From 7a61de64f686ece1052d1abd810abf2c72c812cf Mon Sep 17 00:00:00 2001
From: Anthony Ettinger <anthony@profullstack.com>
Date: Thu, 24 Sep 2026 19:19:57 +0000
Subject: [PATCH] Pad the narration to the length of the picture
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Every narrated render failed validation. The voiceover for a five-second ad is
about two and a half seconds of speech, so the audio stream ended early and the
duration check — which requires picture and sound to agree within one AAC frame
— rejected all of them:

  audio duration expected 5000ms, got 2346.000ms (drift 2654.000ms)

apad extends the audio with silence indefinitely and -shortest then trims the
result back to the video's five seconds, so the track is exactly as long as the
picture. This is worth doing beyond the check: a player handed a track that
ends early is entitled to stop there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 lib/ads/video/encode.ts                  | 10 ++++++++++
 tests/ads-video-compose-validate.test.ts | 17 +++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/lib/ads/video/encode.ts b/lib/ads/video/encode.ts
index aeede367..4aaf348b 100644
--- a/lib/ads/video/encode.ts
+++ b/lib/ads/video/encode.ts
@@ -98,6 +98,16 @@ export function mp4Args(o: Mp4EncodeOptions): string[] {
   }
 
   if (o.audioPath) {
+    // `apad` extends the audio with silence indefinitely, and `-shortest` then
+    // trims the result to the video's five seconds — so the track is exactly as
+    // long as the picture.
+    //
+    // Without the pad, a narration shorter than the ad (which every narration
+    // is: a five-second read is about two and a half seconds of speech) left an
+    // audio stream that ended early. Validation requires the two durations to
+    // match within one AAC frame and rejected every narrated render, and a
+    // player handed a short track is entitled to stop at its end.
+    args.push("-af", "apad");
     args.push("-c:a", "aac", "-profile:a", "aac_low", "-ar", String(AAC_SAMPLE_RATE), "-b:a", "128k", "-ac", "2", "-shortest");
   } else {
     args.push("-an");
diff --git a/tests/ads-video-compose-validate.test.ts b/tests/ads-video-compose-validate.test.ts
index 6a834b4b..800efd76 100644
--- a/tests/ads-video-compose-validate.test.ts
+++ b/tests/ads-video-compose-validate.test.ts
@@ -187,6 +187,23 @@ describe("encoder arguments carry the media contract", () => {
     expect(narrated[narrated.indexOf("-ar") + 1]).toBe("48000");
   });
 
+  it("pads a narration to the length of the picture", () => {
+    // Every narration is shorter than its ad — a five-second read is about two
+    // and a half seconds of speech. Without the pad the audio stream ended
+    // early, which failed the duration check on every narrated render and left
+    // players entitled to stop at the end of the track.
+    const a = mp4Args({
+      framePattern: "f-%04d.png",
+      audioPath: "/tmp/narration.mp3",
+      outPath: "/tmp/out.mp4",
+      profile: "master_1080p",
+      videoKbps: 4000,
+    }).join(" ");
+    expect(a).toContain("-af apad");
+    // apad alone runs forever; -shortest is what trims it back to the video.
+    expect(a).toContain("-shortest");
+  });
+
   it("stream-copies into HLS rather than re-encoding", () => {
     const hls = hlsArgs({ inPath: "/tmp/720.mp4", outDir: "/tmp/hls", name: "720p" });
     // Re-encoding would move the keyframes and break the boundaries the MP4 was