 1st, but in the mean time you can hear a short preview of a few of the songs in this video:

Befaco Crush Delay v2

I usually try to avoid using the word “gnarly”. For me, it will be forever wrapped up in California surf and skateboard culture, drawing memories of Pauly Shore, grunge music, and mushroom cuts. However it must be said: The Crush Delay from Befaco is definitely a gnarly sounding module. Delay is only half of the equation because the module is also a circuit bent digital mangler. As noisy and glitchy as this guy can get, the feedback path thankfully features automatic gain control to protect your ears and speakers.

Along with the satisfyingly big knob to control the delay’s speed, the crush has knobs for feedback, delay input, return level, and wet/dry balance. There are also three switches to engage the circuit bent behaviours and these parameters are all controllable via CV. Interestingly, the speed knob functions like a classic delay clockwise from the 1 O’clock point, while counterclockwise it enters the domain of digital distortion and bit reduction.

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Review: Mutable Instruments Rings

Rings from Mutable Instruments is a physical modeling resonator eurorack synth module with a healthy dose of joyful surprises lurking in its patch points and parameters. It’s designed to mimic the resonant behaviour of vibrating bodies like metal bars, plates, strings, and more, with the ability to blend and bend their attributes. Controls are available for Coarse Frequency, Structure, Damping, Brightness and Excitation Position. Each of these parameters has an input with a handy attenuverter to control the amount and polarity of external modulation, making it easy to integrate with other modules.

A while back I had a look at another resonator module from 4MS (you can read my thoughts here). The 4MS Spectral Multiband Resonator works as a collection of six bandpass filters tuned to selectable frequencies with glassy ringing resonance at high Q values. If you’re asking which one to get, I’m afraid in my case the answer was both.

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