feat(stt): align words with character-level DTW

[EtienneLescot]

Summary

Phase 2 of #948 meets its acceptance on both languages. The helper's DTW pass now teacher-forces the transcript one character per decoder row instead of BPE tokens ("Whisper Has an Internal Word Aligner", arXiv 2509.09987).

What changes

• electron/native/whisper-stt/whisper-patches/char-dtw.cpp replaces whisper.cpp's whisper_exp_compute_token_level_timestamps_dtw. Same signature and same t_dtw meaning (end of the token), so main.cpp and the TS post-pass are unchanged.
• CMakeLists.txt splices it into a build-tree copy of whisper.cpp at configure time. The fetched source is never edited, so a shared FETCHCONTENT_SOURCE_DIR_WHISPER and the Nix build (which uses that override) get the patch too. A WHISPER_REF bump fails the configure if the replaced code moved.
• Inside the pass: per-frame L2 normalisation of the averaged heads (paper) instead of z-score + median filter; French drops one silent final consonant per word (vais, vous, plaît); windows longer than 448 positions as characters are aligned in several runs; t_dtw never steps back between runs.
• Doc (§ Word-level alignment) updated. The harness takes OSC_WORD_TIMING_PORT to run beside another agent.

Measurements

Harness tools/stt-eval/word-timing, Vulkan, ggml-small-q8_0, clean + noisy corpus, post-pass as shipped. Times in ms.

Pipeline	Inner start: median / P90 / within 50 ms	FR inner	EN inner	Phrase-initial within 50 ms	1-word delete: clean / residue / clipping	Phrase delete clean	WER
Phase 1 (main)	31 / 125 / 64%	26 / 70%	40 / 57%	89%	19% / 40 / 45	88%	9.1%
This PR	16 / 60 / 86%	19 / 83%	15 / 89%	89%	39% / 19 / 25	93%	9.1%

Runtime, same 2 builds alternated twice, median per-clip ratio:

• Vulkan (96 clips): −6% and −9%. The upstream median filter cost more than the longer decoder pass.
• CPU, 16 threads (16 clips): +5% and +5%.

Tried and dropped (same harness)

• Spaces and punctuation skipped when looking for the next boundary: 69 ms median. Skipping punctuation only: 21 ms, phrase starts 65%, because a phrase's last word then ends after the pause.
• Space merged with the next letter ( w tokens): 23 ms, P90 tails up to 620 ms in EN.
• Upstream z-score + median filter on characters: 27 ms, FR 40 ms.
• Per-window head selection by attention concentration over all 144 heads (paper): it picks nearly the same 10 every window; top 5/10/20 give 27/24/25 ms and phrase starts 81–86%. That fixed set as custom heads: 20 ms. WHISPER_AHEADS_SMALL stays.
• Dropping punctuation: 20 ms, phrase delete 77%. Lowercasing: 17 ms, no gain.
• French mute final e on top of the consonant rule: FR 18 vs 19 ms but phrase starts 77% vs 80%, kept out.
• Blending BPE and character times (0.3/0.7): 23 ms, worse in EN than characters alone.

Real speech (real-check.mjs, 25 s French take): every phrase start lands on the VAD onset after the post-pass, as on main.

Related issue

Refs #948

Type of change

• Bug fix
• Feature
• Enhancement
• Documentation
• Refactor / maintenance
• Performance
• Security

Release impact

• Patch
• Minor
• Major / breaking change
• No release note needed

Desktop impact

• Windows
• macOS
• Linux
• Installer / packaging
• Not platform-specific

Screenshots / video

None, no UI change.

Testing

• Corpus generated on Windows (make-corpus.mjs, validate-ref.mjs: reference checks as in Transcript word timings: measure them, then make them precise enough for text-based cuts #948).
• run-helper.mjs + evaluate.mjs on the Phase 1 helper built from main and on this branch, Vulkan; runtime on Vulkan and --cpu.
• No TS change, so no unit test. The helper has no C++ tests; build-whisper-stt.yml builds it on the 4 platforms.
• Nix not built locally: it configures through the same CMakeLists.txt with FETCHCONTENT_SOURCE_DIR_WHISPER, which this splice supports.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements

  • Improved word and caption timing by aligning speech at the character level, producing more precise timestamps.
  • Improved alignment for French word endings and longer transcription segments, which are processed in multiple passes.

• Documentation

  • Updated transcription guidance with revised timing measurements, language-specific results, and known limitations.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 4ed6a9c and c9bff0f.

