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fix(audio): hold the export's true peak under -1 dBTP - #1166
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The export limiter capped samples at -1.5 dBFS. On a voice clipped at the source, the squared-off crests reconstruct about 2 dB above their samples, so the limited PCM was already at +0.6 dBTP before the AAC encoder, and the file read +0.5 dBTP (#1157). - Limit on a 4x true-peak envelope (windowed sinc, BS.1770 Annex 2 style) instead of on the samples. - Ceiling at -2 dBTP and AAC at 256 kb/s: the encoder's low-pass and quantisation noise put a limited voice back up by 1.3 dB at 128 kb/s and about 0.5 dB at 256 kb/s. - Give back the loudness the limiter takes with a makeup gain, found by a secant search on the limited loudness and bounded at 3 dB. A clipped voice still lands near -17 LUFS: its crests come every pitch period, so the ceiling holds the voiced passages whatever the gain. The checklist line now says so.
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| Check name | Status | Explanation | Resolution |
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| Linked Issues check |  |
Issue #1157 requires clipped-speech exports to reach approximately -16 LUFS and remain at or below -1 dBTP. The true-peak limiter, 256 kb/s AAC setting, and makeup gain address the peak requirement. T… |
Change the export behavior and automated coverage so the #1157 clipped-speech case reaches approximately -16 LUFS while remaining at or below -1 dBTP. If the peak constraint makes that outcome intentionally impossible, update the open issue… |
✅ Passed checks (4 passed)
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Check name Status Explanation Title check The title clearly identifies the primary change: enforcing a true-peak limit on exported audio. It is concise and directly related to the limiter changes. Description check The description completes all required template sections. It explains the problem, links issue #1157, identifies the change type and release impact, documents platform scope, states that screenshots a…Out of Scope Changes check The changes stay within issue #1157. The true-peak envelope, limiter ceiling, makeup search, AAC bitrate increase, audio tests, and checklist update address the reported export peak and loudness behav…Docstring Coverage Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 1 files. (1 skipped: 1…
Full details: Linked Issues check
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Explanation
Issue
#1157requires clipped-speech exports to reach approximately -16 LUFS and remain at or below -1 dBTP. The true-peak limiter, 256 kb/s AAC setting, and makeup gain address the peak requirement. The reported clipped-voice result remains -17.2 LUFS after AAC, and the implementation documents that the 3 dB makeup cap cannot reach -16 LUFS for this case. This is a concrete unmet coding requirement.Resolution
Change the export behavior and automated coverage so the
#1157clipped-speech case reaches approximately -16 LUFS while remaining at or below -1 dBTP. If the peak constraint makes that outcome intentionally impossible, update the open issue's coding requirement before treating this PR as complete.
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @crates/compositor/src/audio.rs:
- Around line 298-302: Update the limiter flow around true_peak_envelope and
limiter_gains to measure the post-gain waveform and apply a corrective gain when
its true peak exceeds LIMITER_CEILING. Add asymmetric-transient and mixed-click
cases that verify the ceiling after limit_peaks and after AAC encoding.
Review comments at @technical-documentation/testing/manual-e2e-checklist.md:
- Line 617: Update the measurement command in the v2.0.0 checklist item to use
true-peak mode (`peak=true`) so it produces the true-peak reading required by
the check.
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The envelope is read before the gain, and the gain moves across the samples an interpolated point is made of, so a sharp transient could come out a few thousandths of a decibel over the ceiling. One corrective pass on the written result catches it. The checklist's measurement command now prints the true peak too.
1 participant
Summary
The export limiter held samples at -1.5 dBFS, which is not the same as holding the true peak. I ran #1157's voice (clipped at the source, 7 trims) through
finish_audioone stage at a time:The normalisation was right. The limiter caused both problems:
AUDIO_BITRATEis 256 kb/s. The extra 128 kb/s is 6 % of the lowest video bitrate the export asks for.What this does not fix, on purpose. A voice clipped at the source has a crest every pitch period, so the ceiling holds its voiced passages whatever the gain. On #1157's voice, the limited programme reads -17.6 LUFS with no makeup, -17.1 with 3 dB, -16.9 with 6 dB and -16.7 with 12 dB. It levels off at the ceiling minus the waveform's own crest factor, about 15 dB. Past 3 dB, the makeup only lifts pauses and room noise. Reaching -16 would mean clipping the voice again, which is not this stage's job. The true peak is a hard limit, and the loudness is a target. The checklist line now says this, and gives the new numbers for the encoder overshoot.
Result on #1157's source and trims, measured with ffmpeg
ebur128=peak=true:Cost: the envelope and the makeup search take about 0.25 s for this 61 s programme (release build), so about 15 s for an hour.
Candidate for cherry-pick to release/v2.1.0.
Related issue
Fixes #1157
Type of change
Release impact
Desktop impact
finish_audioandAacEncoderare shared by the macOS, Windows and Linux pipelines.Screenshots / video
None (audio only).
Testing
cargo test -p openscreen-compositor --libon macOS 26.5 arm64 with the vendored ffmpeg 8.1.2: 413 passed.crates/compositor/src/audio.rs:a_clipped_voice_exports_under_minus_one_dbtp_and_keeps_what_loudness_it_can: generates a clipped pulse-train voice, records it through AAC (which puts its crests back over full scale, like [Bug]: export of a clipped voice misses -16 LUFS and goes over -1 dBTP #1157's source), and normalises it. It then runsfinish_audioand the export's ownAacEncoderinto an .m4a, decodes that back, and asserts a true peak of at most -1 dBTP. It also asserts that the makeup beats the limiter alone by more than 0.3 LU and never goes louder than the target. The true peak is measured independently of the limiter (8x, a longer Blackman sinc).a_voice_with_a_few_hot_transients_exports_at_the_target_under_minus_one_dbtp: an unclipped voice with plosives over the ceiling exports at -16 ±0.2 LUFS and at most -1 dBTP.a_peak_between_the_samples_is_held_under_the_ceiling: a 12 kHz tone sampled 45 degrees off its crests (every sample under the ceiling, the tone at full scale) is brought down. The old limiter let it through untouched.makeup_is_bounded_on_a_signal_that_is_all_peaks.Manual check left: export #1157's project (or any speech take) from a build of this branch and measure it with
ffmpeg -af ebur128=peak=true.Summary by CodeRabbit