fix(gallery): measure spectral flatness per frame so real speech passes - #1819
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Included review availability: Your plan provides up to 10 included reviews per hour; 1 remains after this review. 📝 WalkthroughWalkthroughThe gallery-preview quality guard now measures mean spectral flatness across short audio frames, filters low-energy frames, and uses a recalibrated threshold. Tests cover tonal-versus-speech separation and stability across repeated clip durations. ChangesSpectral flatness classification
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to Gallery preview validation now evaluates short audio frames to avoid rejecting valid longer or softly voiced speech, while retaining checks for silent and near-tonal output. The calibrated behavior and duration stability are covered by tests, with no remaining merge-blocking risk. Suggested reviewers: 🚥 Pre-merge checks | ✅ 6 | ❌ 3❌ Failed checks (3 warnings)
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| Filename | Overview |
|---|---|
| backend/api/routers/archetypes.py | Replaces whole-clip spectral flatness with energy-filtered framed measurement and recalibrates the tonal rejection threshold. |
| backend/tests/test_archetype_preview_quality.py | Adds measured tonal bounds, shipped-speech calibration coverage, and a clip-length invariance regression test. |
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In `@backend/tests/test_archetype_preview_quality.py`:
- Around line 98-100: The test test_threshold_sits_between_tonal_and_real_speech
currently relies on standalone constants rather than verifying actual
_spectral_flatness measurements. Bind its bounds to checked-in tonal and speech
render fixtures, or first assert that the measured outputs match the expected
constants, then apply the existing tenfold margins around
arch._DEGENERATE_FLATNESS.
In `@CHANGELOG.md`:
- Line 21: Add a concise bullet under the existing **Highlights** section
summarizing the gallery-preview audio fix, before the detail sections, while
preserving the current one-line ### Fixed entry unchanged.
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CHANGELOG.mdbackend/api/routers/archetypes.pybackend/tests/test_archetype_preview_quality.py
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Most gallery previews fail with "the voice engine returned no audible
audio for this archetype". The renders are fine — the guard is not.
Two problems, both in the degenerate-buzz check:
1. `_spectral_flatness` took ONE FFT of the whole clip. Spectral
flatness is defined over short frames; a full-length transform gets
finer frequency resolution the longer the clip is, so voiced
harmonics carve deeper and deeper nulls and the geometric mean
collapses. The number tracked clip length, not timbre.
2. `_DEGENERATE_FLATNESS = 0.015` was calibrated against
`_speech_like()` in the unit test — a synthetic harmonics+noise
stand-in that is far flatter than real speech. Real renders measure
well below it, so the threshold sat inside the speech range.
Measured on this engine's own output (framed, per this patch):
pure tone 80 Hz 2.6e-10 two-tone buzz 3.3e-09
quietest real speech 2.0e-04 (VoxCPM2 ko)
Frame the measurement (1024/512, skipping inter-word frames at the
noise floor) and move the threshold to 1e-5 — ~3000x above the tonal
cases, ~20x below the quietest real render.
Before: 6 of 8 renders rejected; ml_japanese_explainer,
ml_japanese_companion and feat_23_the_explainer all 503 through
GET /archetypes/{id}/preview.
After: 0 false positives across 27 real clips (Japanese, Korean and
English archetypes, cloned voices, human reference recordings), and
those three previews return 200. Every accepted clip was confirmed as
real speech by transcribing it with the app's own ASR.
Not addressed: a render that collapses toward NOISE rather than a tone
still passes (one observed at flatness 0.073, ASR returns a
hallucination). The old threshold missed it too, so this is not a
regression — calibrating an upper bound needs more than one sample.
Tests: frame-based measurement must be clip-length invariant, and the
threshold must sit between the measured tonal ceiling and the measured
real-speech floor. Both fail against the previous implementation.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Problem
Most gallery previews fail with:
The renders are fine. The guard is not.
Through
GET /archetypes/{id}/previewon a cleanmain(OmniVoice, MPS, Apple M4):ml_japanese_explainerml_japanese_companionfeat_23_the_explainerml_japanese_narratorml_korean_explainerNot a language issue:
ml_korean_explainerhas an instruct string identical toml_japanese_explainer(male, young adult, moderate pitch) and passes. English fails too.Cause
Two problems, both in the degenerate-buzz check.
1.
_spectral_flatnesstook one FFT of the whole clip. Spectral flatness is defined over short frames. A full-length transform gets finer frequency resolution the longer the clip is, so voiced harmonics carve ever-deeper nulls and the geometric mean collapses. The result tracked clip length rather than timbre — repeating the same 3 s signal to 12 s moves it from 0.0229 to ~0 (100% drift).2. The threshold was calibrated against a synthetic signal.
_DEGENERATE_FLATNESS = 0.015was set from_speech_like()in the unit test — harmonics + noise + a syllabic envelope, which is far flatter than real speech. The test file says as much: "The real end-to-end render is verified manually." Real renders land below 0.015, so the threshold sat inside the speech range.Measured on the engine's own output, 8 renders across 4 archetypes × 2 seeds — all normalized to peak 0.794, none blank:
Values straddle the threshold, so pass/fail is effectively a coin flip on the seed. Every one of these transcribes correctly with the app's own ASR (MLX Whisper).
Fix
Frame the measurement (1024 / hop 512, skipping inter-word frames sitting at the noise floor — their spectrum is the noise floor, not speech), and recalibrate the threshold against measurements:
1e-5sits ~3000x above the tonal cases and ~20x below the quietest real render. The measured bounds are recorded in the code comment and asserted in a test, so the next change has to re-measure rather than re-guess.Verification
pytest backend/tests— 334 passed, 2 failed; both failures (test_model_load_shutdown.py) reproduce unchanged on cleanmainand are unrelated.New tests, both of which fail against the previous implementation and pass against this one:
test_flatness_is_not_clip_length_dependent— old drifts 99.93% when the signal is repeated 4x, new drifts 0.09%.test_threshold_sits_between_tonal_and_real_speech— pins the threshold between the measured tonal ceiling and the measured real-speech floor.Not addressed
A render that collapses toward noise rather than a tone still passes. One was observed at flatness 0.073 (ASR returns a hallucination —
Продолжение следует...— instead of the script), reproduced withnum_step=16, seed=42on the social sample script. The previous threshold missed it too, so this is not a regression. Calibrating an upper bound from a single sample is what produced the bug being fixed here, so I left it alone; happy to follow up if you have more degenerate samples.Spectral flatness validation now measures 1024-sample frames with a 512-sample hop and skips low-energy frames. This prevents clip-length-dependent false “no audible audio” errors for real speech while preserving degenerate-render detection. Noise-like degenerate renders remain outside this change’s scope.