SoundFont and Sampled Instruments
A sampled instrument makes sound the opposite way from a synthesizer: instead of generating a waveform, it plays back short recordings of a real instrument. Record a piano playing several notes at several volumes, store those clips, and play the closest one back at the right pitch when a key is pressed — that is sampling in a nutshell. The result can be extremely realistic, because it is a real recording.
This page explains what a SoundFont is, how it is addressed, and how libsonare guarantees that a MIDI arrangement always produces sound. It is concepts only — no code.
Sampled vs synthesized
A synthesized instrument computes its sound and can be any pitch or tone, but has to be designed. A sampled instrument replays recordings and sounds true to life, but is fixed to what was recorded and takes far more storage. Many setups use both — samples for realistic instruments, synths for sounds that do not exist acoustically.
What a SoundFont is
A SoundFont (the .sf2 file format) is a single file that bundles a whole library of recorded instrument samples, together with the rules for how to play them back (which sample covers which key range, how loops work, basic envelopes). One .sf2 can hold an entire General MIDI instrument set in a few megabytes.
Inside the file the sounds are organized into banks and programs, and any one instrument is addressed by a (bank, program) pair:
- A program is one playable instrument — "acoustic grand piano", "fingered bass".
- A bank is a numbered shelf of up to 128 programs. Bank 0 holds the main set; higher banks hold variations.
This is exactly the addressing scheme MIDI uses for Program Change and bank select, which is why MIDI and SoundFonts fit together so naturally.
General MIDI program numbers and the drum bank
Because a SoundFont is addressed by (bank, program), a General MIDI-compliant .sf2 lines its programs up with the GM map: program 0 is an acoustic grand piano, 24 a nylon-string guitar, 40 a violin, and so on across all 128 GM instruments. Percussion lives in a separate drum bank, where each note number selects a different drum or cymbal rather than a pitch — matching MIDI's channel-10 drum convention.
The principle underneath is simple: a note list is instrument-agnostic, and the address decides which instrument performs it. The piano roll below makes that tangible — the notes never change; switching the instrument points the same MIDI at a different sound.
How libsonare resolves a note
libsonare can load a SoundFont and play MIDI through it, but it adds one important guarantee. After you supply a .sf2 file, every MIDI program is resolved to a backend:
| Situation | Backend | What you hear |
|---|---|---|
The loaded SoundFont covers the (bank, program) | 'sf2' | The recorded SF2 sample |
| The program is missing, or no SoundFont is loaded | 'synth' | The NativeSynth General MIDI fallback bank |
The key consequence: MIDI never renders silent. If a SoundFont is missing a sound — or you have not loaded one at all — libsonare falls back to its built-in NativeSynth GM bank for that program instead of dropping the note. You always get a usable instrument for every program in the arrangement.
Why the fallback matters
Different SoundFonts cover different instruments, and a small .sf2 might only include a handful of programs. The fallback means you can hand libsonare any arrangement and any SoundFont (or none) and still hear the complete piece — then swap in a richer SoundFont later to upgrade the sounds, with no change to the MIDI.
You can inspect exactly what resolved where: a per-program report tells you, for each (channel, bank, program) the arrangement plays, whether it ended up on the 'sf2' or 'synth' backend and which preset name it matched.
How libsonare implements this
On a Project, loadSoundFont(bytes) registers an .sf2 file from a byte buffer. soundFontManifest() then returns one Sf2ProgramStatus per (channel, bank, program) combination the arrangement uses, each with a backend of 'sf2' or 'synth' and the resolved presetName — so you can see at a glance which programs are covered by your SoundFont and which fell back to the NativeSynth GM bank (drum channels report bank 128). bounceWithSf2Instrument(...) renders the arrangement through the SoundFont player, applying the same GM fallback per note so the output is never silent for an uncovered program. The fallback bank is the data-free floor: even with no SoundFont loaded at all, every program still resolves to a NativeSynth voice.
Related: SoundFont Player, Built-in Synthesizer (NativeSynth), MIDI Basics