@audio/midi-render

MIDI → PCM — General MIDI synth render, no soundfont, no samples

npm install @audio/midi-render

import render, { GM, voiceFor } from '@audio/midi-render'

A synth-GM: every one of the 128 General MIDI Level 1 programs and every percussion key (35-81) maps to an @audio/synth-* generator — FM (Chowning 1973), Karplus-Strong plucked strings (Karplus & Strong 1983), and modal resonator banks (Fletcher & Rossing, The Physics of Musical Instruments, 1998) — instead of a sampled/SF2 bank. Parses with @audio/midi-parse, allocates polyphony per channel with @audio/synth-poly (note-stealing), mixes to stereo. Zero asset weight: the whole instrument set ships as code.

import render from '@audio/midi-render'
import { readFileSync } from 'fs'

let smf = readFileSync('song.mid')
let { channelData, sampleRate, duration } = render(smf)
// channelData: [Float32Array, Float32Array] (L, R) — write to WAV/whatever with @audio/encode-wav

// override an instrument, or inspect the map
import render, { GM, voiceFor } from '@audio/midi-render'
import bell from '@audio/synth-fm'

render(smf, { programs: { 8: (freq, opts) => bell(freq, { ...opts, index: 12 }) } })
console.log(GM.programs[0].name)         // 'Acoustic Grand Piano'
console.log(voiceFor(0)(440, { fs: 44100, duration: 1, velocity: 0.8 }).length)

| Param | Default | | |---|---|---| | fs | 44100 | Sample rate, Hz | | channels | 2 | Output channel count, 1 or 2 | | voices | 32 | Max simultaneous notes per (channel, program) or (channel, drum key) — oldest is stolen with a 5ms fade | | tail | 1.0 | Seconds of silence appended after the render's natural end | | gain | 0.5 | Overall gain multiplier, applied before normalization | | normalize | true | Peak-normalize to −1 dBFS if the mix exceeds it — never boosts a quieter mix | | programs | — | Record<program, Voice> — override the GM program map | | drums | — | Record<key, Voice> — override the GM percussion map | | tempoScale | 1 | Stretches/compresses the whole schedule (2 = twice as fast) | | transpose | 0 | Semitones added to every non-percussion note | | bendRange | 2 | Pitch-bend sensitivity in semitones (GM default) |

The GM map

| Programs | Family | Synthesis | |---|---|---| | 0-3 | Piano (acoustic) | 2-op FM hammer transient + synth-modal 'string' body; t60 scales with pitch (8s @ MIDI36 → 1.5s @ MIDI96, linear) — inharmonicity 0.0002 | | 4-5 | Piano (electric) | synth-fm's epiano() preset, two ratio/index voicings | | 6-7 | Harpsichord/Clavinet | synth-pluck (Karplus-Strong), bright/fast damping | | 8-15 | Chromatic percussion | synth-modal 'bar'/'plate' (Fletcher & Rossing mode tables) or synth-fm's bell(); vibraphone gets a tremolo LFO, marimba/xylophone use strike position | | 16-23 | Organ | 9-partial additive drawbar (Hammond footages 16'-1', ratios 1/2/3/4/6/8/10/12/16) with slow vibrato, sustain-while-held envelope; accordion/harmonica = sawtooth + reed FM | | 24-31 | Guitar | synth-pluck, damping/lowpass/drive per articulation (nylon 0.994 … distortion tanh drive 8) | | 32-39 | Bass | synth-pluck (acoustic/electric/slap, damping 0.993-0.999 + percussive click) or oscillator + ADSR (synth bass 1/2) | | 40-55 | Strings/ensemble | Detuned oscillator pairs/triples (±5-10 cents) through a slow ADSR; pizzicato = pluck, timpani = synth-drum membrane(), choir/voice = a wide detuned pad (no formant/vowel synth in the ecosystem — documented fallback) | | 56-63 | Brass | synth-fm with a rising modulation index (index climbs toward indexFloor over the attack — Chowning 1973's brass patch; fm()'s exponential decay toward a floor runs upward just as well as downward) | | 64-71 | Reed (sax/oboe/bassoon) | synth-fm 2-op, ratio ≈ 2; clarinet is a plain square wave (a cylindrical closed-reed bore suppresses even harmonics — the same spectrum a square wave has) | | 72-79 | Pipe (flute family) | Sine + vibrato + pink-noise breath (−16 to −36 dB below the tone, per instrument) | | 80-87 | Synth lead | Oscillator + fast ADSR, one or two per patch (fifths, sub-oscillator bass+lead) | | 88-95 | Synth pad | Detuned oscillator stack, long attack | | 96-103 | Synth FX | bell()/noise textures — deliberately loose; "keep sane," not a faithful FX patch | | 104-111 | Ethnic | synth-pluck/synth-fm/detuned-pad approximations; bagpipe is a sustained reed drone | | 112-119 | Percussive (tuned) | synth-modal bar/plate or synth-drum membrane(); reverse cymbal = a time-reversed metal() | | 120-127 | Sound effects | Colored-noise bursts (synth-noise), never silent | | 35-81 (ch. 10) | GM percussion key map | synth-drum's membrane()/metal()/noiseDrum() and synth-modal 'tube-closed' (cowbell/agogo), per-key durations/pitches from the spec (kick 35/36, snare 38/40 = noise+membrane blend, hats 42/44/46, crashes/rides 49-59, toms 41-50 at 80-200 Hz, latin percussion 60-81) |

Every program/key is covered — render() never falls silent on a valid GM number, and an out-of-map percussion key gets a short click rather than nothing.

Velocity maps to amplitude through a v^1.5 power curve (perceptually closer to a loudness control than raw linear velocity), applied once before synth-poly's own linear gain stage.

MIDI control handling

• Program changes — honored per note, per channel (mid-track changes apply to every note from that point on).
• CC7 (channel volume) — read per note (the value in effect at the note's start time); default 100/127 per the GM System Level 1 spec's default controller value.
• CC10 (pan) — equal-power, read once per channel at the value in effect at the channel's first note. Mid-channel pan automation is not applied per note — each channel renders to one mono buffer before panning, so a single pan value applies to the whole channel. Default center (64).
• CC64 (sustain pedal) — extends a note's held duration until the pedal releases (or the end of the piece, if never released). Applied per note, before synthesis.
• Pitch bend — read once per note, at the value in effect when the note starts, and applied as a constant per-note transposition (bendRange semitones at full deflection). Bend automation during a held note is not applied — there's no per-sample pitch modulation. Custom RPN 0,0 bend-range messages are not parsed; only the bendRange option changes the default ±2 semitones.
• Percussion (channel 10) — program changes are ignored; the fixed GM1 percussion map always applies (matching GM Level 1, which defines a single kit).

Performance

Rendering is deterministic per (instrument, freq, duration) — repeated notes (very common in real MIDI — the same pitch/duration/instrument recurs constantly) reuse a cached Float32Array within one render() call instead of re-synthesizing. A 2-minute, 8-channel, ~1500-note file renders in ≈4s on a laptop CPU (measured in test.js). Each channel is rendered and mixed down before moving to the next — the renderer never holds every note's audio in memory at once.

Use when: you need audio from a Standard MIDI File and don't want a soundfont/sample-bank dependency — previews, MIDI-to-audio-in-the-browser, test fixtures, procedurally generated music. Not a substitute for a real soundfont/sample player when sample-accurate GM-compatible timbre matters (a game engine's music, a DAW export) — the instrument bank here is a from-scratch synthesis approximation of each GM voice, not a recording of one.

Part of @audio/midi — the midi family umbrella.

MIT © audiojs