preprocess-for-transcription

Preprocess audio for transcription

Single orchestrator that takes a raw recording and emits a transcription-ready file. Source is never modified.

Pipeline (order matters)

Pass	Default	Notes
1. Denoise	off (flag --denoise)	rarely worth it for transcription — modern ASR is robust to moderate noise. See "Should I denoise?" below.
2. Format normalise	on	mono, 16 kHz, opus 24k
3. Loudness normalise	on	EBU R128 via ffmpeg loudnorm
4. Silence cap	on	silero-vad detects speech, every gap capped at 0.4s max (no speech ever clipped). Sub-0.4s natural pauses pass through untouched.

Passes 2+3 run in a single ffmpeg invocation (one decode/encode). Pass 4 runs after.

Folder convention

If invoked inside an existing project structure with audio/ or context/ already present, respect it. Otherwise:

• Source stays where it is, or moves to audio/raw/<basename> if the user is OK with that
• Output: audio/processed/<stem>.preprocessed.opus
• Side-files (loudnorm stats, VAD segment data): audio/processed/<stem>.<ext>

Never overwrite — if <stem>.preprocessed.opus exists, suffix .v2, .v3, etc.

Implementation

The full pipeline is shipped as a reusable script. Invoke it directly rather than reimplementing:

scripts/preprocess-audio.sh [--denoise] [--no-silence-trim] [--max-gap SEC] [--out-dir DIR] INPUT

The script handles all four passes and writes <stem>.preprocessed.opus to the output dir.

Internals (for reference):

• Pass 1 — denoise (optional): ffmpeg -af "afftdn=nf=-25". Off by default. Enable with --denoise for noisy field recordings.
• Passes 2+3 — format + loudness (combined ffmpeg invocation): ffmpeg -af "loudnorm=I=-16:TP=-1.5:LRA=11" -ac 1 -ar 16000 -c:a pcm_s16le (WAV intermediate so the python step doesn't need codec deps).
• Pass 4 — silence cap: scripts/silence-collapse.py — silero-vad detects speech segments (ML, used inside Whisper / NeMo), then every gap between speech segments is capped at --max-gap (default 0.4s). Speech is never clipped; only the dead air between speech is truncated. Sub-max-gap natural pauses pass through untouched.
• Final encode: opus 24k.

Output stats (size, duration, removed seconds, removal %, speech segment count) are written to <stem>.vad-stats.json.

Why --max-gap cap and not >min-gap collapse

Earlier iteration used "collapse silences over 2.5s." That preserved natural cadence but felt loose because medium-length pauses (1–2.5s) survived intact. The cap-gap approach uniformly truncates every gap longer than the threshold, eliminating all dead air without ever cutting speech.

Validated on a 41-minute voice memo: cap@0.4s removes ~14% (vs ~3.6% with the old collapse>2.5s approach), no perceptible word clipping. Auto-editor was tried and rejected — its energy-threshold approach chops mid-syllable on quiet speech.

Output report

After running, print:

Input:        recording.m4a (12m 04s, stereo 48kHz, 14 MB)
Output:       audio/processed/recording.preprocessed.opus (9m 18s, mono 16kHz, 1.6 MB)
Removed:      2m 46s of silence (23%)
Loudness:     normalised to -16 LUFS
Denoise:      skipped (use --denoise to enable)

Flags

• --denoise — enable pass 1
• --no-silence-trim — skip pass 4 (keep all silence)
• --max-gap <seconds> — override default 0.4s gap cap (lower = tighter)
• --out-dir <path> — override default audio/processed/

When to skip this skill

• Audio is already mono 16kHz opus and was recorded in a controlled environment → just transcribe directly
• User explicitly wants raw audio sent (e.g. needs maximum fidelity for diarisation testing)

Should I denoise?

Probably not, for transcription.

POC at https://github.com/danielrosehill/Crying-Baby-Audio-Scrub tested DeepFilterNet vs raw audio (recording with background baby crying) into Whisper. The cleaned and uncleaned transcripts differed by only 5–6 word choices across 119 seconds. Modern ASR (Whisper, AssemblyAI, Gemini) already handles moderate background noise.

Reach for --denoise (or the standalone denoise skill, which has stronger backends) only when:

• The recording has severe noise — gusty wind, heavy traffic, music behind voice — that's clearly degrading even-for-humans intelligibility
• The audio is destined for human listening, not ASR
• You've actually tried ASR on the raw audio first and the transcript was unusable

The --denoise flag here uses ffmpeg afftdn (basic, fast, ~no quality cost). For higher-quality denoise (DeepFilterNet, Auphonic, ElevenLabs Voice Isolator), use the standalone denoise skill before this preprocessor.

Defaults locked against

Validated 2026-04-28 against a 41-minute combined voice memo (Job-Search-Planning-0426). With --max-gap 0.4:

• Input: 41:43, 20 MB
• Output: 35:54, 5.9 MB
• Removed 349s (13.9%) of dead air
• No word clipping (silero is speech-aware, unlike energy-threshold tools)