From 81abc4acca0b18837118b8cff7d2992bd881e3ae Mon Sep 17 00:00:00 2001
From: Deivid Soto <hello@torrentclaw.com>
Date: Thu, 7 May 2026 14:59:43 +0200
Subject: [PATCH] fix(transcoder): force aac stereo 48khz + frag_duration for
 mse compat
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Two transcoder fixes for browser MediaSource Extensions parsing:

1. -ar 48000 -ac 2 on the audio output. Source 5.1 / 7.1 streams produced
   a moov atom Chrome CHUNK_DEMUXER refuses to parse, even when the video
   metadata is fine and a non-MSE video element accepts the same file.
   Forcing AAC-LC stereo 48 kHz makes the moov shape MSE-compatible.

2. -frag_duration 1000000 (1 second) so each moof+mdat fragment caps at
   ~1s of media. Without it, ffmpeg only splits at keyframes and high-
   bitrate 1080p produces 8 MiB+ mdat boxes — MSE waits for the whole
   mdat before parsing the first fragment, so playback never starts.

3. -movflags +negative_cts_offsets so b-frames carry the right pts/dts
   offsets and the playhead doesn't reset every fragment.

4. New range_req debug log to make sizing bugs greppable.
---
 internal/engine/transcoder.go | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/internal/engine/transcoder.go b/internal/engine/transcoder.go
index 1752d6d..215f5bd 100644
--- a/internal/engine/transcoder.go
+++ b/internal/engine/transcoder.go
@@ -259,18 +259,34 @@ func buildFFmpegArgs(filePath string, opts TranscodeOpts) []string {
 			filterChain = "format=yuv420p,setparams=colorspace=bt709:color_trc=bt709:color_primaries=bt709:range=tv"
 		}
 		args = append(args, "-vf", filterChain)
-		args = append(args, "-c:a", "aac", "-b:a", coalesce(opts.AudioBitrate, "192k"))
+		// Force AAC-LC stereo 48 kHz so MSE's CHUNK_DEMUXER accepts the moov.
+		// 5.1 / 7.1 source streams produce a moov shape that MSE refuses to
+		// parse (the <video src=blob:> demuxer is more forgiving), so we
+		// always downmix to stereo and resample to 48 kHz here. Source
+		// material that's already stereo passes through losslessly aside
+		// from the re-encode.
+		args = append(args,
+			"-c:a", "aac",
+			"-b:a", coalesce(opts.AudioBitrate, "192k"),
+			"-ar", "48000",
+			"-ac", "2",
+		)
 	}
 
 	// Common output flags — fragmented MP4 to a single pipe.
-	// NO faststart: that flag rewrites the moov atom to the front of the
-	// file as a SECOND pass after encoding finishes, which means the
-	// browser never sees a moov until ffmpeg exits. For live transcoding
-	// we need empty_moov (write a placeholder up front) so MSE can start
-	// decoding the very first fragment. faststart is only safe for
-	// already-finished files.
+	//
+	//   * empty_moov + default_base_moof: write a header-only init segment
+	//     up front so MSE can start decoding before the file is finished.
+	//   * frag_duration=1s: cap each moof+mdat at ~1 second of media. Without
+	//     this, ffmpeg only splits at keyframes, which on a high-bitrate
+	//     1080p stream produces 8 MiB+ mdat boxes — MSE refuses to parse
+	//     the first fragment until the whole mdat lands, so playback never
+	//     starts.
+	//   * negative_cts_offsets: lets b-frames carry the right pts/dts so
+	//     decoders don't reset the playhead to 0 every fragment.
 	args = append(args,
-		"-movflags", "frag_keyframe+empty_moov+default_base_moof",
+		"-movflags", "+frag_keyframe+empty_moov+default_base_moof+negative_cts_offsets",
+		"-frag_duration", "1000000",
 		"-f", "mp4",
 		"pipe:1",
 	)