Parametric Equalization in Ecasound Explained

Ecasound achieves multi-band parametric equalization by chaining specialized audio processing operators sequentially within an audio processing chain. By utilizing either native IIR peaking filters or integrated LADSPA equalization plugins, the software applies digital biquad filter calculations across distinct center frequencies, bandwidths, and gain levels to shape the frequency spectrum in real time.

The Native Filter Architecture

Ecasound features built-in chain operators designed for sound processing. The native parametric equalization filter is invoked using the -efp operator. The syntax requires three primary parameters:

-efp:center_frequency,width,gain

Center Frequency (\(f_0\)): The midpoint of the target frequency band, expressed in Hertz.
Width (Bandwidth): The range around the center frequency affected by the filter, specified in Hertz or as an equivalent resonance factor.
Gain: The magnitude adjustment applied to the selected band, defined in decibels (dB) where positive values boost and negative values cut.

Under the hood, Ecasound utilizes standard Second-Order Infinite Impulse Response (IIR) biquad filter mathematics. Each filter modifies the amplitude of frequencies surrounding the center point while leaving unaffected frequencies relatively intact.

Chaining Across Multiple Bands

Ecasound does not rely on a monolithic multi-band graphical interface. Instead, multiple bands are formed by cascading individual -efp operators in series within a single chain setup (-a:chain_name). Because digital filters in an Ecasound chain are applied sequentially, the output of one equalization band feeds directly into the input of the next.

For example, a three-band parametric equalizer covering the low-mid, mid, and high-frequency bands is declared directly within the command-line setup:

ecasound -i input.wav -o output.wav \
  -efp:120,40,3 \
  -efp:1500,250,-4 \
  -efp:8000,800,2

In this pipeline:

The 120 Hz band receives a +3 dB boost across a 40 Hz width.
The signal is then passed to the 1500 Hz filter, attenuating that band by -4 dB over a 250 Hz width.
The result enters the final stage, receiving a +2 dB boost centered at 8000 Hz.

Integrating LADSPA Parametric Equalizers

For complex mastering tasks requiring precise Q-factor controls or pre-packaged multi-band layouts, Ecasound supports the LADSPA (Linux Audio Developer's Simple Plugin API) framework via the -el operator.

Third-party plugin suites (such as SWH or TAP plugins) expose dedicated multi-band equalizers that can be embedded directly into an Ecasound chain. Using LADSPA allows an engineer to apply algorithms such as constant-Q filtering, phase-aligned band splits, and shelf stages within the same native stream routing.

Dynamic Control via the Interactive Mode

Multi-band parametric filters can be modulated during playback using Ecasound Interactive Access Mode (EIAM). By assigning unique identifiers to individual chain operators or addressing them by their position index, users can query and update parameters without interrupting the audio stream.

Through standard control commands like cop-select and cop-set, automated scripts or external controllers can sweep frequencies, adjust bandwidths, and automate gains dynamically across all active equalization bands.