fix(audio): preserve channels in resampleStream

.changeset/steady-stereo-resampling.md

@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+Preserve audio channels during stream resampling and reconfigure the resampler when the input channel count changes.

agents/src/utils.test.ts

@@ -923,6 +923,42 @@ world
       }
     });
 
+    it('preserves stereo channels and duration when resampling', async () => {
+      const inputFrame = createAudioFrame(48000, 960, 2);
+      const inputStream = new ReadableStream<AudioFrame>({
+        start(controller) {
+          controller.enqueue(inputFrame);
+          controller.close();
+        },
+      });
+      const frames = await streamToArray(
+        resampleStream({ stream: inputStream, outputRate: 16000 }),
+      );
+      expect(frames.length).toBeGreaterThan(0);
+      expect(frames.every((frame) => frame.channels === 2)).toBe(true);
+      expect(frames.reduce((total, frame) => total + frame.samplesPerChannel, 0)).toBe(320);
+    });
+
+    it('reconfigures the resampler when the channel count changes', async () => {
+      const inputStream = new ReadableStream<AudioFrame>({
+        start(controller) {
+          controller.enqueue(createAudioFrame(48000, 960, 1));
+          controller.enqueue(createAudioFrame(48000, 960, 2));
+          controller.close();
+        },
+      });
+      const frames = await streamToArray(
+        resampleStream({ stream: inputStream, outputRate: 16000 }),
+      );
+      for (const channels of [1, 2]) {
+        expect(
+          frames
+            .filter((frame) => frame.channels === channels)
+            .reduce((total, frame) => total + frame.samplesPerChannel, 0),
+        ).toBe(320);
+      }
+    });
+
     it('should handle same input and output rate', async () => {
       const sampleRate = 44100;
       const inputFrame = createAudioFrame(sampleRate, 1024);

agents/src/utils.ts

@@ -764,6 +764,7 @@ export function resampleStream({
 }): ReadableStream<AudioFrame> {
   let resampler: AudioResampler | null = null;
   let currentInputRate = 0;
+  let currentChannels = 0;
   const transformStream = new TransformStream<AudioFrame, AudioFrame>({
     transform(chunk: AudioFrame, controller: TransformStreamDefaultController<AudioFrame>) {
       if (chunk.samplesPerChannel === 0) {
@@ -783,15 +784,20 @@ export function resampleStream({
         return;
       }
 
-      if (!resampler || currentInputRate !== chunk.sampleRate) {
+      if (
+        !resampler ||
+        currentInputRate !== chunk.sampleRate ||
+        currentChannels !== chunk.channels
+      ) {
         if (resampler) {
           for (const frame of resampler.flush()) {
             controller.enqueue(frame);
           }
           resampler.close();
         }
-        resampler = new AudioResampler(chunk.sampleRate, outputRate);
+        resampler = new AudioResampler(chunk.sampleRate, outputRate, chunk.channels);
         currentInputRate = chunk.sampleRate;
+        currentChannels = chunk.channels;
       }
 
       for (const frame of resampler.push(chunk)) {