EVOC 20 Filterbank

The EVOC 20 Filterbank consists of two formant filter banks. The input signal passes through the two filter banks in parallel. Each bank features level faders for up to 20 frequency bands, allowing independent level control of each band. Setting a level fader to its minimum value completely suppresses the formants in that band. You can control the position of the filter bands with the Formant Shift parameter. You can also crossfade between the two filter banks.

A Short Primer on Formants

A formant is a peak in the frequency spectrum of a sound. When the term is used in relation to human voices, formants are the key component that enables humans to distinguish between different vowel sounds—based purely on the frequency of these sounds. Formants in human speech and singing are produced by the vocal tract, with most vowel sounds containing four or more formants.

Getting to Know the EVOC 20 Filterbank Interface

The EVOC 20 Filterbank interface is divided into three main sections:  the Formant Filter parameters section in the center of the window, the Modulation parameters section at the bottom center, and the Output parameters section along the right side.

Figure. EVOC20 Filterbank window.

EVOC 20 Filterbank Formant Filter Parameters

The parameters in this section provide precise level and frequency control of the filters.

Figure. Formant Filter parameters.
  • High and Low Frequency parameters: Determine the lowest and highest frequencies allowed to pass by the filter banks. Frequencies that fall outside these boundaries will be cut.

    • The length of the horizontal blue bar at the top represents the frequency range. You can move the entire frequency range by dragging the blue bar. The silver handles on either end of the blue bar set the Low Frequency and High Frequency values, respectively.

    • You can also use the numeric fields to adjust the frequency values separately.

  • Frequency band faders: Set the level of each frequency band in Filter Bank A (upper blue faders) or Filter Bank B (lower green faders). You can quickly create complex level curves by dragging horizontally (“drawing”) across either row of faders.
  • Formant Shift knob: Moves all bands in both filter banks up or down the frequency spectrum.

    Note: The use of Formant Shift can result in the generation of unusual resonant frequencies—when high Resonance settings are used.

  • Bands value field: Sets the number of frequency bands—up to 20—in each filter bank.
  • Lowest button: Click to determine whether the lowest filter band acts as bandpass or highpass filter. In the Bandpass setting, the frequencies below the lowest bands and above the highest bands are ignored. In the Highpass setting, all frequencies below the lowest bands are filtered.
  • Highest button: Click to determine whether the highest filter band acts as bandpass or lowpass filter. In the Bandpass setting, the frequencies below the lowest bands and above the highest bands are ignored. In the Lowpass setting, all frequencies above the highest bands are filtered.
  • Resonance knob: Determines the basic sonic character of both filter banks. Increasing Resonance emphasizes the center frequency of each band. Low settings result in a softer character; high settings result in a sharper, brighter character.
  • Boost A and B knobs: Set the amount of boost—or cut—applied to the frequency bands in Filter Bank A or B. This allows you to compensate for the reduction in volume caused by lowering the level of one or more bands. If you use Boost to set the (level) mix relationship between the filter banks, you can use Fade A/B (see “Fade AB slider” below) to alter the tonal color, but not the levels.
  • Slope pop-up menu: Sets the amount of filter attenuation applied to all filters in both filter banks. Choices are 1 (6 dB/Oct.) and 2 (12 dB/Oct.). 1 sounds softer, 2 sounds tighter.
  • Fade AB slider: Crossfades between Filter Bank A and Filter Bank B. At the top position, only Bank A is audible. At the bottom position, only Bank B is audible. In the middle position, the signals passing through both banks are evenly mixed.

EVOC 20 Filterbank Modulation Parameters

The Modulation section offers two LFOs. The LFO Shift parameters on the left side control the Formant Shift parameter. The LFO Fade parameters on the right side control the Fade AB parameter.

Figure. Modulation parameters.
  • LFO Shift Intensity slider: Controls the amount of Formant Shift modulation by the Shift LFO.
  • Rate knobs and fields: Determine the speed of modulation. Values to the left of the center positions are synchronized with the host application tempo and include bar values, triplet values, and more. Values to the right of the center positions are non synchronized and are displayed in Hertz (cycles per second).

    Note: The ability to use synchronous bar values could be used to perform a formant shift every four bars on a cycled one-bar percussion part, for example. Alternately, you could perform the same formant shift on every eighth-note triplet within the same part. Either method can generate interesting results, and can lead to new ideas, or add new life to old audio material.

  • Waveform buttons: Set the waveform type used by the Shift LFO on the left side or Fade LFO on the right side. You can choose between triangle, falling and rising sawtooth, square up and down around zero (bipolar, good for trills), square up from zero (unipolar, good for changing between two definable pitches), a random stepped waveform (S&H), and a smoothed random waveform for each LFO.
  • LFO Fade Intensity slider: Controls the amount of Fade AB modulation by the Fade LFO.

    Tip: LFO modulations are the key to some extraordinary effects that can be obtained with the EVOC 20 Filterbank. Set up either completely different or complementary filter curves in both filter banks. You can use rhythmic material—such as a drum loop—as an input signal, and set up tempo-synchronized modulations, with different rates for each LFO. Feel free to try a tempo-synchronized delay effect—such as Tape Delay—after the EVOC 20 Filterbank to produce unique polyrhythms.

EVOC 20 Filterbank Output Parameters

The Output parameters provide control over the level and stereo width. The Output section also incorporates an integrated overdrive (distortion) circuit.

Figure. Output parameters.
  • Overdrive button: Click to turn the overdrive circuit on or off.

    Note: To hear the overdrive effect, you may need to boost the level of one or both filter banks.

  • Level slider: Controls the volume of the EVOC 20 Filterbank output signal.
  • Stereo Mode pop-up menu: Sets the input/output mode of the EVOC 20 Filterbank. The choices are m/s (mono input/stereo output) and s/s (stereo input/stereo output).

    • In s/s mode, the left and right channels are processed by separate filter banks.

    • In m/s mode, a stereo input signal is first summed to mono before being routed to the filter banks.

  • Stereo Width knob: Distributes the output signals of the filter bands in the stereo field.

    • At the left position, the outputs of all bands are centered.

    • At the centered position, the outputs of all bands ascend from left to right.

    • At the right position, the bands are output—alternately—to the left and right channels.

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