Bolt: Optimize Mel Filterbank with Sparse Matrix Multiplication

[ysdede]

What: Implemented sparse matrix optimization for the Mel filterbank application step.
Why: The Mel filterbank matrix is ~98.5% sparse (triangular filters). Iterating over all elements (including zeros) was a major performance bottleneck (32k ops/frame vs ~500 ops/frame needed).
Impact:

• 5s audio processing: ~76ms -> ~18ms (~4.2x faster)
• 10s audio processing: ~148ms -> ~62ms (~2.4x faster)
• Incremental processing: ~80ms -> ~18ms (~4.4x faster)
  Measurement: Verify with npm test which includes performance benchmarks.

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PR created automatically by Jules for task 7908949670163162976 started by @ysdede

Summary by CodeRabbit

Release Notes

• Refactor
• Optimized Mel spectrogram computation for improved performance by streamlining the filterbank calculation process.

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR optimizes Mel filterbank computation by precomputing sparse index ranges (_fbStart and _fbEnd) for each Mel band to identify non-zero coefficient spans, then uses these ranges during computation to iterate only over relevant indices instead of processing the entire filterbank.

Changes

Cohort / File(s)	Summary
Mel Filterbank Optimization src/mel.js	Adds precomputed sparse index ranges for Mel bands in the constructor and refactors computeRawMel to iterate only over non-zero spans, improving computational efficiency of Mel filterbank application.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 Sparse indices hop with glee,
No more full vectors we shall see,
Each Mel band now knows where to play,
Filterbanks computed the nimble way! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly aligns with the main objective: optimizing the Mel filterbank using sparse matrix multiplication. It is specific, concise, and clearly summarizes the primary change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bolt/sparse-mel-filterbank-7908949670163162976

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