Real-Time LADSPA Modulation in Ecasound

Ecasound fully supports real-time modulation of LADSPA plugin control parameters. Users can dynamically alter audio effects during playback or processing using internal automation operators, external MIDI continuous controllers, or live commands sent through Ecasound’s interactive mode (ECI). This guide explains how Ecasound interacts with LADSPA controls and outlines the primary methods available for real-time parameter modulation.

How Ecasound Integrates LADSPA Plugins

LADSPA plugins are attached to an Ecasound chain using the -el (or -eli) chain operator, followed by the plugin name and its static default parameters:

ecasound -i input.wav -o alsa -el:ladspa_plugin_name,param1,param2

In LADSPA architecture, parameters designated as "control inputs" can be adjusted on the fly without interrupting the audio stream. Ecasound numbers these control ports sequentially, allowing you to attach control sources directly to them.

Method 1: Internal Control Oscillators (LFOs and Envelopes)

Ecasound includes built-in controller operators that can modulate any chain operator parameter automatically at runtime. These controllers attach to the target parameter by specifying its index number.

Sine LFO (-kos): Modulates a parameter using a low-frequency sine wave.
```
-kos:fx_param,min_val,max_val,frequency,initial_phase
```
Linear Ramp / Envelope (-kog): Generates a time-based linear envelope or ramp to transition parameter values gradually.

For example, to attach a low-pass filter and oscillate its cutoff frequency between 500 Hz and 3000 Hz at a rate of 0.5 Hz:

ecasound -i input.wav -o alsa \
  -el:lowpass_iir,1000,2 \
  -kos:1,500,3000,0.5,0

In this command, -kos:1,... targets parameter 1 (the cutoff frequency of the filter) in real-time.

Method 2: MIDI Hardware Controllers

You can map physical MIDI continuous controllers (knobs, sliders, or pedals) directly to LADSPA plugin parameters using the -km operator.

The syntax for MIDI continuous controller mapping is:

-km:fx_param,min_val,max_val,midi_controller_number,midi_channel

Example: To modulate the resonance or cutoff of a filter using MIDI controller #74 on channel 1:

ecasound -i input.wav -o alsa \
  -el:lowpass_iir,1000,2 \
  -km:1,200,8000,74,1

Ecasound translates the 0–127 MIDI control values smoothly into the target range (200 Hz to 8000 Hz) while audio continues running.

Method 3: Interactive Mode (ECI)

Ecasound includes the Ecasound Control Interface (ECI), which enables direct command-line or programmatic manipulation while the audio engine runs.

Launch Ecasound in interactive mode:

ecasound -c -i input.wav -o alsa -el:lowpass_iir,1000,2

Start playback with the start command.
Use operator control commands to inspect and update parameters in real-time:
- cop-select 1 (select the LADSPA plugin operator)
- cop-set 1,2500 (instantly update parameter 1 to 2500)

Because ECI commands can be issued over standard input, Python bindings, or via a TCP/IP network socket (--server), you can build external GUIs or automated scripts to modulate LADSPA parameters dynamically.

Control Rate vs. Audio Rate

LADSPA control parameters are updated at the block control rate rather than the sample rate. While this allows for extremely low CPU overhead when automating complex filters, delays, and reverbs, very rapid modulation may cause audible stepping if the buffer size (-b) is set too high. Reducing the buffer size improves modulation responsiveness and resolution.