Mapping MIDI CC to LADSPA Ports in Ecasound

This article explains how Ecasound maps incoming MIDI Continuous Controller (CC) data to the input control ports of LADSPA audio plugins. By reading MIDI streams through its controller subsystem, Ecasound dynamically scales raw 7-bit MIDI control messages (0–127) to match the floating-point parameter ranges expected by LADSPA plugins, enabling real-time automation and physical hardware control inside a terminal-based audio chain.

Configuring the MIDI Input Device

Before assigning control parameters, Ecasound must be configured to receive MIDI events from a valid interface. This is done using the -Md option, which attaches a MIDI control device to the current engine setup. Common drivers include ALSA sequencer ports or raw MIDI devices:

ecasound -Md:alsaseq,default ...

Once the MIDI device is declared, Ecasound listens for incoming channel events, including Continuous Controller messages.

The -km Controller Operator

Ecasound attaches parameter automation to audio effects using "controllers." The specific operator used for MIDI Continuous Controllers is -km.

When defining an audio processing chain, the -km option attaches directly to the immediately preceding chain operator (such as a LADSPA effect loaded via -el). The syntax is structured as follows:

-km:fx_param,midichannel,cc_number,low_value,high_value

The parameters function as follows:

• fx_param: The numerical index of the target parameter on the LADSPA plugin (starting from 1). This matches the control port index of the plugin.
• midichannel: The MIDI channel to listen to (1 to 16).
• cc_number: The MIDI Continuous Controller number (0 to 127), such as CC 1 (Modulation Wheel) or CC 7 (Channel Volume).
• low_value: The target floating-point value assigned when the MIDI CC is at its minimum (0).
• high_value: The target floating-point value assigned when the MIDI CC is at its maximum (127).

Parameter Scaling and Value Translation

LADSPA control ports operate using standard single-precision floating-point numbers, while MIDI CC messages deliver integer values between 0 and 127.

Ecasound maps the two by performing linear interpolation between low_value and high_value. When an incoming MIDI message is received on the configured channel and CC number, Ecasound computes the new port value using the following formula:

\[\text{Port Value} = \text{low\_value} + \left( \frac{\text{MIDI CC Value}}{127} \right) \times (\text{high\_value} - \text{low\_value})\]

Inverted control is also supported by defining low_value higher than high_value, allowing maximum controller positions to correspond to minimum plugin values.

Practical Implementation Example

The following command demonstrates routing an audio input through a LADSPA low-pass filter while mapping MIDI CC 74 (standard brightness/cutoff) on MIDI channel 1 to control the filter's cutoff frequency between 200 Hz and 8000 Hz:

ecasound -i:sound.wav -o:alsa \
  -Md:alsaseq,128:0 \
  -el:lowpass_iir,200,2 \
  -km:1,1,74,200,8000

In this command:

1. -el:lowpass_iir,200,2 initializes the plugin with default parameters (cutoff = 200, stages = 2).
2. -km:1,1,74,200,8000 binds parameter 1 (cutoff frequency) of that plugin to MIDI Channel 1, Controller 74, scaling the output continuously across 200 to 8000.