Lumotia/docs/architecture-map/03-audio-transcription/audio-file-decoding.md

name, type, slice, last_verified

name	type	slice	last_verified
Audio file decoding (symphonia)	architecture-map-page	03-audio-transcription	2026/05/09

Audio file decoding (symphonia)

Where you are: Architecture map → Audio + Transcription → Audio file decoding

Plain English summary. When the user imports an audio file, decode_audio_file reads it through symphonia, downmixes to mono, and returns f32 PCM at the file's native rate. decode_and_resample chains this with resample_to_16khz on a blocking task so the Tauri command stays async. A short probe_audio_duration_secs helper reads the duration without decoding the body, used for "is this longer than the import limit?" preflight.

At a glance

• Crate: magnotia-audio
• Paths: crates/audio/src/decode.rs (283 LOC), crates/audio/src/concurrency.rs (19 LOC).
• External deps: symphonia 0.5 with features mp3, aac, flac, pcm, vorbis, wav, ogg, isomp4.
• Internal callers (best effort, slice 2 reconciles): the import-file Tauri command and the file-mode transcription command call decode_and_resample. Standalone decode_audio_file is exposed for tests and any path that wants the native rate.

Public surface:

• pub fn decode_audio_file(path: &Path) -> Result<AudioSamples> — crates/audio/src/decode.rs:23.
• pub fn decode_audio_file_limited(path: &Path, max_duration_secs: Option<f64>) -> Result<AudioSamples> — crates/audio/src/decode.rs:27.
• pub fn probe_audio_duration_secs(path: &Path) -> Result<Option<f64>> — crates/audio/src/decode.rs:43.
• pub async fn decode_and_resample(path: &Path) -> Result<AudioSamples> — crates/audio/src/concurrency.rs:11.

What's in here

decode_audio_file and friends

decode_audio_file (decode.rs:23) delegates to decode_audio_file_limited(path, None). The _limited variant accepts a hard cap in seconds; once the per-track sample count crosses (max_duration_secs * sample_rate).ceil() it returns MagnotiaError::AudioDecodeFailed("Audio is longer than the {:.0} minute import limit", ...). Used by the Tauri import command to enforce the configured maximum.

Both call into decode_media_stream (decode.rs:77), which is the actual decode loop and the seam tests inject a MediaSource into. Steps:

1. Probe the format (symphonia::default::get_probe()).
2. Pick the default track; reject if there is none or the codec sample rate is 0.
3. Build a decoder via symphonia::default::get_codecs().
4. Loop:
   • format.next_packet() — UnexpectedEof is the natural EOF marker; any other error is propagated as AudioDecodeFailed. ResetRequired is treated as a discontinuity error.
   • Skip packets that don't belong to the chosen track.
   • Decode into a SampleBuffer<f32>.
   • Average channels into mono if channels > 1; otherwise extend the output verbatim.
   • Apply the duration cap if one was provided.
5. Reject empty output (No audio data decoded) and return AudioSamples::new(samples, sample_rate, 1).

probe_audio_duration_secs

decode.rs:43. Re-runs the probe step only, reads track.codec_params.n_frames and sample_rate, returns Some(seconds) if both are available. Cheaper than a full decode for "is this file too long?" gating.

decode_and_resample

concurrency.rs:11. Wraps decode_audio_file + resample_to_16khz in tokio::task::spawn_blocking. Used by every async caller that needs ready-to-transcribe samples. Maps JoinError to MagnotiaError::AudioDecodeFailed("Task join error: {e}").

Data flow

Path
  └─ File::open
      └─ MediaSourceStream
          └─ Hint(extension)
              └─ probe → format + codec_params
                  └─ Decoder
                      └─ next_packet loop
                          ├─ packet → decoded → SampleBuffer<f32>
                          └─ if channels > 1: mean across channels (mono mixdown)
                              → samples (f32, native rate)

decode_and_resample chains the above with resample_to_16khz so the caller gets AudioSamples already at 16 kHz.

Watch-outs

• RB-09 regression (2026-04-22 review). The previous loop did Err(_) => break and silently returned partial audio on a mid-stream I/O error. Current code distinguishes EOF, ResetRequired, and other errors. Test at decode.rs:262 injects a flaky MediaSource to keep this honest.
• AAC and ISO MP4 are pulled in unconditionally by the symphonia feature set. Patent-licence implications for AAC are flagged in the brief; this is not a per-build decision today.
• Channels are averaged, not mixed psychoacoustically. Stereo to mono goes via mean of all channels per sample. Fine for ASR; lossy for any future task that needs spatial cues.
• Whole-file decode loads everything into memory. The duration cap is the only guard. Long imports exceed RAM long before they exceed disk.
• Hint only carries extension. A .mp3 named .bin will fail to probe even if the bytes are valid MP3. Caller responsibility to keep extensions truthful.
• decode_and_resample returns the resampled output. If a downstream caller wanted the original rate (for a non-ASR purpose), it must call decode_audio_file directly.

See also

• Audio resampling — resample_to_16khz is the second half of decode_and_resample.
• Audio WAV I/O — read_wav is the WAV-only fast path that uses hound instead of symphonia.
• magnotia_core::types::AudioSamples (slice 5) — the value type returned.
• Tests at decode.rs:175 (RB-09 regression among others).