fix(render): trim AAC packet padding exactly

packages/producer/src/services/distributed/assemble.ts

@@ -291,7 +291,7 @@ export async function assemble(
     // ── 3. Audio: pad-or-trim then mux ────────────────────────────────────
     let audioForMux: string | null = null;
     if (audioPath !== null && existsSync(audioPath)) {
-      const paddedAudioPath = join(workDir, "audio-padded.aac");
+      const paddedAudioPath = join(workDir, "audio-padded.m4a");
       const padTrimResult = await padOrTrimAudioToVideoFrameCount({
         videoPath: postConcatPath,
         audioPath,

packages/producer/src/services/render/audioPadTrim.test.ts

@@ -81,15 +81,20 @@ describe("buildPadTrimAudioArgs", () => {
     expect(args[args.indexOf("-t") + 1]).toBe("1.000000");
   });
 
-  it("emits -t when audio is longer than target", () => {
-    const { args, operation } = buildPadTrimAudioArgs("/tmp/in.aac", "/tmp/out.aac", 6.123, 5.0);
+  it("filter-trims and re-encodes AAC packet padding beyond the target", () => {
+    const { args, operation } = buildPadTrimAudioArgs(
+      "/tmp/in.aac",
+      "/tmp/out.m4a",
+      15.018667,
+      15.0,
+    );
     expect(operation).toBe("trim");
-    const tIdx = args.indexOf("-t");
-    expect(tIdx).toBeGreaterThan(-1);
-    expect(args[tIdx + 1]).toBe("5.000000");
-    // Trim preserves AAC stream copy.
+    const filterIdx = args.indexOf("-af");
+    expect(args[filterIdx + 1]).toBe("atrim=duration=15.000000,asetpts=PTS-STARTPTS");
     const codecIdx = args.indexOf("-c:a");
-    expect(args[codecIdx + 1]).toBe("copy");
+    expect(args[codecIdx + 1]).toBe("aac");
+    expect(args[args.indexOf("-b:a") + 1]).toBe("192k");
+    expect(args.at(-1)).toBe("/tmp/out.m4a");
   });
 
   it("emits a plain copy when source duration matches target within ~1ms", () => {
@@ -243,8 +248,8 @@ describe("padOrTrimAudioToVideoFrameCount", () => {
     expect(result.operation).toBe("trim");
     expect(result.targetDurationSeconds).toBe(4);
     expect(captured.args).toHaveLength(1);
-    const tIdx = captured.args[0]!.indexOf("-t");
-    expect(captured.args[0]![tIdx + 1]).toBe("4.000000");
+    const filterIdx = captured.args[0]!.indexOf("-af");
+    expect(captured.args[0]![filterIdx + 1]).toBe("atrim=duration=4.000000,asetpts=PTS-STARTPTS");
   });
 
   it("emits a copy when audio duration already equals frameCount/fps", async () => {

packages/producer/src/services/render/audioPadTrim.ts

@@ -14,8 +14,9 @@
  *     "audio cuts off early" or "video shows a frozen final frame" bugs.
  *
  * The fix: post-pad/trim audio to *exactly* `frameCount / fps` seconds at
- * assemble time. Pad by concat-copying a generated silence tail, trim with
- * `-t`, and avoid re-encoding the already mixed source AAC in either case.
+ * assemble time. Pad by concat-copying a generated silence tail. For trim,
+ * decode and filter to the exact target before re-encoding into an M4A
+ * container; packet-copying AAC can only cut on packet boundaries.
  */
 
