No Strain, No Gain
May 1, 2006 12:00 PM, By Jason Scott Alexander
Audio filters have come a long way since their earliest utilitarian use in studios to cut out problematic low frequencies and annoying tape hiss. Still, too few of us ever delve deep enough beyond presets to discover the true potential behind everyday filters, and that's a shame. With a bit of tweaking know-how, a simple filter can fix a problematic mix, revitalize an old synth and perhaps even give your next track its signature sound.
FILTER PRIMER
It's likely that most readers know what filters are, but here's a quick crash course on the main differences between the more popular varieties. Filters come in four general classes: lowpass (LP), highpass (HP), bandpass (BP) and band-stop (BS), also known as a band-reject (BR) or notch filter. As its name suggests, a lowpass filter allows low frequencies to pass through while attenuating frequencies above a certain point, called the cutoff frequency. It is sometimes also called a high-cut or treble-cut filter. A highpass filter does exactly the opposite, and a bandpass filter is a combination of the two, passing frequencies within a certain range (or band) and rejecting frequencies outside that range. Conversely, a band-stop/band-reject filter passes all frequencies except for those within a chosen frequency range. When that stop band becomes extremely narrow, it creates a notch in the frequency spectrum.
Filter roll-off, or slope, describes the rate at which the audio level is boosted/attenuated as the frequency is increased/decreased and is typically annotated as a fraction of decibels per octave. You'll hear this intensity being attributed to a number of poles. A single-pole filter has a soft, round character due to its gentle 6-dB-per-octave (first order) slope. Increasing the number of poles can achieve lush results — such as what Oberheim's two-pole, 12-dB-per-octave (second order) filter designs were known for — or it can deliver an intense bite like Robert Moog's classic 24-dB-per-octave (fourth order) four-pole filter cascade did in the Minimoog. It's not uncommon to find extremely steep 48-dB-per-octave or 96-dB-per-octave “brick wall” slopes in today's filter plug-ins.
Even the simplest modern filter design will typically have controls for frequency cutoff and variable resonance around the cutoff peak, called Q. The greater the Q, the sharper the cutoff slope of the filter. An extremely high resonance setting can cause some filters to self-oscillate, which is a form of feedback caused when the filter amplitude increases all by itself until it hits the headroom and stays there, producing a characteristic scream. Complex filter arrangements may consist of many poles of different classes (known as multimode filters), or they can take the form of multiband vocoders, spectral delay lines and denoisers; those, however, are not the focus of this article.
PARALLEL UNIVERSES
Quite often you'll see filters presented as pairs that can be routed in series or in parallel with each other. Naturally, the sonic results of each configuration can be either advantageous or ineffective, depending on your intent. For example, placing a highpass and lowpass filter in series with each other (where the highpass cutoff frequency is set below that of the lowpass cutoff frequency) can effectively create a bandpass filter with independently controllable cutoffs and resonance peaks, which is quite handy. Conventionally placing a lowpass and a highpass in series, however, would nullify and result in complete silence, which is not so handy. In parallel, filters process a signal in an isolated fashion, which can be desirable in creating split treatments such as definable and sweepable band-reject or notch filters.
CHAIN GANG
Another consideration is where to place a filter in an effects chain. Although the only rule to filter placement is that there are no rules, there are a few conventions you can follow. First, filters are typically set as a prefade channel insert with 100-percent wet output so that what goes in comes out completely processed with no dry signal remaining. Of course, nothing says you can't put a filter on an aux send and fold its output back onto the original signal for more subtle effects. Nondynamic filters (those without an envelope following) can be placed anywhere in the effects chain, but envelope-controlled filters (ECFs) should be first in the signal chain, ensuring that they respond to the full dynamics of the signal before other processors alter it. The same reasoning suggests that you place an envelope filter before a compressor, so that it can have the widest range of dynamics to treat. However, placing an ECF after effects such as delay or flanger can be cool because it tends to create a blurred effect as the filter responds to trails of repeated notes or washed-out dynamics. Also keep in mind that filters arranged as inserts will behave serially and in the order in which they are placed.