📒 Files selected for processing (2)

• electron/native/whisper-stt/CMakeLists.txt
• tools/stt-eval/word-timing/lib.mjs

🚧 Files skipped from review as they are similar to previous changes (2)

• tools/stt-eval/word-timing/lib.mjs
• electron/native/whisper-stt/CMakeLists.txt

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📝 Walkthrough

Walkthrough

The Whisper build now uses character-level DTW alignment in place of token-level alignment. The architecture documentation describes the method and reports timing measurements. The word-timing helper now supports a configurable starting port for its 100-port range.

Changes

Whisper character-level alignment

Layer / File(s)	Summary
Character-level alignment and timing electron/native/whisper-stt/whisper-patches/char-dtw.cpp, technical-documentation/architecture/transcription-and-captions.md	The alignment code splits eligible vocabulary tokens into character pieces, handles specified French final consonants, decodes windows, and assigns nondecreasing timestamps from DTW paths. The documentation describes the alignment method and reports timing results.
Build-tree patch integration electron/native/whisper-stt/CMakeLists.txt	CMake generates and validates a patched build-tree copy of whisper.cpp, then directs the whisper target to compile it.

Word-timing helper port range

Layer / File(s)	Summary
Configurable helper port range tools/stt-eval/word-timing/lib.mjs, tools/stt-eval/word-timing/README.md	The helper uses OSC_WORD_TIMING_PORT as the starting port when set, with a default of 20500, and selects a random offset from 0 through 99. The README documents the range.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Alignment as whisper_exp_compute_token_level_timestamps_dtw
  participant Decoder as Whisper decoder
  participant DTW
  Alignment->>Decoder: Decode alignment window
  Decoder-->>Alignment: Return cross-attention data
  Alignment->>DTW: Compute path from normalized cross-attention
  DTW-->>Alignment: Return path entries for timestamp assignment

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Merge Risk: ⚪ Minimal · up to c9bff

The build integration and configurable helper port preserve their intended contracts. No actionable merge-blocking issue remains, subject to normal build and test checks.

Architecture Summary

Architecture risk: 🔵 Low · up to 4ed6a

The change affects 3 systems.

Changed systems: electron, tools, technical-documentation

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — electron (service) was modified; 2 changed files map to changed impact.
• observed — tools (service) was modified; 2 changed files map to changed impact.
• observed — technical-documentation (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in electron/native/whisper-stt/CMakeLists.txt: CMake now generates a patched build-tree copy of whisper.cpp, replacing the region from median_filter_user_data up to whisper_log_set with the character-level DTW fragment. It normalizes source line endings and verifies that the region and expected DTW function exist; if not, configuration fails with a fatal error. It tracks both input files for reconfiguration and redirects the whisper target’s source to the generated copy.
• observed — Modified behavior in electron/native/whisper-stt/whisper-patches/char-dtw.cpp: The new comments describe character-level teacher-forced DTW, its timestamp convention, French consonant handling, window splitting, and nondecreasing timestamps.
• observed — Modified behavior in electron/native/whisper-stt/whisper-patches/char-dtw.cpp: The replacement function gathers non-EOT tokens from the requested segments, returns when none are present, and builds the decoder prefix from the start-of-transcription token, optional language token, and not token.
• observed — Modified behavior in electron/native/whisper-stt/whisper-patches/char-dtw.cpp: Each text token is split into character pieces using vocabulary lookups, falling back to individual bytes when a character has no direct token; if any byte cannot be found, the original token is retained unsplit.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: character-level DTW alignment for STT word timing.
Description check	✅ Passed	The description follows the required template and includes a summary, related issue, change type, release impact, platform impact, screenshots status, measurements, and testing details.
Docstring Coverage	✅ Passed	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 2 functions across 2 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @electron/native/whisper-stt/CMakeLists.txt:
- Around line 175-177: Add a configure-time check after the source
transformation in the `whisper` target setup to verify `osc_whisper_srcs`
contains the patched `whisper.cpp` path; if it does not, stop configuration with
a fatal error. Keep the existing `set_property` behavior for a successful
replacement.

Review comments at @tools/stt-eval/word-timing/lib.mjs:
- Line 30: Update the port selection in startHelper to read OSC_WORD_TIMING_PORT
from the supplied env object instead of process.env, preserving the existing
default and random offset.

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Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: aded4efe-12e0-4fa1-9d44-0290abd0431b

📥 Commits

Reviewing files that changed from the base of the PR and between e04943a and 4ed6a9c.

📒 Files selected for processing (5)

• electron/native/whisper-stt/CMakeLists.txt
• electron/native/whisper-stt/whisper-patches/char-dtw.cpp
• technical-documentation/architecture/transcription-and-captions.md
• tools/stt-eval/word-timing/README.md
• tools/stt-eval/word-timing/lib.mjs

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