Sonix Overview

Sonix is the audio engine module of VoxaTrace, providing:

• Audio Playback — Play audio files with pitch shifting and tempo control
• Recording — Record audio to M4A, MP3, or WAV formats
• Multi-track Mixing — Mix multiple audio tracks with individual controls
• Audio Utilities — Concatenation, normalization, offline mixing, and tone generation
• Metronome — Programmable metronome with visual feedback
• MIDI Synthesis — Play MIDI notes using SoundFont instruments

Sonix in Action

Mixing Multiple Tracks

Track 1 (Backing):  ████████████████████████████████  vol: 0.8
Track 2 (Vocals):   ░░░░████████████████░░░░░░░░░░░░  vol: 1.0
Track 3 (Guide):    ████████████████████████████████  vol: 0.3 (muted)
Track 4 (Click):    █░░░█░░░█░░░█░░░█░░░█░░░█░░░█░░░  vol: 0.5
                    ─────────────────────────────────
Output:             Mixed audio to speakers

Pitch Shifting

Original:     C4 ──── D4 ──── E4 ──── F4 ────
              262 Hz  294 Hz  330 Hz  349 Hz

+2 semitones: D4 ──── E4 ──── F#4 ─── G4 ────
              294 Hz  330 Hz  370 Hz  392 Hz

The melody stays the same, just in a different key.

Recording Pipeline

Microphone
    │
    ▼
┌─────────────────┐
│ SonixRecorder   │──── audioBuffers (real-time for visualization)
│                 │──── level (VU meter)
│ ┌─────────────┐ │
│ │ MP3 Encoder │ │──── output.mp3
│ └─────────────┘ │
└─────────────────┘

Key Features

Playback

val player = SonixPlayer.create("song.mp3")
player.play()
player.pitch = 2f     // +2 semitones (higher key)
player.tempo = 0.8f   // 80% speed (slower)
player.seek(30_000)   // Jump to 30 seconds

Feature	Range
Formats	M4A, MP3, WAV
Pitch shifting	-12 to +12 semitones
Tempo control	0.25× to 4×

Recording

val config = SonixRecorderConfig.Builder()
    .format(AudioFormat.MP3)
    .build()
val recorder = SonixRecorder.create("recording.mp3", config)
recorder.start()
// ... user sings ...
recorder.stop()

Feature	Options
Output formats	M4A (AAC), MP3, WAV
Sample rates	16 kHz (VOICE), 44.1 kHz mono (STANDARD), 44.1 kHz stereo (HIGH); custom via Builder
Background recording	Supported
Hardware-clock timing	AudioBuffer.timestamp is monotonic-nanos with input latency already subtracted (1.0.1+)

Echo cancellation is exposed via Builder.echoCancellation(true). As of 1.0.0, AEC is not requested on Android (unsupported in practice on most devices); on iOS it depends on the configured audio mode.

Multi-track Mixing

For karaoke apps with backing track + vocal guide + click:

val mixer = SonixMixer.create(SonixMixerConfig.DEFAULT)
mixer.addTrack("backing", "backing.mp3")
mixer.addTrack("guide", "vocal_guide.mp3")
mixer.setTrackVolume("backing", 0.8f)
mixer.setTrackVolume("guide", 0.3f)
mixer.play()

Tracks are keyed by string name (not int index). Per-track controls: volume (setTrackVolume, fadeTrackVolume); tracks are added and removed by name (addTrack, removeTrack). There is no mute/solo API; set a track's volume to 0f to silence it.

Metronome

val metronome = SonixMetronome.create(
    samaSamplePath = "sama.wav",   // downbeat (first beat of the cycle)
    beatSamplePath = "beat.wav",   // regular beat
    bpm = 120f
)
metronome.currentBeat.collect { beat -> updateUI(beat) }
metronome.start()

Feature	Range
BPM	30 – 300
Beats per cycle	Any positive integer (set at build time)
Custom sounds	Sama / beat sample paths via SonixMetronome.Builder

MIDI Synthesis

Synthesize vocal guides or accompaniment from MIDI files / pitch files:

val synth = SonixMidiSynthesizer.create("piano.sf2")   // sample rate set via Builder (default 44100)
val ok = synth.synthesize(midiPath = "song.mid", outputPath = "out.wav")

Feature	Specification
SoundFont support	.sf2 / .sf3 files
Inputs	MIDI files (synthesize), raw MIDI bytes (synthesizeMidi), MidiNote lists (synthesizeFromNotes), pitch files (synthesizeFromPitchFile)
Output	WAV file written to outputPath; every synthesis method returns Boolean. Synthesizer is stateless; no noteOn/noteOff event API

Next Steps

API Reference

• SonixPlayer — Audio playback with pitch shifting and tempo control
• SonixRecorder — Record audio to M4A, MP3, or WAV
• SonixMixer — Multi-track mixing with per-track volume
• SonixMetronome — Programmable click track
• SonixDecoder — Decode audio files to raw PCM
• SonixEncoder — Encode PCM to M4A, MP3, or WAV
• SonixAudioUtils — Concatenation, normalization, offline mixing
• SonixToneGenerator — Generate sine, square, sawtooth, and triangle waveforms
• SonixResampler — Sample rate conversion
• SonixMidiSynthesizer — MIDI to audio synthesis
• SonixLessonSynthesizer — Lesson audio from note sequences
• AudioSessionManager — Audio session / focus coordination (usually automatic)
• Utilities — Parser, frames, and error types

Guides

• Playing Audio Guide — In-depth SonixPlayer guide