AUTOMATION
Filters left untouched get boring quickly. That's why you'll want to learn how to automate every parameter on your filter plug-ins before going any further. Aside from real-time automation, most plug-ins come with built-in envelope generators that can be triggered manually or from the audio itself. The simplest approach is to use the envelope generator to control the filter's cutoff frequency and possibly set a threshold, above which the resonance starts kicking in. That works best for staccato sounds because the envelope has clearly defined triggers from which to fire. When things aren't so clear, such as with dense drum loops, making use of your DAW's automation facilities makes sense. Here, it's possible to create entirely complex automation sequences for the parameters and then simply copy them to as many bars as you like.
If your filter can be triggered by MIDI note, you will be able to “play” or sequence the cutoff frequency and resonance settings in time to your material. It's a dance vocal trick as dated as the hills, but it never seems to get old. It's also cool to play a filter's cutoff frequency tracked across the keyboard, much as you would an oscillator. When used to filter white noise from a soft synth, for example, the result is a wonderful, airy sound that would make a perfect ethereal transition or build for trance and dance production.
THIS IS A MODDIN' WORLD
For times when automation isn't appropriate or requires too much work, turn to your filter's built-in LFO modulators. Whether tempo-synched, or not, LFOs are your secret weapon for animated filter fun. Taking the output from both a lowpass and highpass filter, for example, and panning the signals hard left and right and then sweeping the filters from the LFO, you can create rich, swirling auto-panning effects in no time. You can also chop beats and create entirely new rhythms by setting a lowpass filter to be modulated by a square-wave LFO synched to various note resolutions in your DAW.
RINGY DING
Another exciting variety of filter modulation comes in the form of a ring modulator. Your typical lunchbox-variety filter plug-in probably won't have a ring-mod section, but many DAW plug-ins (such as Logic Pro's Ringshifter) and specialty filters (like Waldorf's D-Pole) offer excellent facilities. Synths, guitars and percussion all sound great processed, yielding everything from grungy and metallic noises to spacey sweeps and swooshes, underwater burbling sounds and other sci-fi effects.
A ring modulator works by using the amplitude of one signal (the “ring”) to modulate the amplitude of another. Just as a tremolo pedal rhythmically fades a signal in and out using an LFO sine wave, a ring modulator produces a cyclical change in amplitude whose frequency is way up in the audible range. Because the modulation state is constantly in flux as the amplitudes meander around, that produces sideband frequencies that are the sum and difference of the frequencies present in the two signals at any given time. It may be boring to read, but it's exhilarating to hear.
Though most ring mods provide a built-in oscillator with clean waveforms such as square, sine, triangle and saw, it is particularly interesting to run two soft synths into the modulator — as carrier and modulator signals, respectively — and play different notes on each synth. Alternatively, try feeding only one synth into the ring modulator with the signal from the synth cloned and sent through a digital delay. Now when you play a sequence of notes, the ones you have just played will come back to haunt you and modulate your current note. That works well when the delay is synched to the bpm of your music and sounds spooky with a pitch bender.
FORMING FORMANTS
Some soft synths and filter plug-ins provide what are known as formant filters. Although the term formant is most often associated with human speech, a formant is really nothing more than a fixed resonance in any object that creates a sound, including all acoustic instruments. If you have formant filters in your arsenal of goodies, they can be used to add unique and consistent tones to just about any synth sound or sample loop. For example, you can extract specific vowel sounds from an ethnic percussion loop or key a particular formant color from a piano.
Interestingly, you can roughly emulate a formant filter by assembling two or more resonant bandpass filters in parallel and tune them to specific formants while increasing their resonance levels to taste.
