fix(audio): renumber timestamps between apad and atrim in mixed branches

packages/engine/src/services/audioMixer.test.ts

@@ -296,7 +296,7 @@ describe("processCompositionAudio", () => {
 
     expect(filter).toContain("volume=0");
     expect(filter).toContain("[mixed]volume=1[out]");
-    expect(filter).toContain("apad,atrim=0:2");
+    expect(filter).toContain("apad,asetpts=N/SR/TB,atrim=0:2");
     expect(filter).not.toContain("whole_dur");
     expect(filter).not.toContain("normalize=");
     expect(filter).not.toContain("weights=");
@@ -464,7 +464,7 @@ describe("processCompositionAudio", () => {
     // 2 s clip + the 1.9 s tail 0.6 + size * 2.6 generates.
     expect(filter).toContain("atrim=0:3.9,");
     // And still cut at the composition's end, so a tail cannot extend the video.
-    expect(filter).toContain("apad,atrim=0:8");
+    expect(filter).toContain("apad,asetpts=N/SR/TB,atrim=0:8");
   });
 
   it("hands the volume envelope to the FX pass instead of ducking the file after it", async () => {
@@ -1078,6 +1078,60 @@ describe("processCompositionAudio", () => {
     expect((filter?.match(/apad,/g) ?? []).length).toBe(trackCount);
   });
 
+  it("renumbers timestamps between apad and atrim on every mixed branch", async () => {
+    // Regression: `apad` then `atrim` is the portable pad-to-length shape --
+    // #2769 moved off `apad=whole_dur=` because some builds reject that option.
+    // But on FFmpeg 5.x-8.0.x the padded samples carry timestamps `atrim`
+    // misreads, so a delayed branch sounds at t=0 and, past three branches, the
+    // last one vanishes from the mix. `asetpts=N/SR/TB` between the two rebuilds
+    // the timestamps from the sample count and costs no portability, since all
+    // three filters exist in every build we support.
+    const baseDir = mkdtempSync(join(tmpdir(), "hf-audio-base-"));
+    const workDir = mkdtempSync(join(tmpdir(), "hf-audio-work-"));
+    tempDirs.push(baseDir, workDir);
+    writeFileSync(join(baseDir, "a.wav"), "stub");
+    writeFileSync(join(baseDir, "b.wav"), "stub");
+
+    // Two branches, the second delayed: the shape that misplaced audio.
+    await processCompositionAudio(
+      [
+        {
+          id: "a",
+          src: "a.wav",
+          start: 0,
+          end: 1,
+          mediaStart: 0,
+          layer: 0,
+          volume: 1,
+          type: "audio",
+        },
+        {
+          id: "b",
+          src: "b.wav",
+          start: 4,
+          end: 5,
+          mediaStart: 0,
+          layer: 1,
+          volume: 1,
+          type: "audio",
+        },
+      ],
+      baseDir,
+      workDir,
+      join(baseDir, "out.m4a"),
+      8,
+    );
+
+    const filter = capturedFilterScripts.at(-1) ?? "";
+    const branches = filter.match(/apad[^;]*/g) ?? [];
+    expect(branches).toHaveLength(2);
+    for (const branch of branches) {
+      expect(branch).toMatch(/^apad,asetpts=N\/SR\/TB,atrim=0:/);
+    }
+    // The portability constraint #2769 established still holds.
+    expect(filter).not.toContain("whole_dur");
+  });
+
   it("retries with the current file-valued filter option when a nightly removes the legacy alias", async () => {
     const baseDir = mkdtempSync(join(tmpdir(), "hf-audio-base-"));
     const workDir = mkdtempSync(join(tmpdir(), "hf-audio-work-"));

packages/engine/src/services/audioMixer.ts

@@ -709,8 +709,19 @@ async function mixAudioTracks(
       // can run over what follows but never past the end of the video.
       const trimDuration = track.end - track.start + (track.tailSeconds ?? 0);
       const volumeFilter = buildVolumeExpression(track, ignoreAutomation);
+      // `apad` then `atrim` is the portable pad-to-length shape: PR #2769 moved
+      // off `apad=whole_dur=` because some FFmpeg builds reject that option
+      // outright ("Error applying option 'whole_dur': Option not found").
+      // But on FFmpeg 5.x through 8.0.x the samples `apad` appends carry
+      // timestamps the following `atrim` misreads, so a delayed branch lands at
+      // t=0 and, once four or more branches are mixed, the last one disappears
+      // entirely. `asetpts=N/SR/TB` renumbers the padded stream from the sample
+      // count before the trim reads it, which fixes the misplacement while
+      // keeping the filter set every build supports. Verified correct on 4.2.7,
+      // 7.0.2, an 8.x nightly and 8.1.1; the un-reset form is wrong on the
+      // middle two.
       filterParts.push(
-        `[${i}:a]atrim=0:${formatFilterNumber(trimDuration)},${volumeFilter},adelay=${delayMs}|${delayMs},apad,atrim=0:${formatFilterNumber(totalDuration)}[a${i}]`,
+        `[${i}:a]atrim=0:${formatFilterNumber(trimDuration)},${volumeFilter},adelay=${delayMs}|${delayMs},apad,asetpts=N/SR/TB,atrim=0:${formatFilterNumber(totalDuration)}[a${i}]`,
       );
     });
 

packages/producer/src/services/audioExtractor.ts

@@ -211,8 +211,9 @@ async function mixTracks(
     const delayMs = Math.round(track.start * 1000);
     const trimDuration = track.duration > 0 ? track.duration : totalDuration;
 
+    // See audioMixer.ts for why asetpts sits between apad and atrim.
     filterParts.push(
-      `[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},apad,atrim=0:${totalDuration}[a${i}]`,
+      `[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},apad,asetpts=N/SR/TB,atrim=0:${totalDuration}[a${i}]`,
     );
   });