fix: mix L/R channels when converting system audio to mono

[werpoz]

What

This PR fixes mono conversion in the macOS system-audio path so both stereo channels are used.

Why

Root cause: convertStereoToMono() was reading only the left channel and discarding the right channel.
On some setups, useful signal is mostly on the right channel, which caused very low/empty audio input.

Change

Updated convertStereoToMono() in src/utils/gemini.js to downmix stereo frames as:
mono = round((left + right) / 2)

Scope

• File: src/utils/gemini.js
• Function: convertStereoToMono
• Commit: 97f23eb

Validation

• Verified code path in macOS SystemAudioDump flow.
• Confirmed only this change is included (isolated fix).

Risk

Low. Change is localized to stereo->mono conversion and does not alter IPC/session flow.

Summary by CodeRabbit

• Bug Fixes
  • Enhanced stereo to mono audio conversion to properly blend both channels for improved audio quality, replacing the previous method that used only a single channel.

[coderabbitai]

📝 Walkthrough

Walkthrough

Modified the convertStereoToMono function in src/utils/gemini.js to average both left and right audio channels instead of using only the left channel, improving mono audio conversion quality.

Changes

Cohort / File(s)	Summary
Audio Processing Utility src/utils/gemini.js	Updated convertStereoToMono to read both left and right channels, average them, and write the resulting mixed sample to the mono buffer instead of using only the left channel.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A stereo song now sings in mono grace,
Left and right channels embrace,
Averaged together with care,
Audio balanced, perfectly fair! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: mixing left and right channels when converting stereo to mono audio, which is the core fix in the changeset.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

🧹 Nitpick comments (1)

src/utils/gemini.js (1)

743-745: Optional: guard against non-multiple-of-4 buffer lengths.

convertStereoToMono is exported and called by external paths. If stereoBuffer.length is not a multiple of 4, samples is fractional, the loop runs one extra iteration, and both readInt16LE and writeInt16LE will throw a RangeError at the boundary. Current callers always pass CHUNK_SIZE = 9600 (a multiple of 4), so this is safe today, but a small guard would make the function robust against future callers.

🛡️ Proposed defensive guard

 function convertStereoToMono(stereoBuffer) {
-    const samples = stereoBuffer.length / 4;
+    const samples = Math.floor(stereoBuffer.length / 4);
     const monoBuffer = Buffer.alloc(samples * 2);

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@src/utils/gemini.js` around lines 743 - 745, convertStereoToMono currently
assumes stereoBuffer.length is a multiple of 4 which can cause RangeError when
callers pass other sizes; update the convertStereoToMono function to guard
against non-multiple-of-4 lengths by computing samples =
Math.floor(stereoBuffer.length / 4) (or truncating/slicing the input to samples
* 4) before allocating monoBuffer and entering the readInt16LE/writeInt16LE
loop, and ensure any extra trailing bytes are ignored or handled safely so
readInt16LE/writeInt16LE never read past the buffer end.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@src/utils/gemini.js`:
- Around line 743-745: convertStereoToMono currently assumes stereoBuffer.length
is a multiple of 4 which can cause RangeError when callers pass other sizes;
update the convertStereoToMono function to guard against non-multiple-of-4
lengths by computing samples = Math.floor(stereoBuffer.length / 4) (or
truncating/slicing the input to samples * 4) before allocating monoBuffer and
entering the readInt16LE/writeInt16LE loop, and ensure any extra trailing bytes
are ignored or handled safely so readInt16LE/writeInt16LE never read past the
buffer end.