High-Pass Filter Chain Operators in Ecasound

This article explains how to implement high-pass filtering on audio signals using the Ecasound command-line audio processor. While Ecasound provides built-in internal operators for low-pass and band-pass filtering, it does not feature a dedicated native internal filter operator for high-pass processing. Instead, high-pass filtering in Ecasound is implemented using the LADSPA plugin chain operator (-el or -eli) or preset operators (-pn). The sections below cover the specific operators, their syntax, and practical examples for applying high-pass filters to your audio chains.

The LADSPA Chain Operator (-el and -eli)

Ecasound offloads high-pass filtering to the LADSPA (Linux Audio Developer's Simple Plugin API) ecosystem. To apply a high-pass filter, you use the standard LADSPA effect chain operator:

• -el:plugin_label,param1,param2,...: Loads a LADSPA plugin by its unique textual label and passes configuration parameters.
• -eli:plugin_unique_id,param1,param2,...: Loads a LADSPA plugin by its unique numerical identifier.

The most widely used LADSPA high-pass filter plugins integrated with Ecasound come from the SWH (Steve Harris) plugin suite:

1. highpass_iir (Single or multi-pole IIR high-pass filter):

   • Syntax: -el:highpass_iir,cutoff_frequency,stages
   • Parameters:
     • cutoff_frequency: The cutoff frequency in Hertz (Hz).
     • stages: The filter roll-off steepness (each stage adds 6 dB/octave; for example, 2 stages produce a 12 dB/octave slope).

2. highpass_1804 (Alternative IIR high-pass implementation):

   • Syntax: -el:highpass_1804,cutoff_frequency

Practical Command Examples

To filter out low frequencies below 150 Hz with a 2-stage (12 dB/octave) cutoff from an input file and output to a new file, execute:

ecasound -i:input.wav -el:highpass_iir,150,2 -o:output.wav

To apply a 300 Hz high-pass filter in real-time between your ALSA sound card input and output:

ecasound -i:alsa -el:highpass_iir,300,2 -o:alsa

Using Preset Chain Operators (-pn)

Ecasound supports custom presets defined in ecasoundrc or global preset files via the -pn:preset_name operator. If you frequently use high-pass filtering, you can define an alias in your .ecasound/ecasoundrc configuration:

preset highpass {
    -el:highpass_iir,%1,%2
}

Once defined, you can invoke the high-pass filter directly with the preset chain operator:

ecasound -i:input.wav -pn:highpass,200,2 -o:output.wav