MIDI Note Gating for Audio in Ecasound

This article explains how Ecasound utilizes MIDI note-on and note-off messages to gate audio signals in real time. By routing incoming MIDI events to chain operator parameters—specifically volume and amplification controls—Ecasound can open the audio path upon receiving a note-on command and silence it immediately when a corresponding note-off command is detected.

The Underlying Control Mechanism

Ecasound treats dynamic signal adjustments through its chain controller subsystem. Rather than operating as a full-featured digital audio workstation with built-in graphical MIDI routing, Ecasound relies on mapping MIDI triggers directly to audio processing parameters using command-line arguments or interactive control commands.

Gating an audio track via MIDI requires binding an amplitude operator to a MIDI controller input. This is typically achieved using the -ea (amplify) effect coupled with the -km (MIDI continuous controller) or note-tracking controller options.

Capturing MIDI Events

To accept MIDI data, Ecasound must be bound to a MIDI driver, commonly the ALSA sequencer (alsaseq) or raw MIDI interface (rawmidi):

-Md:alsaseq,default

Once initialized, Ecasound registers an ALSA client port, allowing external hardware controllers, software sequencers, or virtual keyboards to route events directly into the processing engine.

Mapping Note-On and Note-Off to Gate Audio

A standard gate requires two states: closed (silence) and open (pass-through). In Ecasound, this behavior is established using the following principles:

1. Volume Modulation Operator: The chain must contain an amplify effect, such as -ea:0, which initializes the channel in a muted or closed state.
2. Controller Attachment (-km): The -km option maps incoming MIDI messages to the target effect parameter. The syntax establishes the target parameter, minimum value, maximum value, MIDI channel, and controller type.
3. Note-On Behavior: When a MIDI note-on event arrives, Ecasound translates the event into a parameter change. If velocity sensitivity is enabled, the velocity value (1–127) scales the parameter between the minimum (0%) and maximum (e.g., 100% or higher) amplitude. If configured as a static gate, any note-on event sets the gain directly to the designated upper threshold.
4. Note-Off Behavior: Standard MIDI protocol sends a note-off event (or a note-on event with a velocity of 0) when a key is released. Ecasound interprets this change by resetting the mapped amplitude parameter back to the baseline minimum value (0%), instantaneously closing the gate and silencing the track.

Practical Implementation

In an Ecasound processing chain, the setup typically looks like this:

ecasound -B:rt -b:128 -Md:alsaseq,default \
  -a:1 -i:audio_track.wav -ea:0 -km:1,0,100,1,1 -o:alsa

In this routing setup:

• -ea:0 initializes the audio track at 0% gain (gate closed).
• -km:1,0,100,1,1 attaches a MIDI control mapping to parameter 1 (amplitude), scaling from 0 (closed) to 100 (open) based on incoming MIDI channel 1 messages.

Latency and Buffer Sizing

Because gating is an amplitude-based envelope operation, gate responsiveness depends directly on Ecasound’s internal buffer configuration. By reducing the buffer size (using the -b flag) and running in real-time mode (-B:rt), you minimize the delay between the physical MIDI event and the audio gate opening or closing, ensuring tight temporal synchronization between MIDI triggers and audio playback.