Mixbus can record, edit and render multitimbral MIDI files, often used for backing tracks or “sweetening” tracks to fill-out a live recording with additional instruments.

What is “Multitimbral”?

MIDI performance data ( such as notes from a MIDI file ) may be provided in up to 16 different channels on a single MIDI connection. Typically, each of the channels represents an instrument. A channel can change instruments anytime by sending a “patch change” message, which the receiving synth may (or may not) respond to.

At the start of the file, a “patch change” MIDI message will typically be sent for each of the channels in use, and these patch messages will define what instrument should be sounded on that channel … perhaps a saxophone, guitar, or piano.

What is General MIDI?

General MIDI is an agreed-on set of patches to render performance data with a variety of common instruments from various ethnicities. As MIDI can only send messages like “Channel 7, patch 21”, it was found necessary to define some instruments for these generic patch messages. In this way, a .mid file that follows the “General MIDI” specification can be reliably played in a synthesizer that is also General MIDI compliant.

For example, it is generally agreed that drums will be played on Channel 10.

How does Mixbus implement Multitimbral and General-MIDI synthesis?

Mixbus is optimized for high-quality mixing of audio. Support for General MIDI, multitimbral MIDI, and backing tracks is not a high priority for us. Nevertheless, it is a feature that is sometimes desirable, and with Mixbus v4 we have added some mechanisms to import, edit, and render multitimbral MIDI data.

Mixbus provides 2 multitimbral synths:

  • a-FluidSynth, a multitimbral MIDI-capable synthesizer that loads .sf2 soundfonts.
  • General MIDI Synth, a simplified version of FluidSynth that automatically loads the GeneralUser soundfont by S. Christian Collins. GMSynth provides a very thorough implementation of the General MIDI specifications.

What is a soundfont?

A “soundfont” is a file that contains audio samples and a “mapping” of each sample which defines the MIDI notes it will be played. Fully-featured soundfont players, such as Fluid Synth, provide full support for many MIDI features such as pitch-bend and modulation; as well as a built-in chorus and reverb to improve the sometimes lackluster sounds of General MIDI.

How do I use the General MIDI Synth?

To use the General MIDI Synth, simply add a new MIDI track ( or Bus, if you intend to feed it from multiple tracks ), and leave General MIDI Synth as the Instrument selection.

How do I choose a different patch/sound in the GeneralUser soundfont?

See: MIDI Banks and Patches

How do I use other soundfonts?

There are many Soundfont (.sf2) files online, from both commercial and “free” vendors.

You may download any .sf2 file, and use the selector menu inside a-Fluid Synth to navigate to, and load, the desired soundfont file. Note that the quality of sound fonts can vary widely, and you might need to check the soundfont’s documentation to learn which patches and note ranges are implemented.

Some soundfont files are optimized for General MIDI, and multitimbral operation. While other soundfonts are intended to provide only a single instrument, or several instruments, and do not implement the General MIDI specification. For example you can download a soundfont file that provides only a high-quality piano, a virtual orchestra, or a vintage electronic drum machine. A full investigation of .sf2 files is outside the scope of this manual, but you can find more information by searching the internet for “sf2” or “soundfont”.


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