Filters operate on frequency rejection; however, this method of subtractive processing has interesting effects when combined with resonance, drive, sidechain and/or modulation.
Filters manipulate frequency content based on their shapes…low pass, band pass, band reject (notch) and high pass.
Each shape has a particular characteristic used for various reasons.
Filters can clean sounds up or enhance a particular band of frequencies for creative sound design.
When should I use a filter?
Removing low frequency rumble – HPF
Common with poor recordings due to outside ambience or mechanical pick up from appliances, central air systems and vehicles on the road.
Creating movement in sustained sounds – any available shape
With the use of a resonance boost, the frequency cutoff parameter is accentuated creating a tonal characteristic depending what band of frequencies the filter shape is affecting. Combine with LFO modulation for evolving sounds.
Removing brightness – LPF
At 10kHz and above, frequencies can be removed to darken a sound. Adjust the slope for subtle or more drastic effects.
Creating depth – HPF
When a source of sound moves farther away from our ears, we experience a greater loss with low frequency transmission. Recreating this phenomenon will push a sound away in the mix. Conversely, to bring a sound closer to your proximity, reintroducing low frequencies will simulate that.
Generating genre specific techniques – any available shape
Filter shapes were largely experimented with upon initial introduction to the music industry. Synthesizers are often sought after due to their filter circuitry and every circuit has its own character. Some synthesizers and their filters were used to create specific genres and the popular methods of filtering are the ones that defined the genre itself. For example, using a modulating band pass filter on long bass textures can create sounds associated with genres like ‘Nuero’, a form of hybrid dubstep. The acid music genre utilizes a very high resonance value on a modulating HP/LPF which is a distinct sound almost exclusively associated to the genre.
Changing the height of a sound – HP/LPF
In terms of stereo imaging, frequency can create a sense of height. Low frequencies typically feel low in height while high frequencies feel higher in the stereo field. Rolling off the low end can raise a sound and conversely rolling off the top end can bring a sounds height perception lower.
Shifting energy – HP/LPF
Utilize the characteristic of certain frequencies to create a particular feeling. For instance, low frequency can be used for power and size while high frequency can be used for energy and excitement. Think about the standard DJ build-up with a filter on a mixer; the HPF continually rolls off the low end and upon the drop, the filter returns all the low frequency information to create a powerful impact.
Filter Circuits
Most of the circuits available are modelled after analog synthesizers providing familiar textures and tones of the analog world.
- CLEAN: This is a high-quality, CPU-efficient design that is the same as the filters used in EQ Eight.
- OSR: Models the filter on the classic British OSCar monosynth introduced in 1983 by Oxford Synthesiser Company. This circuit is a state-variable type filter where the resonance gets limited by a hard-clipping diode.
- MS2: Models the filter on Korg’s MS-20 monosynth introduced in 1978. This circuit option uses a Sallen-Key design and soft clipping to limit resonance.
- SMP: This circuit is a custom design not based on any particular hardware. It shares characteristics of both the MS2 and PRD circuits.
- PRD: Models the filter on Moog's Prodigy monosynth introduced in 1979. This circuit uses a ladder filter design and has no explicit resonance limiting.
Using the drive parameter with these circuits gives the sound a specific harmonic distortion modelled after the listed above. Circuitry has a great effect on how distortion is applied to a signal.
Modulation
LFO
Synchronized LFO modulation on the frequency cutoff can help to create movements in static sounds while keeping the movement locked to a tempo.
A popular method of LFO modulation is a LPF on a distorted bass note. While the note is sustained the LPF cutoff will sweep through the harmonic series providing dynamic movement while avoiding the fundamental frequency that is carrying the tone itself.
Envelope/Sidechain
The sidechain may be enabled to have the frequency cutoff react to an external signal. This operation is identical to the sidechain compression method except you are manipulating frequency content rather than overall amplitude.
Removing frequency content with the capabilities of digital filters has never been so fun!