Almost like a sculptor's clay, audio filters possess the ability to shape basic tones. Beyond that, these tragically hip devices can create swelling crescendos and practically define entire genres of club music, such as acid-house and trance. Filters can also perform day-to-day tasks such as eliminating an annoying 60-cycle hum or highlighting a desirable frequency. Yet fundamentally simple as they often are, the terminology associated with filters can sound enigmatic to newbies. For instance, what do the terms lowpass, notch and Q denote? If you own any synth or other piece of gear with one or more filters, it's important to understand what's under the hood and the various terminology. Then you can then boldly preach to your friends while sounding like an electrical engineering sage. Or at least you can get down to the business of creating more dynamic and expressive music.
A MALLEABLE MATERIAL
The electronic filters that we musicians tweak for days are signal processors that are designed to remove or enhance specific frequencies or frequency bands from a given audio signal. The fundamental musical attraction to filters is their frequency-agile ability (usually controlled via one or more continuously variable knobs or sliders), which explains the term “sweep-able filter.” Just like the synthesizers that have housed them for the past few decades, electronic filters can be broadly categorized into two types: analog or digital. Analog filters rely on physical matrices of electrical components such as capacitors, inductors or resistors to weave their magic, whereas digital filters perform algorithmic mathematical functions on signals in the digital domain. The advantages to each are clear: Digital filters are capable of being far more frequency-accurate, whereas analog filters sound smoother because they act on fluid electronic waveforms rather than “staircaselike,” finite samples that represent those waveforms.
The most common varieties of filters, whether digital or analog, are lowpass, highpass, bandpass and notch. A lowpass filter (also known as a high-cut filter) employs a specified cutoff frequency as a threshold, above which a passing signal is attenuated (gradually reduced or eliminated). The names lowpass and high-cut are somewhat self-explanatory: A lowpass filter passes the low frequencies through while cutting the highs. Conversely, a highpass filter (or low-cut, bass-cut or rumble filter) also employs a cutoff frequency, below which a passing signal is attenuated. Simply put, a bandpass filter combines a highpass with a lowpass filter; it is as if the “pass-band” is surrounded by two cutoff frequencies — one for the highs above and one for the lower frequencies below. Finally, the opposite of a bandpass filter is a notch filter, also known as a band-stop or band-rejection filter. If you envision a typical multiband EQ such as those you might find on a modern car stereo, pulling a single band of the EQ down could be thought of as a notch filter. Pulling down all but one band that is situated somewhere in the middle roughly simulates the effect of a bandpass filter.
The degree of attenuation provided by a filter is known as the filter's rolloff and is typically expressed in a decibels-per-octave measurement. Thus, you will often see 12 dB or 24 dB (also respectively known as 2-pole and 4-pole) fixed filters or encounter comparable settings on variable filters. If you have a 24 dB lowpass filter, for instance, that means that all frequencies higher than the cutoff frequency will be attenuated at a rate of -24 decibels (an octave equals 12 semitones). Because the higher numbers represent a steeper rolloff slope, 24 dB filters often have a more pronounced effect on sound than 12 dB filters. A key filter characteristic is resonance. In fact, many synthesizer filters are composed of just two adjustable controls: frequency and resonance. A resonant filter has the capability of enhancing the frequencies immediately surrounding the cutoff frequency; in other words, the target frequencies are played with greater amplitude and are often responsible for a filter's characteristic “whistling” sound. In addition, some filters employ a Q control, which — like the same control on certain parametric equalizers — allows for a wider or narrower pass-band and consequently a smooth adjustment of the filter's rolloff slope. The more you raise the Q setting, the more a diagram of the frequency rolloff would look like a steep cliff instead of a rolling hill.
THE BASIC CLAY
The most fundamental filter type in terms of electronic-music gear is the VCF, a voltage-controlled filter. VCFs use controlled amounts of voltage to accomplish their filtering. A true VCF operates purely in the analog domain and is designed such that its cutoff frequency and Q can be continuously shifted. Simpler versions have a fixed pole, while others sport switchable poles, or degrees of signal rolloff. Simpler VCFs are also fixed in lowpass, highpass or another mode, while more flexible filters boast a variety of user-selectable modes. In tone generators such as synths, the VCF is positioned in the electronic path after the oscillators, which generate the basic tones and harmonics. By tweaking the VCF controls, you can coax the sound module to produce an endless variety of thinner to fatter tones, and by continuously “playing” the controls over time, musicians can create dramatic, sweeping effects. This is the secret ingredient to much dancefloor music. Some examples of gear with famous filters include the Moog Minimoog, the ARP 2600 and the Roland TB-303, which spawned and absolutely defined the acid-house movement.
It's pretty rare that an instrument defines a genre of music. The electric guitar probably defines rock music. Rock also has drums, bass, organ/piano and sometimes even more, but what would rock be without the electric guitar? It's even rarer that just a component of an instrument can have such a huge impact on a musical genre as to define it, but that is arguably what the filter has done for large swaths of electronic music. You owe it to yourself to become intimate with this tool.