 import { spawn } from "node:child_process";
@@ -121,8 +122,8 @@ export interface PadTrimAudioPlan {
  *     tail, then concat-copy the source AAC plus that tail. This avoids
  *     re-encoding the already mixed `audio.aac`; the pad branch remains the
  *     inverse of trim instead of becoming a second full-source AAC encode.
- *   - `sourceDuration > targetDuration` → trim with `-t target`. `-c:a copy`
- *     is preserved when the input is already AAC.
+ *   - `sourceDuration > targetDuration` → filter to the exact target and
+ *     re-encode AAC so packet padding cannot outlast the video.
  *   - `|Δ| < AUDIO_DURATION_TOLERANCE_SECONDS` → no-op `copy`, but we still
  *     run ffmpeg with `-c:a copy` to materialize the output path.
  */
@@ -187,13 +188,27 @@ export function buildPadTrimAudioPlan(
       cleanupPaths: [silencePath, concatListPath],
     };
   }
-  // Trim. `-t` truncates AAC without re-encoding because AAC frames are
-  // independently decodable; ffmpeg snaps the cut point to the nearest
-  // packet boundary, fine for the ±1ms tolerance we care about here.
+  // Packet-copy trimming snaps to AAC frame boundaries (typically 1024
+  // samples), which can leave ~20ms beyond the target. Decode/filter/re-encode
+  // into M4A so ffmpeg records the exact presentation duration.
   return {
     operation: "trim",
     steps: [
-      { kind: "trim", args: ["-i", audioPath, "-t", targetSec, "-c:a", "copy", "-y", outputPath] },
+      {
+        kind: "trim",
+        args: [
+          "-i",
+          audioPath,
+          "-af",
+          `atrim=duration=${targetSec},asetpts=PTS-STARTPTS`,
+          "-c:a",
+          "aac",
+          "-b:a",
+          "192k",
+          "-y",
+          outputPath,
+        ],
+      },
     ],
     cleanupPaths: [],
   };

packages/producer/src/services/render/stages/assembleStage.test.ts

@@ -49,7 +49,7 @@ describe("runAssembleStage audio duration parity", () => {
     muxVideoWithAudioMock.mockResolvedValue({ success: true });
     padOrTrimAudioMock.mockResolvedValue({
       success: true,
-      outputPath: "/tmp/audio.duration-normalized.aac",
+      outputPath: "/tmp/audio.duration-normalized.m4a",
       targetDurationSeconds: 1,
       sourceDurationSeconds: 1.024,
       operation: "trim",
@@ -62,11 +62,11 @@ describe("runAssembleStage audio duration parity", () => {
     expect(padOrTrimAudioMock).toHaveBeenCalledWith({
       videoPath: "/tmp/video-only.mp4",
       audioPath: "/tmp/audio.aac",
-      outputPath: "/tmp/audio.duration-normalized.aac",
+      outputPath: "/tmp/audio.duration-normalized.m4a",
     });
     expect(muxVideoWithAudioMock).toHaveBeenCalledWith(
       "/tmp/video-only.mp4",
-      "/tmp/audio.duration-normalized.aac",
+      "/tmp/audio.duration-normalized.m4a",
       "/tmp/output.mp4",
       undefined,
       { audioCodec: "aac" },
@@ -80,14 +80,14 @@ describe("runAssembleStage audio duration parity", () => {
     expect(padOrTrimAudioMock).toHaveBeenCalledWith({
       videoPath: "/tmp/video-only.mp4",
       audioPath: "/tmp/audio.m4a",
-      outputPath: "/tmp/audio.duration-normalized.aac",
+      outputPath: "/tmp/audio.duration-normalized.m4a",
     });
   });
 
   it("fails instead of muxing an unnormalized AAC tail", async () => {
     padOrTrimAudioMock.mockResolvedValue({
       success: false,
-      outputPath: "/tmp/audio.duration-normalized.aac",
+      outputPath: "/tmp/audio.duration-normalized.m4a",
       targetDurationSeconds: 1,
       sourceDurationSeconds: 1.024,
       operation: "trim",

packages/producer/src/services/render/stages/assembleStage.ts

@@ -61,7 +61,7 @@ export async function runAssembleStage(input: AssembleStageInput): Promise<Assem
     const audioStem = audioExtension
       ? audioOutputPath.slice(0, -audioExtension.length)
       : audioOutputPath;
-    const normalizedAudioPath = `${audioStem}.duration-normalized.aac`;
+    const normalizedAudioPath = `${audioStem}.duration-normalized.m4a`;
     const normalizeResult = await padOrTrimAudioToVideoFrameCount({
       videoPath: videoOnlyPath,
       audioPath: audioOutputPath,