TWIN PEAKS
Early digital synths often boasted raw and completely grating oscillators — perfect for industrial and synth-metal styles but lacking true muscle power in the filtering department. If you have any old, cheap digital synths hanging around (such as a Kawai K4 or a Casio CZ-101), take one and set it to a thin, lifeless sawtooth, square or PWM preset. Then insert an analog-style filter plug-in with two 12dB filters running in parallel (that is what made the Oxford OSCar synth sound so killer back in 1983). Using any additive or FM soft-synthesis plug-in will do the trick as well. Boosting the resonant peaks of the twin filters and then separating their cutoffs through modulation can breathe life into cold plastic again. With more metallic or bell-like PCM programs, adding resonance while detuning and spreading the two filter cutoffs can have extraordinary, liquid results as the harmonics begin to fold in on each other with every pass.
DOUBLE TROUBLE
If you want to lend a growing sense of urgency to a loop or a full mix, try doubling it and placing a lowpass filter with emphasized Q on the original and a notch filter on the double. The lowpass builds a more powerful bottom end around the kick and bass, which you can keep centered, while sweeping a gentle notch across the double can create padlike harmonic movement across the full frequency range. That creates a high-energy, euphoric effect without losing the bass drive. Experiment by opening the notch width as you sweep the frequencies.
Along similar lines, an excellent way of adding more presence to an electric guitar solo or a lead vocal is by mixing a bandpass version of the sound back in with the original. Theoretically, that can be achieved through doubling or by using an aux send (as discussed earlier in this article), although the filter frequency needs to be chosen with care, and level blending is key.
LEFT OF THE MIDDLE
Hard-panning the vocal track has long been a trick of mix engineers for creating momentary special attention. But you can make a vocal noticeable more continuously throughout a song by applying dynamic filtering across the stereo field. Rather than panning a single track, try duplicating your lead vocal a few times and panning each of the newly created tracks to different pan positions from left to right. Now insert a basic filter plug-in on each track and call up whatever filter mode catches your fancy. Sometimes the same filter set to different cutoffs for each track works best for subtle effect; other times, completely different filter types sound good.
In your DAW's arrange page, selectively chop up the vocals across all duplicated tracks simultaneously, and mute (don't cut) the regions you want silenced. Experiment with different filter/pan combinations, and try automating fade-in/fade-outs so that transitions between tracks containing radically different filter settings sound right. Don't be afraid to have more than one track active at a time, as multiple filter types blended together at different pan positions can sound great. Also try macro-splicing (stutter editing) your vocals in key spots and flipping them between the filtered tracks for some really wild, dynamically filtered bouncing effects.
METALLURGY
Though complex multiband processing is usually limited to dedicated tools such as spectral gates and vocoders, working with only a handful of frequency bands at a time can produce easily manageable results along the same lines. Simple multiband filters, such as McDSP's FilterBank, allow you to separate the audio into several frequency bands for separate processing. FilterFreak is unique in that it gives you independent control of frequency, gain, peak, slope and dip. By adjusting those values, you can emulate the response curve of any shelving EQ ever made.
Splitting a signal up in that manner and applying different effects to each band can generate sounds that appear almost resynthesized. Throwing bit-reducers or ring modulators on only specific bands can create metallic, electronic-sounding textures from extremely conventional, acoustic input material. I've even placed as many as six bandpass filters in parallel to create incredible glassy, orchestra-type sounds with the resonance set high.
SHARE THE WEALTH
Way before plug-ins, many hardware synths made their filter blocks available for processing external audio. Most of today's virtual-analog synths also allow that, often providing unique results because you can incorporate the rest of their synthesis engines as part of the sound.
The Access Virus, for example, allows you to tap into its amazing analog saturation module, which adds delicious distorted overtones during filtering. Since the beloved Korg Wavestation EX lacks resonant filters, I often add balls to its wave sequences by combining them with the complementary-sounding digital wave oscillators in the Virus C's Multi mode. With the two synth clocks synched, I can play the Virus and have LFOs or envelopes control filters over both sound sources — as if they were one — for a distinctly hybrid sound. When working with filters, the more unconventional you are, the better it sounds.
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